Professor David Sampson
Professor David Sampson is the Pro-Vice-Chancellor, Research & Innovation. David’s portfolio includes: research and innovation strategy and services; research training (through the Doctoral College); knowledge exchange and commercialisation; and the Surrey Research Park.
The Pro-Vice-Chancellor, Research and Innovation works closely with and is supported by the three Faculty Associate Deans, Research and Innovation, the four Research Theme Champions, and the Academic Lead, Research Culture and Integrity.
He is a member of Council, director of the University of Surrey Seed Fund and of SETsquared Limited, serves on the board of the Surrey Research Park, the editorial board of The Conversation (UK), and on the boards of SETsquared Partnership, and SPRINT.
Previously, David was with the University of Western Australia (UWA), Perth, Australia.
David's research interests are described below under Research. David is heavily involved in the global optics & photonics community, serving as an elected Director of the SPIE – The International Society for Optics & Photonics (2017-2019). He is a fellow of the optics societies, SPIE and OSA, and the electrical engineering society, IEEE. He serves on various society committees and editorial boards, including currently as adviser to the Board of Directors of SPIE.
24 JUN 2021
University of Surrey joins renowned networks to strengthen its impact on research and policy
04 JUN 2021
Surrey subjects ranked amongst the best in the world by the 2021 Global Ranking of Academic Subjects
31 MAR 2021
Surrey marks strong performance as part of the first Knowledge Exchange Framework dashboards
26 MAR 2021
Professor David Sampson elected to the prestigious American Institute for Medical and Biological Engineering's College of Fellows.
24 MAR 2021
University of Surrey awarded funding to build a new radiochemistry laboratory for environmental research into microplastics
12 FEB 2021
University of Surrey announces new Centre for Innovation and Commercialisation, as report finds the University is already contributing nearly £2bn in value and 19,430 jobs to the UK
19 JAN 2021
Surrey project to address the problem of indoor air pollution affecting millions in developing countries
21 OCT 2020
Surrey’s open research position statement promises to lift barriers to accessing knowledge
13 AUG 2020
Surrey Professor awarded first NPL Fellowship in Nuclear and Radiation Science and Metrology
11 MAR 2020
The University of Surrey has been awarded €1.4 million to undertake cutting edge research into infectious disease and antimicrobial resistance
08 JAN 2020
University of Surrey’s Faculty of Arts and Social Sciences holds Festival of Research 2020
19 JUN 2019
University of Surrey and its Research Park launch collaborative initiatives to help accelerate business growth in Guildford
16 MAY 2019
University of Surrey research that helped halve global meningitis death rate ranked as pioneering health breakthrough
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Professor Sampson’s research interests are in the science and applications of light in medicine and biology, fields known as biomedical optics and biophotonics, and he is an authority on optical coherence tomography (OCT), having made contributions to underlying understanding, new methods and translational applications in surgical guidance and biopsy. His current interests are in the microscope-in-a-needle (for which he was awarded the IEEE Distinguished Lecturer Award and several other prizes); optical elastography, the micro-scale imaging of the mechanical properties of tissue; and other parametric extensions of OCT, such as polarisation-sensitive OCT and OCT angiography. The microscope-in-a-needle and elastography are both undergoing commercialisation. He has published more than 180 journal papers, attracting in excess of 9,000 citations, presented more than 100 invited and plenary lectures and raised more than £27.5M in research funding.
Three-dimensional optical coherence tomography (3D-OCT) is used to evaluate the structure and pathology of regenerating mouse skeletal muscle autografts for the first time. The death of myofibers with associated inflammation and subsequent new muscle formation in this graft model represents key features of necrosis and inflammation in the human disease Duchenne muscular dystrophy. We perform 3D-OCT imaging of excised autografts and compare OCT images with coregistered histology. The OCT images readily distinguish the necrotic and inflammatory tissue of the graft from the intact healthy muscle fibers in the underlying host tissue. These preliminary findings suggest that, with further development, 3D-OCT could be used as a tool for the evaluation of small-animal muscle morphology and pathology, in particular, for analysis of mouse models of muscular dystrophy.
Deciphering the role of cell-to-cell communication in acquisition of cancer traits such as metastasis is one of the key challenges of integrative biology and clinical oncology. In this context, extracellular vesicles (EVs) are important vectors in cell-to-cell communication and serve as conduits in the transfer of cellular constituents required for cell function and for the establishment of cellular phenotypes. In the case of malignancy, they have been shown to support the acquisition of common traits defined as constituting the hallmarks of cancer. Cellular biophysics has contributed to our understanding of some of these central traits with changes in tissue biomechanics reflective of cell state. Indeed, much is known about stiffness of the tissue scaffold in the context of cell invasion and migration. This article advances this knowledge frontier by showing for the first time that EVs are mediators of tissue biomechanical properties and, importantly, demonstrates a link between the acquisition of cancer multidrug resistance and increased tissue stiffness of the malignant mass. The methodology used in the study employed optical coherence elastography and atomic force microscopy on breast cancer cell monolayers and tumor spheroids. Specifically, we show here that the acquired changes in tissue stiffness can be attributed to the intracellular transfer of a protein complex comprising ezrin, radixin, moesin, CD44, and P-glycoprotein. This has important implications in facilitating mechano-transduced signaling cascades that regulate the acquisition of cancer traits, such as invasion and metastasis. Finally, this study also introduces novel targets and strategies for diagnostic and therapeutic innovation in oncology, with a view to prevention of metastatic spread and personalized medicine in cancer treatment.
Visualizing stiffness within the local tissue environment at the cellular and subcellular level promises to provide insight into the genesis and progression of disease. In this Letter, we propose ultrahigh-resolution optical coherence elastography (UHROCE), and demonstrate 3D imaging of local axial strain of tissues undergoing compressive loading. We combine optical coherence microscopy (OCM) and phase-sensitive detection of local tissue displacement to produce strain elastograms with resolution (
Assessment of vasculature is an important aspect of monitoring healing of cutaneous burn injuries. Recent advances in optical coherence tomography (OCT) have enabled it to be used to perform high‐resolution imaging of the cutaneous vasculature in vivo, with the potential to provide a superior alternative to the conventional assessment of scoring skin color. The goal of this study is to investigate the feasibility of OCT angiography for longitudinal monitoring of vasculature and identification of vascular features in human cutaneous burns. We integrate several OCT imaging protocols and image‐processing techniques into a systematic method for longitudinal monitoring and automatic quantification. The demonstration of this method on a partial‐thickness burn shows the accurate co‐location of longitudinal scans; characteristic vascular features in different healing phases; and eventual decrease of the elevated vasculature area density and vessel diameter to normal levels. Such a method holds promise for longitudinal monitoring of vasculature in burn injures as well as in other cutaneous vascular pathologies and responses to treatment.
We demonstrate polarization sensitive OCT using miniaturized needle probes. Employing the Mueller-formalism, we reconstruct tissue birefringence and retrieve the depolarization index of ex vivo tissue samples, providing contrast complementary to the structural intensity signal.
Stromal collagen organization has been identified as a potential prognostic indicator in a variety of cancers and other diseases accompanied by fibrosis. Changes in the connective tissue are increasingly considered for grading dysplasia and progress of oral squamous cell carcinoma, investigated mainly ex vivo by histopathology. In this study, polarization-sensitive optical coherence tomography (PS-OCT) with local phase retardation imaging is used for the first time to visualize depth-resolved (i.e., local) birefringence of healthy human oral mucosa in vivo. Depth-resolved birefringence is shown to reveal the expected local collagen organization. To demonstrate proof-of-principle, 3D image stacks were acquired at labial and lingual locations of the oral mucosa, chosen as those most commonly affected by cancerous alterations. To enable an intuitive evaluation of the birefringence images suitable for clinical application, color depth-encoded en-face projections were generated. Compared to en-face views of intensity or conventional cumulative phase retardation, we show that this novel approach offers improved visualization of the mucosal connective tissue layer in general, and reveals the collagen fiber architecture in particular. This study provides the basis for future prospective pathological and comparative in vivo studies non-invasively assessing stromal changes in conspicuous and cancerous oral lesions at different stages.
This study describes a framework for characterizing resolution and sensitivity in optical coherence micro-elastography, and presents a means of optimizing these parameters through spatial filtering. Results show improved axial resolution with no loss in sensitivity.
Assessment of lymph node involvement is a key prognostic marker in early breast cancer. This paper demonstrates the ability of optical coherence tomography (OCT) to characterise the micro-architecture of healthy, non-cancerous lymph nodes. OCT is shown to differentiate stroma, cortex and adipose tissue. Characteristic patterns are also identified for germinal centres and blood vessels within the node. Results are correlated against a histopathological gold standard.
We show for the first time, to our knowledge, high-resolution wide-field images of biological samples recorded using coherent aperture-synthesis Fourier holography. To achieve this, we combined off-axis plane-wave polarized illumination with an axial sample rotation and polarization-sensitive collection of backscat- tered light. We synthesized 180 Fourier holograms using an efficient postdetection phase-matching correlation scheme. The result was an annular spatial frequency-space synthetic aperture (NA=0.93) with an effective area 25 times larger than that due to a single hologram. A high-resolution high-contrast microscopic reconstruction of biological tissue was computed over a sample area of 9mm2 from holograms acquired at 34 mm working distance.
Probing the mechanical properties of tissue on the microscale could aid in the identification of diseased tissues that are inadequately detected using palpation or current clinical imaging modalities, with potential to guide medical procedures such as the excision of breast tumours. Compression optical coherence elastography (OCE) maps tissue strain with microscale spatial resolution and can delineate microstructural features within breast tissues. However, without a measure of the locally applied stress, strain provides only a qualitative indication of mechanical properties. To overcome this limitation, we present quantitative micro-elastography, which combines compression OCE with a compliant stress sensor to image tissue elasticity. The sensor consists of a layer of translucent silicone with well-characterized stress-strain behaviour. The measured strain in the sensor is used to estimate the two-dimensional stress distribution applied to the sample surface. Elasticity is determined by dividing the stress by the strain in the sample. We show that quantification of elasticity can improve the ability of compression OCE to distinguish between tissues, thereby extending the potential for inter-sample comparison and longitudinal studies of tissue elasticity. We validate the technique using tissue-mimicking phantoms and demonstrate the ability to map elasticity of freshly excised malignant and benign human breast tissues.
Optical fibers are widely used in medicine and biology for routine tasks such as beam delivery and video endoscopy, to more advanced systems such as nonlinear endoscopy and the Microscope-in-a-Needle platform. This talk will review basic principles and survey state-of-the-art applications.
An optical pulse sequence derived from a single pulse can be summed coherently by a delay network provided that delay lengths are strictly controlled. We demonstrate coherent correlation experimentally, using an electronic feedback circuit to interferometrically stabilize fiber delay lengths. The potential for application to code-division multiple access (CDMA) networks is demonstrated by using coherent correlation to distinguish a data sequence from an incoherent back-ground signal.
Optical elastography is aimed at using the visco-elastic properties of soft tissue as a contrast mechanism, and could be particularly suitable for high-resolution differentiation of tumour from surrounding normal tissue. We present a new approach to measure the effect of an applied stimulus in the kilohertz frequency range that is based on optical coherence tomography. We describe the approach and present the first in vivo optical coherence elastography measurements in human skin at audio excitation frequencies.
Incomplete excision of malignant tissue is a major issue in breast-conserving surgery, with typically 20 - 30% of cases requiring a second surgical procedure arising from postoperative detection of an involved margin. We report advances in the development of a new intraoperative tool, optical coherence micro-elastography, for the assessment of tumor margins on the micro-scale. We demonstrate an important step by conducting whole specimen imaging in intraoperative time frames with a wide-field scanning system acquiring mosaicked elastograms with overall dimensions of ~50 × 50 mm, large enough to image an entire face of most lumpectomy specimens. This capability is enabled by a wide-aperture annular actuator with an internal diameter of 65 mm. We demonstrate feasibility by presenting elastograms recorded from freshly excised human breast tissue, including from a mastectomy, lumpectomies and a cavity shaving.
Histologic assessment is the gold standard technique for the identification of metastatic involvement of lymph nodes in malignant disease, but can only be performed ex vivo and often results in the unnecessary excision of healthy lymph nodes, leading to complications such as lymphedema. Optical coherence tomogra-phy (OCT) is a high-resolution, near-IR imaging modality capable of visualizing microscopic features within tissue. OCT has the potential to provide in vivo assessment of tissue involvement by cancer. In this morpho-logic study, we show the capability of OCT to image nodal microarchitecture through an assessment of fresh, unstained ex vivo lymph node samples. Examples include both benign human axillary lymph nodes and nodes containing metastatic breast carcinoma. Through accurate correlation with the histologic gold standard, OCT is shown to enable differentiation of lymph node tissue from surrounding adipose tissue, reveal nodal struc-tures such as germinal centers and intranodal vessels, and show both diffuse and well circumscribed patterns of metastatic node involvement.
Visualizing stiffness within the local tissue environment at the cellular and sub-cellular level promises to provide insight into the genesis and progression of disease. In this paper, we propose ultrahigh-resolution optical coherence elastography, and demonstrate three-dimensional imaging of local axial strain of tissues undergoing compressive loading. The technique employs a dual-arm extended focus optical coherence microscope to measure tissue displacement under compression. The system uses a broad bandwidth supercontinuum source for ultrahigh axial resolution, Bessel beam illumination and Gaussian beam detection, maintaining sub-2 μm transverse resolution over nearly 100 μm depth of field, and spectral-domain detection allowing high displacement sensitivity. The system produces strain elastograms with a record resolution (x,y,z) of 2×2×15 μm. We benchmark the advances in terms of resolution and strain sensitivity by imaging a suitable inclusion phantom. We also demonstrate this performance on freshly excised mouse aorta and reveal the mechanical heterogeneity of vascular smooth muscle cells and elastin sheets, otherwise unresolved in a typical, lower resolution optical coherence elastography system.
The advances reported herein form part of a larger project that has as its objective the development of a full flow-structure-interaction model of the human upper airway. Here we first briefly report on a two-dimensional (saggital section) model built using published CT-scan geometric data. For the development of our three-dimensional capability, we use the unique data captured in vivo by an endoscopic optical technique that we have developed. This measurement system, described as anatomical optical coherence tomography (aOCT), allows quantitative real-time imaging of the internal anatomy of the human upper airway with minimal invasiveness. Moreover, the system permits motions of the internal geometry at a fixed location to be recorded. The aOCT data set is insufficient by itself to construct a complete geometry because only the polar coordinates are obtained in a local reference frame. Accordingly, the locus described by the endoscope, in which the aOCT is housed, is obtained by orthogonal CT scans. The combination of CT scans and aOCT measurements then provides the required geometric information for the construction of the computational model. Results of a twodimensional model show how the soft palate responds to the mean-flow variations of the breathing cycle. For the threedimensional work, the key results of this paper rest in the reconstruction of the time-dependent geometry of the upper airway, the first time that this has been accomplished using direct internally-based measurement.
A scheme for all-optical conversion of codes for coherent code-division multiplexing is proposed. The key functionality of the scheme is demonstrated by error-free conversion of an encoded data stream at 10 Gbit/s from one code to another. The scheme may enable routing and add/drop multiplexing based on optical codes.
The capacity of optical coherence tomography to characterize biological tissues can be augmented by extensions to detect motion (blood and lymph flow), response to load (stiffness) and birefringence (stress and sub-structure). This talk will review these extensions and describe example applications in cancer, the eye and skin.
The papers in this special issue focus on the topic of nanobiophotonics, an advanced field of modern science and biomedical nanotechnology. This new field continues to vastly expend with state-of-the-art developments across the entire spectrum of biomedical applications ranging from fundamental studies of light-nanobiomaterial interactions to clinical diagnostics and therapeutics with nanophotonics. In nanobiophotonics research areas, there has been great impetus recently for noninvasive imaging and sensing intracellular structures and functions as well as for obtaining quantitative information for light-tissue interactions at the cellular, intracellular and molecular level. The papers in this issue offer some of the latest leading-edge developments in nanobiophotonics. Some of these developments include advanced optical nanoimaging and nanosensing techniques based on nanoprobes enhanced imaging and sensing principles employing various highly effective nanobiomaterials such as nanoparticles, quantum dots, and plasmonic nanostructures. These techniques offer an effective and fast way for sensing and monitoring various biomedical quantities in-vivo with a nanoresolution beyond the diffraction limit. Other new developments include optical manipulation of nanoparticles, single molecule spectroscopy and imaging, integrated nanoprobe-enhanced diagnostics and therapeutics, and novel nanobiophotonics devices.
The lymphatic system is a common route for the spread of cancer and the identification of lymph node metastases is a key task during cancer surgery. This paper demonstrates the use of optical coherence tomography to construct parametric images of lymph nodes. It describes a method to automatically estimate the optical attenuation coefficient of tissue. By mapping the optical attenuation coefficient at each location in the scan, it is possible to construct a parametric image indicating variations in tissue type. The algorithm is applied to ex vivo samples of human axillary lymph nodes and validated against a histological gold standard. Results are shown illustrating the variation in optical properties between cancerous and healthy tissue.
This study presents the first in vivo longitudinal assessment of scar vasculature in ablative fractional laser treatment using optical coherence tomography (OCT). A method based on OCT speckle decorrelation was developed to visualize and quantify the scar vasculature over the treatment period. Through reliable co‐location of the imaging field of view across multiple imaging sessions, and compensation for motion artifact, the study was able to track the same scar tissue over a period of several months, and quantify changes in the vasculature area density. The results show incidences of occlusion of individual vessels 3 days after the first treatment. The subsequent responses ˜20 weeks after the initial treatment show differences between immature and mature scars. Image analysis showed a distinct decrease (25 ± 13%, mean ± standard deviation) and increase (19 ± 5%) of vasculature area density for the immature and mature scars, respectively. This study establishes the feasibility of OCT imaging for quantitative longitudinal monitoring of vasculature in scar treatment.
The lymphatic system is a common route for the spread of cancer and the identification of lymph node metastases is a key task during cancer surgery. This paper demonstrates the use of optical coherence tomography to construct parametric images of lymph nodes. It describes a method to automatically estimate the optical attenuation coefficient of tissue. By mapping the optical attenuation coefficient at each location in the scan, it is possible to construct a parametric image indicating variations in tissue type. The algorithm is applied to ex vivo samples of human axillary lymph nodes and validated against a histological gold standard. Results are shown illustrating the variation in optical properties between cancerous and healthy tissue.
This paper reviews recent research undertaken in the Optical+Biomedical Engineering Laboratory on the application of anatomical optical coherence tomography to endoscopic imaging. The technology is demonstrated for the assessment of pathologies of the human lower airway.
Airway dimensions are difficult to quantify bronchoscopically because of optical distortion and a limited ability to gauge depth. Anatomical optical coherence tomography (aOCT), a novel imaging technique, may overcome these limitations. This study evaluated the accuracy of aOCT against existing techniques in phantom, excised pig and in vivo human airways. Three comparative studies were performed: 1) micrometer-derived area measurements in 10 plastic tubes were compared with aOCT-derived area; 2) aOCT-derived airway compliance curves from excised pig airways were compared with curves derived using an endoscopic technique; and 3) airway dimensions from the trachea to subsegmental bronchi were measured using aOCT in four anaesthetised patients during bronchoscopy and compared with computed tomography (CT) measurements. Measurements in plastic tubes revealed aOCT to be accurate and reliable. In pig airways, aOCT-derived compliance measurements compared closely with endoscopic data. In human airways, dimensions measured with aOCT and CT correlated closely. Bland - Altman plots showed that aOCT diameter and area measurements were higher than CT measurements by 7.6% and 15.1%, respectively. Airway measurements using aOCT are accurate, reliable and compare favourably with existing imaging techniques. Using aOCT with conventional bronchoscopy allows real-time measurement of airway dimensions and could be useful clinically in settings where knowledge of airway calibre is required.
Identifying tumour margins during breast-conserving surgeries is a persistent challenge. We have previously developed miniature needle probes that could enable intraoperative volume imaging with optical coherence tomography. In many situations, however, scattering contrast alone is insufficient to clearly identify and delineate malignant regions. Additional polarization-sensitive measurements provide the means to assess birefringence, which is elevated in oriented collagen fibres and may offer an intrinsic biomarker to differentiate tumour from benign tissue. Here, we performed polarization-sensitive optical coherence tomography through miniature imaging needles and developed an algorithm to efficiently reconstruct images of the depth-resolved tissue birefringence free of artefacts. First ex vivo imaging of breast tumour samples revealed excellent contrast between lowly birefringent malignant regions, and stromal tissue, which is rich in oriented collagen and exhibits higher birefringence, as confirmed with co-located histology. The ability to clearly differentiate between tumour and uninvolved stroma based on intrinsic contrast could prove decisive for the intraoperative assessment of tumour margins.
Background: Previous histological and imaging studies have shown the presence of variability in the degree of bronchoconstriction of airways sampled at different locations in the lung (i.e., heterogeneity). Heterogeneity can occur at different airway generations and at branching points in the bronchial tree. Whilst heterogeneity has been detected by previous experimental approaches, its spatial relationship either within or between airways is unknown.Methods: In this study, distribution of airway narrowing responses across a portion of the porcine bronchial tree was determined in vitro. The portion comprised contiguous airways spanning bronchial generations (#3-11), including the associated side branches. We used a recent optical imaging technique, anatomical optical coherence tomography, to image the bronchial tree in three dimensions. Bronchoconstriction was produced by carbachol administered to either the adventitial or luminal surface of the airway. Luminal cross sectional area was measured before and at different time points after constriction to carbachol and airway narrowing calculated from the percent decrease in luminal cross sectional area.Results: When administered to the adventitial surface, the degree of airway narrowing was progressively increased from proximal to distal generations (r = 0.80 to 0.98, P < 0.05 to 0.001). This 'serial heterogeneity' was also apparent when carbachol was administered via the lumen, though it was less pronounced. In contrast, airway narrowing was not different at side branches, and was uniform both in the parent and daughter airways.Conclusions: Our findings demonstrate that the bronchial tree expresses intrinsic serial heterogeneity, such that narrowing increases from proximal to distal airways, a relationship that is influenced by the route of drug administration but not by structural variations accompanying branching sites.
Quantitative elasticity imaging seeks to retrieve spatial maps of elastic moduli of tissue. Unlike strain, which is commonly imaged in compression elastography, elastic moduli are intrinsic properties of tissue, and therefore, this approach reconstructs images that are largely operator and system independent, enabling objective, longitudinal, and multisite diagnoses. Recently, novel quantitative elasticity imaging approaches to compression elastography have been developed. These methods use a calibration layer with known mechanical properties to sense the stress at the tissue surface, which combined with strain, is used to estimate the tissue's elastic moduli by assuming homogeneity in the stress field. However, this assumption is violated in mechanically heterogeneous samples. We present a more general approach to quantitative elasticity imaging that overcomes this limitation through an efficient iterative solution of the inverse elasticity problem using adjoint elasticity equations. We present solutions for linear elastic, isotropic, and incompressible solids; however, this method can be employed for more complex mechanical models. We retrieve the spatial distribution of shear modulus for a tissue-simulating phantom and a tissue sample. This is the first time, to our knowledge, that the iterative solution of the inverse elasticity problem has been implemented on experimentally acquired compression optical coherence elastography data.
Regulation of airway caliber by lung volume or bronchoconstrictor stimulation is dependent on physiological, structural, and mechanical events within the airway wall, including airway smooth muscle (ASM) contraction, deformation of the mucosa and cartilage, and tensioning of elastic matrices linking wall components. Despite close association between events in the airway wall and the resulting airway caliber, these have typically been studied separately: the former primarily using histological approaches, the latter with a range of imaging modalities. We describe a new optical technique, anatomical optical coherence tomography (aOCT), which allows changes at the luminal surface (airway caliber) to be temporally related to corresponding dynamic movements within the airway wall. A fiber-optic aOCT probe was inserted into the lumen of isolated, liquid-filled porcine airways. It was used to image the response to ASM contraction induced by neural stimulation and to airway inflation and deflation. Comparisons with histology indicated that aOCT provided highresolution images of the airway lumen including mucosal folds, the entire inner wall (mucosa and ASM), and partially the cartilaginous outer wall. Airway responses assessed by aOCT revealed several phenomena in "live" airways (i.e., not fixed) previously identified by histological investigations of fixed tissue, including a geometric relationship between ASM shortening and luminal narrowing, and sliding and bending of cartilage plates. It also provided direct evidence for distensibility of the epithelial membrane and anisotropic behavior of the airway wall. Findings suggest that aOCT can be used to relate changes in airway caliber to dynamic events in the wall of airways.
We present a novel sample arm arrangement for dynamic optical coherence elastography based on excitation by a ring actuator. The actuator enables coincident excitation and imaging to be performed on a sample, facilitating in vivo operation. Sub-micrometer vibrations in the audio frequency range were coupled to samples that were imaged using optical coherence tomography. The resulting vibration amplitude and microstrain maps are presented for bilayer silicone phantoms and multiple skin sites on a human subject. Contrast based on the differing elastic properties is shown, notably between the epidermis and dermis. The results constitute the first demonstration of a practical means of performing in vivo dynamic optical coherence elastography on a human subject.
Anatomical optical coherence tomography (aOCT) is a long-range, fibre-optic endoscopic imaging modality capable of quantifying the size and shape of the human airway lumen. This paper presents the first application of respiratory gating to 3D aOCT volumetric data. A sequence of time-gated data volumes are generated, characterising the dynamic behaviour of a segment of the lower airway over an averaged respiratory cycle. The technique is demonstrated on in vivo data acquired from three human subjects.
We report a theoretical investigation of the crosstalk performance of photonic code-division multiple-access (CDMA) networks that are based on coherent matched filtering of optical pulses. We describe the importance of time gating in the reduction of noise in spread-time CDMA schemes. We give guidelines for the selection of codes in coherent matched filtering and give a code set that produces low crosstalk. We present calculated bitor rates (BER's) that show for individual user data rates in the gigabit per second range that crosstalk limits aggregate bit rates to the tens of gigabits per second range. This level of performance is a significant improvement over purely incoherent spread-time approaches. Such low crosstalk suggests that this scheme may be the first spread-time photonic CDMA scheme that is not crosstalk-limited.
Imaging of the human upper airway is widely used in medicine, in both clinical practice and research. Common imaging modalities include video endoscopy, X-ray CT, and MRI. However, no current modality is both quantitative and safe to use for extended periods of time. Such a capability would be particularly valuable for sleep research, which is inherently reliant on long observation sessions. We have developed an instrument capable of quantitative imaging of the human upper airway, based on endoscopic optical coherence tomography. There are no dose limits for optical techniques, and the minimally invasive imaging probe is safe for use in overnight studies. We report on the design of the instrument and its use in preliminary clinical studies, and we present results from a range of initial experiments. The experiments show that the instrument is capable of imaging during sleep, and that it can record dynamic changes in airway size and shape. This information is useful for research into sleep disorders, and potentially for clinical diagnosis and therapies.
We employ optical coherence tomography (OCT) and optical coherence microscopy (OCM) to study conjunctival lymphatics in porcine eyes ex vivo. This study is a precursor to the development of in vivo imaging of the collecting lymphatics for potentially guiding and monitoring glaucoma filtration surgery. OCT scans at 1300 nm and higher‐resolution OCM scans at 785 nm reveal the lymphatic vessels via their optical transparency. Equivalent signal characteristics are also observed from blood vessels largely free of blood (and devoid of flow) in the ex vivo conjunctiva. In our lymphangiography, vessel networks were segmented by compensating the depth attenuation in the volumetric OCT/OCM signal, projecting the minimum intensity in two dimensions and thresholding to generate a three‐dimensional vessel volume. Vessel segmentation from multiple locations of a range of porcine eyes (n = 21) enables visualization of the vessel networks and indicates the varying spatial distribution of patent lymphatics. Such visualization provides a new tool to investigate conjunctival vessels in tissue ex vivo without need for histological tissue processing and a valuable reference on vessel morphology for the in vivo label‐free imaging studies of lymphatics to follow.
We investigate a photonic CDMA system based on coherent matched filtering in ladder networks, employing both time-spreading and selective interference to discriminate wanted from unwanted signals. A theory is presented for time-resolved pulses which takes account of arbitrary source coherence. We examine the key issues of optical source requirements and phase and polarization control of en/decoder networks, and demonstrate key concepts experimentally. The crosstalk-limited signal-to-noise ratio is shown to be much higher than previous, purely incoherent systems. CDMA systems using coherent matched filtering therefore offer renewed promise for practical high capacity, multigigabit/s, multiuser networks.
Background and Objectives: Sutures are currently the gold standard for wound closure but they are still unable to seal tissue and may induce scarring or inflammation. Biocompatible glues, based on polysaccharides such as chitosan, are a possible alternative to conventional wound closure. In this study, the adhesion of laser-activated chitosan films is investigated in vitro and in vivo. In particular we examine the effect of varying the laser power, as well as adding a natural cross-linker (genipin) to the adhesive composition. Study Design/Materials and Methods: Flexible and insoluble strips of chitosan films (surface area ∼34 mm2, thickness ∼20 µm) were bonded to sheep intestine using several laser powers (0, 80, 120, and 160 mW) at 808-nm wavelength. The strength of repaired tissue was tested by a calibrated tensiometer to select the best power. A natural cross-linker (genipin) was also added to the film and the tissue repair strength compared with the strength of plain films. The adhesive was also bonded in vivo to the sciatic nerve of rats and the thermal damage induced by the laser assessed 4 days post-operatively. Results: Chitosan adhesives successfully repaired intestine tissue, attaining a maximum repair strength of 14.7 ± 4.3 kPa (n = 30) at the laser power of 120 mW. The chitosan-genipin films achieved lower repair strength (9.1 ± 2.9 kPa). The laser caused partial demyelination of axons at the site of operation, but the myelinated axons retained a normal morphology proximally and distally. Conclusions: The chitosan adhesive effectively bonded to tissue causing only localized thermal damage in vivo, when the appropriate laser parameters were selected.
A static Michelson interferometer and optical spectral analysis have been used to perform optical coherence-domain reflectometry (OCDR). Direct compensation for non-Gaussian source spectrum and dispersion are shown. Reflections indistinguishable using an EDFA source and conventional scanning OCDR are clearly distinguished with this method.
We propose and demonstrate a novel detection technique, based on a modified electronic phase-locked loop, for Doppler optical coherence tomography. The technique permits real-time simultaneous reflectivity and continuous, bidirectional velocity mapping in turbid media over a wide velocity range with minimal sensitivity penalty compared with conventional optical coherence tomography, which is a major advance over current postprocessing and discrete parallel detection techniques.
We present a novel needle-based device for the measurement of refractive index and scattering using low-coherence interferometry. Coupled to the sample arm of an optical coherence tomography system, the device detects the scattering response of, and optical path length through, a sample residing in a fixed-width channel. We report use of the device to make near-infrared measurements of tissues and materials with known optical properties. The device could be used to exploit the refractive index variations of tissue for medical and biological diagnostics accessible by needle insertion.
A static Michelson interferometer coupled with optical spectral analysis has been used to perform optical coherence-domain reflectometry (OCDR). Compensation of both the coherence sidelobes caused by a non-Gaussian source spectrum and the coherence broadening caused by dispersion is shown. Reflections indistinguishable using an EDFA source and conventional scanning OCDR are clearly distinguished with this method.
We report a new synthetic aperture optical microscopy in which high-resolution, wide-field amplitude and phase images are synthesized from a set of Fourier holograms. Each hologram records a region of the complex two-dimensional spatial frequency spectrum of an object, determined by the illumination field's spatial and spectral properties and the collection angle and solid angle. We demonstrate synthetic microscopic imaging in which spatial frequencies that are well outside the modulation transfer function of the collection optical system are recorded while maintaining the long working distance and wide field of view.
We present a low-complexity optical ranging system intended for distance ranging to moving biological tissue. The optical ranging technique used is short tuning range, optical FMCW interferometry, in a Fabry-Perot configuration, with a common-path downlead. We demonstrate ranging to a bovine muscle tissue sample over a 250 mm range with an average resolution of 0.55 mm.
Imaging of alveoli in situ has for the most part been infeasible due to the high resolution required to discern individual alveoli and limited access to alveoli beneath the lung surface. In this study, we present a novel technique to image alveoli using optical coherence tomography (OCT). We propose the use of OCT needle probes, where the distal imaging probe has been miniaturized and encased within a hypodermic needle (as small as 30-gauge, outer diameter 310 Î¼m), allowing insertion deep within the lung tissue with minimal tissue distortion. Such probes enable imaging at a resolution of ∼12 μm within a three-dimensional cylindrical field of view with diameter ~1.5 mmm centered on the needle tip. The imaging technique is demonstrated on excised lungs from three different species: adult rats, fetal sheep, and adult pigs. OCT needle probes were used to image alveoli, small bronchioles, and blood vessels, and results were matched to histological sections. We also present the first dynamic OCT images acquired with an OCT needle probe, allowing tracking of individual alveoli during simulated cyclical lung inflation and deflation.
Optical coherence elastography employs optical coherence tomography (OCT) to measure the displacement of tissues under load and, thus, maps the resulting strain into an image, known as an elastogram. We present a new improved method to measure vibration amplitude in dynamic optical coherence elastography. The tissue vibration amplitude caused by sinusoidal loading is measured from the spread of the Doppler spectrum, which is extracted using joint spectral and time domain signal processing. At low OCT signal-to-noise ratio (SNR), the method provides more accurate vibration amplitude measurements than the currently used phasesensitive method. For measurements performed on a mirror at OCT SNR = 5 dB, our method introduces 20% using the phasesensitive method. We present elastograms of a tissue-mimicking phantom and excised porcine tissue that demonstrate improvements, including a 50% increase in the depth range of reliable vibration amplitude measurement.
We propose and demonstrate a novel wavelength-division multiplexed (WDM) ring-network for wide-area, local-area, and access networks. The network employs a single, shared, multiwavelength incoherent source that is remotely located from network nodes. The use of a single incoherent source provides a straight-forward means of wavelength stabilization and potential for a cost-effective network. We demonstrate less than 10 -9 biterror-rate (BER) operation of a four-node network at per-node data rates of 622 Mb/s.
OBJECTIVE. The purpose of this study was to evaluate a new imaging technique for the assessment of breast cancer tumor margins. The technique entails deployment of a high-resolution optical imaging needle under ultrasound guidance. Assessment was performed on fresh ex vivo tissue samples. CONCLUSION. Use of the ultrasound-guided optical needle probe allowed in situ assessment of fresh tissue margins. The imaging findings corresponded to the histologic findings.
Colour Doppler optical coherence tomography (CDOCT) is important for two-dimensional, high-spatial-resolution tomographic velocity mapping of blood in living tissue. CDOCT is based on optical interference in a scanning Michelson interferometer. A novel detection scheme, phase-locked loop (PLL) has the ability to establish and maintain phase-lock to the interferogram's periodic signal over a wide range of frequency and amplitude variations. Only the values of frequency and reflectivity must be stored on a computer representing a massive reduction in stored data to the conventional approach. The CDOCT system is implemented in bulk optics to test the detection scheme.
We report the effects on two-photon excitation microscopy of applying optical clearing agents to human skin tissue samples. We demonstrate that the agents glycerol, propylene glycol and glucose in aqueous solution are all effective in enhancing penetration depth (by up to a factor of 2) and in increasing image contrast (by up to a factor of 90 at 80 μm depth) in 150 μm thick sections. We analysed the dynamics of the clearing process, by developing a simple theoretical model based on the free diffusion of the agent into the tissue. In experiments employing simultaneous two-photon excitation and second harmonic generation microscopy similar contrast was produced. A preliminary measurement of the clearing effect on a bulk skin sample is also presented. All three agents are potentially biocompatible and effective in reducing scattering; hence, in improving light penetration depth and image contrast. As such, they could be suitable for in vivo application in two-photon microscopy, as well as in other techniques performing optical biopsy of human skin tissue.
An extended range (up to 26 mm), rapid scanning (1 m/s) optical delay line which is applicable to the dynamic optical coherence tomography of macroscopic biological subjects is demonstrated. The double-pass polarizing reflector incorporated into the delay line confers several important advantages. The delay line has demonstrated excellent linearity and uniformity.
Image formation in optical coherence elastography (OCE) results from a combination of two processes: the mechanical deformation imparted to the sample and the detection of the resulting displacement using optical coherence tomography (OCT). We present a multiphysics model of these processes, validated by simulating strain elastograms acquired using phase- sensitive compression OCE, and demonstrating close correspondence with experimental results. Using the model, we present evidence that the approximation commonly used to infer sample displacement in phase-sensitive OCE is invalidated for smaller deformations than has been previously considered, significantly affecting the measurement precision, as quantified by the displacement sensitivity and the elastogram signal-to-noise ratio. We show how the precision of OCE is affected not only by OCT shot-noise, as is usually considered, but additionally by phase decorrelation due to the sample deformation. This multiphysics model provides a general framework that could be used to compare and contrast different OCE techniques.
Needle-based devices, which are in wide clinical use for needle biopsy procedures, may be augmented by suitable optical techniques for the localization and diagnosis of diseased tissue. Tissue refractive index is one optical contrast mechanism with diagnostic potential. In the case of mammary tissue, for example, recent research indicates that refractive index variations between tissue types may be useful for the identification of cancerous tissue. While many coherence-based forward-sensing devices have been developed to detect scattering changes, none have demonstrated refractive index measurement capabilities. We present a novel needle-based device that is capable of simultaneously measuring refractive index and scattering. Coupled to the sample arm of an optical coherence tomography system, the needle device detects the scattering response and optical pathlength through tissue residing in a fixed-width channel. Near-infrared measurements of tissues and materials with known optical properties using a prototype device will be presented. This work demonstrates the feasibility of integrated in vivo measurement of refractive index and scattering in conjunction with existing clinical needle-based devices.
We demonstrate wideband reduction of excess intensity noise in an incoherent light source by an optoelectronic feed forward technique. The technique allows significantly narrower optical bandwidths to be used in high-bit rate spectrum-sliced communication systems. We use this technique to successfully transmit data at 2.5 Gb/s over 40 km of standard fiber using a 0.23-nm bandwidth slice of incoherent light. The scheme requires the addition of only a few components to the transmitting node and uses a single optical modulator for data modulation and feedforward noise reduction.
We propose and demonstrate, theoretically and experimentally, a novel achromatic optical phase shifter-modulator based on a frequency-domain optical delay line configured to maintain zero group delay as variable phase delay is generated by means of tilting a mirror. Compared with previously reported phase shifter-modulators, e.g., based on the Pancharatnam (geometric) phase, our device is high speed and polarization insensitive and produces a large, bounded phase delay that, uniquely, is one-to-one mapped to a measurable parameter, the tilt angle.
Investigators have found new ways to provide the biological community with more powerful tools
This paper presents a novel method based on a fiducial marker for correction of motion artifacts in 3D, in vivo, optical coherence tomography (OCT) scans of human skin and skin scars. The efficacy of this method was compared against a standard cross-correlation intensity-based registration method. With a fiducial marker adhered to the skin, OCT scans were acquired using two imaging protocols: direct imaging from air into tissue; and imaging through ultrasound gel into tissue, which minimized the refractive index mismatch at the tissue surface. The registration methods were assessed with data from both imaging protocols and showed reduced distortion of skin features due to motion. The fiducial-based method was found to be more accurate and robust, with an average RMS error below 20 Î¼m and success rate above 90%. In contrast, the intensity-based method had an average RMS error ranging from 36 to 45 Î¼m, and a success rate from 50% to 86%. The intensity-based algorithm was found to be particularly confounded by corrugations in the skin. By contrast, tissue features did not affect the fiducial-based method, as the motion correction was based on delineation of the flat fiducial marker. The average computation time for the fiducial-based algorithm was approximately 21 times less than for the intensity-based algorithm.
Surgical treatment of breast cancer aims to identify and remove all malignant tissue. Intraoperative assessment of tumor margins is, however, not exact; thus, re-excision is frequently needed, or excess normal tissue is removed. Imaging methods applicable intraoperatively could help to reduce re-excision rates whilst minimizing removal of excess healthy tissue. Optical coherence elastography (OCE) has been proposed for use in breast-conserving surgery; however, intraoperative interpretation of complex OCE images may prove challenging. Observations of breast cancer on multiple length scales, by OCE, ultrasound elastography, and atomic force microscopy, have shown an increase in the mechanical heterogeneity of malignant breast tumors compared to normal breast tissue. In this study, a micro-scale mechanical heterogeneity index is introduced and used to form heterogeneity maps from OCE scans of 10 ex vivo human breast tissue samples. Through comparison of OCE, optical coherence tomography images, and corresponding histology, malignant tissue is shown to possess a higher heterogeneity index than benign tissue. The heterogeneity map simplifies the contrast between tumor and normal stroma in breast tissue, facilitating the rapid identification of possible areas of malignancy, which is an important step towards intraoperative margin assessment using OCE. (Figure presented.).
We present the assessment of ex vivo mouse muscle tissue by quantitative parametric imaging of the near-infrared attenuation coefficient Î¼t using optical coherence tomography. The resulting values of the local total attenuation coefficient Î¼t (mean Â± standard error) from necrotic lesions in the dystrophic skeletal muscle tissue of mdx mice are higher (9.6 Â± 0.3 mm-1) than regions from the same tissue containing only necrotic myofibers (7.0 Â± 0.6 mm-1), and significantly higher than values from intact myofibers, whether from an adjacent region of the same sample (4.8 Â± 0.3 mm-1) or from healthy tissue of the wild-type C57 mouse (3.9 Â± 0.2 mm-1) used as a control. Our results suggest that the attenuation coefficient could be used as a quantitative means to identify necrotic lesions and assess skeletal muscle tissue in mouse models of human Duchenne muscular dystrophy.
In situ imaging of alveoli and the smaller airways with optical coherence tomography (OCT) has significant potential in the assessment of lung disease. We present a minimally invasive imaging technique utilizing an OCT needle probe. The side-facing needle probe comprises miniaturized focusing optics consisting of no-core and GRIN fiber encased within a 23-gauge needle. 3D-OCT volumetric data sets were acquired by rotating and retracting the probe during imaging. The probe was used to image an intact, fresh (not fixed) sheep lung filled with normal saline, and the results validated against a histological gold standard. We present the first published images of alveoli acquired with an OCT needle probe and demonstrate the potential of this technique to visualize other anatomical features such as bifurcations of the bronchioles.
Transbronchial needle aspiration (TBNA) of small lesions or lymph nodes in the lung may result in nondiagnostic tissue samples. We demonstrate the integration of an optical coherence tomography (OCT) probe into a 19-gauge flexible needle for lung tissue aspiration. This probe allows simultaneous visualization and aspiration of the tissue. By eliminating the need for insertion and withdrawal of a separate imaging probe, this integrated design minimizes the risk of dislodging the needle from the lesion prior to aspiration and may facilitate more accurate placement of the needle. Results from in situ imaging in a sheep lung show clear distinction between solid tissue and two typical constituents of nondiagnostic samples (adipose and lung parenchyma). Clinical translation of this OCT-guided aspiration needle holds promise for improving the diagnostic yield of TBNA.
The spatial presentation of mechanical information is a key parameter for cell behavior. We have developed a method of polymerization control in which the differential diffusion distance of unreacted cross-linker and monomer into a prepolymerized hydrogel sink results in a tunable stiffness gradient at the cell–matrix interface. This simple, low-cost, robust method was used to produce polyacrylamide hydrogels with stiffness gradients of 0.5, 1.7, 2.9, 4.5, 6.8, and 8.2 kPa/mm, spanning the in vivo physiological and pathological mechanical landscape. Importantly, three of these gradients were found to be nondurotactic for human adipose-derived stem cells (hASCs), allowing the presentation of a continuous range of stiffnesses in a single well without the confounding effect of differential cell migration. Using these nondurotactic gradient gels, stiffness-dependent hASC morphology, migration, and differentiation were studied. Finally, the mechanosensitive proteins YAP, Lamin A/C, Lamin B, MRTF-A, and MRTF-B were analyzed on these gradients, providing higher-resolution data on stiffness-dependent expression and localization.
We present an optical technique to image the frequency-dependent complex mechanical response of a viscoelastic sample. Three-dimensional hyperspectral data, comprising two-dimensional B-mode images and a third dimension corresponding to vibration frequency, were acquired from samples undergoing external mechanical excitation in the audio-frequency range. We describe the optical coherence tomography (OCT) signal when vibration is applied to a sample and detail the processing and acquisition techniques used to extract the local complex mechanical response from three-dimensional data that, due to a wide range of vibration frequencies, possess a wide range of sample velocities. We demonstrate frequency-dependent contrast of the displacement amplitude and phase of a silicone phantom containing inclusions of higher stiffness. Measurements of an ex vivo tumor margin demonstrate distinct spectra between adipose and tumor regions, and images of displacement amplitude and phase demonstrated spatially-resolved contrast. Contrast was also observed in displacement amplitude and phase images of a rat muscle sample. These results represent the first demonstration of mechanical spectroscopy based on B-mode OCT imaging. Spectroscopic optical coherence elastography (SOCE) provides a high-resolution imaging capability for the detection of tissue pathologies that are characterized by a frequency-dependent viscoelastic response.
High-resolution tactile imaging, superior to the sense of touch, has potential for future biomedical applications such as robotic surgery. In this paper, we propose a tactile imaging method, termed computational optical palpation, based on measuring the change in thickness of a thin, compliant layer with optical coherence tomography and calculating tactile stress using finite-element analysis. We demonstrate our method on test targets and on freshly excised human breast fibroadenoma, demonstrating a resolution of up to 15–25 µm and a field of view of up to 7 mm. Our method is open source and readily adaptable to other imaging modalities, such as ultrasonography and confocal microscopy.
It is challenging to recover local optic axis orientation from samples probed with fiber-based polarization-sensitive optical coherence tomography (PS-OCT). In addition to the effect of preceding tissue layers, the transmission through fiber and system elements, and imperfect system alignment, need to be compensated. Here, we present a method to retrieve the required correction factors from measurements with depth-multiplexed PS-OCT, which accurately measures the full Jones matrix. The correction considers both retardation and diattenuation and is applied in the wavenumber domain, preserving the axial resolution of the system. The robustness of the method is validated by measuring a birefringence phantom with a misaligned system. Imaging ex-vivo lamb trachea and human bronchus demonstrates the utility of reconstructing the local optic axis orientation to assess smooth muscle, which is expected to be useful in the assessment of airway smooth muscle thickness in asthma, amongst other fiber-based applications.
Significance: Optical coherence tomography (OCT) provides cross-sectional and volumetric images of backscattering from biological tissue that reveal the tissue morphology. The strength of the scattering, characterized by an attenuation coefficient, represents an alternative and complementary tissue optical property, which can be characterized by parametric imaging of the OCT attenuation coefficient. Over the last 15 years, a multitude of studies have been reported seeking to advance methods to determine the OCT attenuation coefficient and developing them toward clinical applications. Aim: Our review provides an overview of the main models and methods, their assumptions and applicability, together with a survey of preclinical and clinical demonstrations and their translation potential. Results: The use of the attenuation coefficient, particularly when presented in the form of parametric en face images, is shown to be applicable in various medical fields. Most studies show the promise of the OCT attenuation coefficient in differentiating between tissues of clinical interest but vary widely in approach. Conclusions: As a future step, a consensus on the model and method used for the determination of the attenuation coefficient is an important precursor to large-scale studies. With our review, we hope to provide a basis for discussion toward establishing this consensus.
A simple scheme for data transmission by direct modulation of a gain-switched semiconductor laser is demonstrated. Experimental results are presented that show low error rate and low pattern dependence. The sensitivity of the error rate to changes in drive conditions is also measured. Comparison of our direct modulation scheme with external modulation of the same gain-switched laser shows nearly penalty-free operation.
In optical coherence elastography, images are formed by mapping a mechanical property of tissue. Such images, known as elastograms, are formed on the microscale, intermediate between that of cells and whole organs. Optical coherence elastography holds great promise for detecting and monitoring the altered mechanical properties that accompany many clinical conditions and pathologies, particularly in cancer, cardiovascular disease and eye disease. In this review, we first consider how the mechanical properties of tissue are linked with tissue function and pathology. We then describe currently prominent optical coherence elastography techniques, with emphasis on the methods of mechanical loading and displacement estimation. We highlight the sensitivity to microstrain deformations at tens of micrometer resolution. Throughout, optical coherence elastography is considered in the context of other elastography methods, mainly ultrasound elastography and magnetic resonance elastography. This context serves to highlight its advantages, early stage of development of applications, and strong prospects for future impact.
The majority of existing models of image formation in optical coherence tomography make simplifying assumptions. For example, those based on the extended Huygens-Fresnel formalism make the first-order Born approximation and consider ensemble average, rather than deterministic, scatterer distributions. Monte Carlo solutions of the radiative transport equation also consider ensemble average scatterer distributions and do not explicitly model interferometric detection. Although such models have been successful in answering many questions, there is a growing number of applications where the ability to predict image formation based upon a full wave treatment is needed, including, for example, image formation in turbid tissue. Such a rigorous model of image formation, based upon three-dimensional solutions of Maxwell's equations offers a number of tantalising opportunities. For example, shedding light on image formation for features near or below the resolution of an optical coherence tomography system, allowing for full wave inverse scattering methods to be developed and providing gold standard verification of quantitative imaging techniques. We have developed the first such model and will present simulated B-scans and C-scans, the principal features of our model, and comparisons of experimental and simulated image formation for phantoms.
In this paper, we describe a technique capable of visualizing mechanical properties at the cellular scale deep in living tissue, by incorporating a gradient-index (GRIN)-lens micro-endoscope into an ultrahigh-resolution optical coherence elastography system. The optical system, after the endoscope, has a lateral resolution of 1.6 µm and an axial resolution of 2.2 µm. Bessel beam illumination and Gaussian mode detection are used to provide an extended depth-of-field of 80 µm, which is a 4-fold improvement over a fully Gaussian beam case with the same lateral resolution. Using this system, we demonstrate quantitative elasticity imaging of a soft silicone phantom containing a stiff inclusion and a freshly excised malignant murine pancreatic tumor. We also demonstrate qualitative strain imaging below the tissue surface on in situ murine muscle. The approach we introduce here can provide high-quality extended-focus images through a micro-endoscope with potential to measure cellular-scale mechanics deep in tissue. We believe this tool is promising for studying biological processes and disease progression in vivo.
Optical coherence elastography (OCE), as the use of OCT to perform elastography has come to be known, began in 1998, around ten years after the rest of the field of elastography – the use of imaging to deduce mechanical properties of tissues. After a slow start, the maturation of OCT technology in the early to mid 2000s has underpinned a recent acceleration in the field. With more than 20 papers published in 2015, and more than 25 in 2016, OCE is growing fast, but still small compared to the companion fields of cell mechanics research methods, and medical elastography. In this review, we describe the early developments in OCE, and the factors that led to the current acceleration. Much of our attention is on the key recent advances, with a strong emphasis on future prospects, which are exceptionally bright.
The capacity and number of users that can be supported by code-division multiple access (CDMA) networks that use coherence multiplexing were theoretically determined. The analysis show good agreement with the measured performance of the experimental demonstration. It is believed that this is the first comparison between experiment and theory for the performance of an optical CDMA network.
Purpose: To evaluate the impact of image magnification correction on superficial retinal vessel density (SRVD) and foveal avascular zone area (FAZA) measurements using optical coherence tomography angiography (OCTA). Methods: Participants with healthy retinas were recruited for ocular biometry, refraction, and RTVue XR Avanti OCTA imaging with the 3 × 3-mm protocol. The foveal and parafoveal SRVD and FAZA were quantified with custom software before and after correction for magnification error using the Littman and the modified Bennett formulae. Relative changes between corrected and uncorrected SRVD and FAZA were calculated. Results: Forty subjects were enrolled and the median (range) age of the participants was 30 (18–74) years. The mean (range) spherical equivalent refractive error was −1.65 (−8.00 to +4.88) diopters and mean (range) axial length was 24.42 mm (21.27–28.85). Images from 13 eyes were excluded due to poor image quality leaving 67 for analysis. Relative changes in foveal and parafoveal SRVD and FAZA after correction ranged from −20% to +10%, −3% to +2%, and −20% to +51%, respectively. Image size correction in measurements of foveal SRVD and FAZA was greater than 5% in 51% and 74% of eyes, respectively. In contrast, 100% of eyes had less than 5% correction in measurements of parafoveal SRVD. Conclusions: Ocular biometry should be performed with OCTA to correct image magnification error induced by axial length variation. We advise caution when interpreting interocular and interindividual comparisons of SRVD and FAZA derived from OCTA without image size correction.
We simulate transmission of a spectrum-sliced WDM channel operating at high bit rates (e.g., 622 to 2488 Mb/s). We calculate the bit error rate using the non-Gaussian statistics of thermal light sources that are commonly used in spectrum slicing and account for the effects of fiber dispersion. We evaluate the tradeoff in optical slice lint-width between signal-to-excess optical noise ratio and dispersion penalty in spectrum-sliced WDM systems, and determine the channel slicewidth that minimizes transmission penalty for a given link length and bit rate. We compare our simulations against the measured performance of a 1244 Mb/s channel over 20 km of fiber. The results in this paper provide useful information for the design of spectrum-sliced WDM networks.
Endoscopic imaging using optical coherence tomography (OCT) has been demonstrated as clinically useful in the assessment of human airways. These airways have a complex 3D structure, bending, tapering and bifurcating. Previously published 3D OCT reconstructions have not accounted for changes in the orientation and trajectory of the endoscopic probe as it moves through the airway during imaging. We propose a novel endoscopic setup incorporating a magnetic tracking system that accounts for these changes, yielding reconstructions that reveal the true 3D nature of the imaged anatomy. We characterize the accuracy of the system, and present the first published magnetic tracker-assisted endoscopic OCT reconstructions using a phantom airway.
Depth-encoded optical coherence elastography (OCE) enables simultaneous acquisition of two three-dimensional (3D) elastograms from opposite sides of a sample. By the choice of suitable path-length differences in each of two interferometers, the detected carrier frequencies are separated, allowing depth-ranging from each interferometer to be performed simultaneously using a single spectrometer. We demonstrate depth-encoded OCE on a silicone phantom and a freshly excised sample of mouse liver. This technique minimizes the required spectral detection hardware and halves the total scan time. Depth-encoded OCE may expedite clinical translation in time-sensitive applications requiring rapid 3D imaging of multiple tissue surfaces, such as tumor margin assessment in breast-conserving surgery.
The physical mechanism attributed to producing 'rabbit-ears' on the chirped multimode optical spectra of gain-switched Fabry-PÃ©rot semiconductor lasers is investigated thoroughly. It has been observed experimentally that the short-wavelength rabbit-ears are dominant in the short-wavelength longitudinal modes. However, the long-wavelength rabbit-ears are dominant in the modes near the lasing wavelength, while the short-wavelength rabbit-ears are again dominant in the longest-wavelength modes. In this paper, the unequal rabbit-ears of each mode and the asymmetric chirped spectrum are shown to be a result of the dynamic power transfer between the modes during the gain-switched optical pulse due to carrier-induced modal intermodulation. Such an understanding, and the ability to model accurately this characteristic, is also shown to be essential for designing Fabry-PÃ©rot laser systems, for short-haul applications such as fibre-to-the-home.
We evaluate the trade-off in optical bandwidth between signal-to-excess optical noise ratio and dispersion penalty in Gbit/s spectrum-sliced WDM systems. Furthermore, we demonstrate for the first time that, apart from producing intersymbol interference, dispersion also increases the excess optical noise.
The dependence on seeding power and seeding wavelength of the key parameters of an externally injection-seeded Fabry-Perot laser was investigated. It is shown that the optimum power is determined by a tradeoff between low timing jitter/high side-mode suppression, and short pulse widths. It is demonstrated experimentally that by injecting -17 dBm of narrowband cw light, the timing jitter can be reduced by 60% and a side-mode suppression ratio of 17 dB can be achieved without noticeably increasing the pulse width. The results show that careful setting of the wavelength and power level of the injected seed are required in a tunable picosecond source based on external injection seeding.
We experimentally verify the impact of optical beat noise on a coherence-multiplexed optical communication system. We then show that optical beat noise can be significantly reduced, and transmission capacity increased, by using differential detection. We demonstrate transmission at low bit error rate of four coherence multiplexed channels, each having a capacity of 1 Gb/s, over 8 km of dispersion-shifted fiber. This is the highest capacity demonstration to date for any code-division multiplexing scheme.
Optical elastography, the use of optics to characterize and map the mechanical properties of biological tissue, involves measuring the deformation of tissue in response to a load. Such measurements may be used to form an image of a mechanical property, often elastic modulus, with the resulting mechanical contrast complementary to the more familiar optical contrast. Optical elastography is experiencing new impetus in response to developments in the closely related fields of cell mechanics and medical imaging, aided by advances in photonics technology, and through probing the microscale between that of cells and whole tissues. Two techniques — optical coherence elastography and Brillouin microscopy — have recently shown particular promise for medical applications, such as in ophthalmology and oncology, and as new techniques in cell mechanics.
Flexible bronchoscopy is a common procedure that is used in both diagnostic and therapeutic settings but does not readily permit measurement of central airway dimensions. Anatomic optical coherence tomography (aOCT), a modification of conventional optical coherence tomography (OCT), is a novel light-based imaging tool with the capacity to measure the diameter and lumen area of the central airways accurately during bronchoscopy. This study describes the first clinical use of aOCT imaging in the lower airways in three individuals with common endobronchial pathologies. During bronchoscopy, a specialized fiberoptic probe was passed through the biopsy channel of a standard flexible bronchoscope to the site of airway pathology. Airway dimensions were measured from the generated cross-sectional images in three subjects, one with subglottic tracheal stenosis (subject 1), one with malignant left main bronchus (LMB) obstruction (subject 2), and another with severe tracheomalacia (subject 3). Measured dimensions included internal airway diameter, cross-sectional area, and, in subject 1, stenosis length. Tracheal stenosis dimensions, measured using aOCT imaging, correlated with chest CT scan findings and guided the choice of airway stent (subject 1). The airway beyond a malignant obstruction of the LMB, and beyond bronchoscopic view, could be imaged using aOCT, and the distal extent of obstructing tumor identified (subject 2). The severity of newly diagnosed tracheomalacia was able to be quantified using aOCT imaging (subject 3). aOCT imaging during bronchoscopy allows accurate real-time airway measurements and may assist bronchoscopic assessment.
Optical coherence tomography (OCT) needle probes use miniaturized focusing optics encased in a hypodermic needle. Needle probes can scan areas of the body that are too deep to be imaged by other OCT systems. This paper presents an OCT needle probe-based system that is capable of acquiring three-dimensional scans of tissue structures. The needle can be guided to a target area and scans acquired by rotating and pulling-back the probe. The system is demonstrated using ex vivo human lymph node and sheep lung samples. Multiplanar reconstructions are shown of both samples, as well as the first published 3D volume rendering of lung tissue acquired with an OCT needle probe.
We report on the design and implementation of a gradient-index microendoscope suitable for accessing tissues deep within the body using confocal fluorescence imaging. The 350-μm diameter microendoscope has a length of 27 mm, which enables it to be inserted through a 22-gauge hypodermic needle. A prototype imaging system is demonstrated to obtain images of tissue samples at depths of ~15 mm with a lateral resolution of ~700 nm. To the best of our knowledge, this is the highest resolution and imaging depth reported for a confocal probe of these dimensions. We employ a scanning arrangement using a lensed fiber that can conveniently control the input beam parameters without causing off-axis aberrations typically present in the optical relay lenses used in galvanometer-mirror scanning systems.
Three-dimensional optical coherence tomography (3D-OCT) was used to image the structure and pathology of skeletal muscle tissue from the treadmill-exercised mdx mouse model of human Duchenne muscular dystrophy. Optical coherence tomography (OCT) images of excised muscle samples were compared with coregistered hematoxylin and eosin-stained and Evans blue dye fluorescence histology. We show, for the first time, structural 3D-OCT images of skeletal muscle dystropathology well correlated with co-located histology. OCT could identify morphological features of interest and necrotic lesions within the muscle tissue samples based on intrinsic optical contrast. These findings demonstrate the utility of 3D-OCT for the evaluation of small-animal skeletal muscle morphology and pathology, particularly for studies of mouse models of muscular dystrophy.
Practical micro-imaging deep in solid tissues will open up new avenues in research on clinical diagnosis and treatment of disease. We report on our recent advances in optical microscope-in-a-needle technology capable of 3D micro-imaging in situ.
We present a technique to reduce speckle in optical coherence tomography images of soft tissues. An average is formed over a set of B-scans that have been decorrelated by viscoelastic creep strain. The necessary correction for the deformation-induced spatial distortions between B-scans is achieved through geometrical co-registration using an affine transformation. Speckle reduction by up to a factor of 1.65 is shown in images of tissue-mimicking soft fibrin phantoms and excised human lymph node tissue with no observable loss of spatial resolution.
The year 2010 has seen many major developments in optical bioimaging - too many to fully survey here. We concentrate on breakthroughs that impact on future bioimaging of live animals and humans, including advances in resolution, imaging depth, speed, and function.
Rationale: Our understanding of how airway remodeling affects regional airway elastic properties is limited due to technical difficulties in quantitatively measuring dynamic, in vivo airway dimensions. Such knowledge could help elucidate mechanisms of excessive airway narrowing. Objectives: To use anatomical optical coherence tomography (aOCT) to compare central airway elastic properties in control subjects and those with obstructive lung diseases. Methods: After bronchodilation, airway lumen area (Ai) was measured using aOCT during bronchoscopy in control subjects (n = 10) and those with asthma (n = 16), chronic obstructive pulmonary disease (COPD) (n = 9), and bronchiectasis (n = 8). Ai was measured in each of generations 0 to 5 while airway pressure was increased from -10 to 20 cm H2O. Airway compliance (Caw) and specific compliance (sCaw) were derived from the transpulmonary pressure (PL) versus Ai curves. Measurements and Main Results: Caw decreased progressively as airway generation increased, but sCaw did not differ appreciably across the generations. In subjects with asthma and bronchiectasis, Caw and sCaw were similar to control subjects and the PL-Ai curves were left-shifted. No significant differences were observed between control and COPD groups. Conclusions: Proximal airway elastic properties are altered in obstructive lung diseases. Although central airway compliance does not differ from control subjects in asthma, bronchiectasis, or COPD, Ai is lower in asthma and the PL-Ai relationship is left-shifted in both asthma and bronchiectasis, suggesting that airways are maximally distended at lower inflating pressures. Such changes reflect alteration in the balance between airway wall distensibility and radial traction exerted on airways by surrounding lung parenchyma favoring airway narrowing.
We examine the effects of scattering and absorption in skin tissue upon the axial resolution of ultrahigh-resolution optical coherence tomography. By modeling the frequencydependence of the optical properties, we quantify the depth-dependent axial resolution.
In this paper, we demonstrate in vivo volumetric quantitative micro-elastography of human skin. Elasticity is estimated at each point in the captured volume by combining local axial strain measured in the skin with local axial stress estimated at the skin surface. This is achieved by utilizing phase-sensitive detection to measure axial displacements resulting from compressive loading of the skin and an overlying, compliant, transparent layer with known stress/strain behavior. We use an imaging probe head that provides optical coherence tomography imaging and compression from the same direction. We demonstrate our technique on a tissue phantom containing a rigid inclusion, and present in vivo elastograms acquired from locations on the hand, wrist, forearm and leg of human volunteers.
We report on the design and implementation of a 350 Î¼m-diameter confocal imaging probe based on gradient-index (GRIN) optics and a fiber-based scanning arrangement. The form factor of the probe is such that it can potentially be inserted into a 22-gauge hypodermic needle to perform high-resolution confocal fluorescence imaging in solid tissues. We introduce a simple scanning arrangement based on lensed fiber, which eliminates off-axis aberrations induced by conventional scanning optics and is suitable for integration into a compact hand-held unit. We present the details of the optical design and experimental verification of the performance of the optical system. The measured lateral resolution of ~700 nm is in agreement with the optical design and is the highest resolution reported for a confocal fluorescence imaging probe of this size. Further, we demonstrate the imaging capability of the probe by obtaining high-resolution images of fluorescently labeled muscle fibers.
We analyze a code-division multiple access technique where information is optically encoded by manipulating the coherence between a pair of transmitted signals. Key features are intrinsic security, operation of the receiver at only the bit rate of a single channel, and reconfiguration without switching optical delays, which are considerable advantages compared to previously proposed CDMA schemes. Experimental results demonstrating the basic operation of the scheme are presented. The performance limitations are calculated and novel implementations are proposed.
The mechanical properties of tissues and cells have proven to be of great importance in biology and medicine, spawning major research efforts and commercial outcomes. The interplay between mechanical and biochemical cues and the mechanical properties of subcellular and cellular constituents has come to be appreciated as key to understanding many fundamental aspects of biology as well as the genesis and progression of disease. At the same time, at the other extreme, the mechanical properties of whole organs and their constituents have been shown to be effective markers of disease on the scale of the human body, as probed by ultrasound elastography and magnetic resonance elastography, both of which, after 20-year gestations, have reached commercial markets. The role of optics in these companion fields of medical elastography and cell mechanics has been mixed. In cell mechanics, extensions of optical microscopy for the examination of single cells, such as traction force microscopy, have been important, as has atomic force microscopy, that uses optical interferometric detection of nanoindentation. In medical elastography, optics has been much less prominent. All of this has started to change over the last five years or so, as new techniques in optics emerge, and other techniques begin to mature.
We utilize synthetic-aperture Fourier holographic microscopy to resolve micrometer-scale microstructure over millimeter-scale fields of view. Multiple holograms are recorded, each registering a different, limited region of the sample object's Fourier spectrum. They are "stitched together" to generate the synthetic aperture. A low-numerical-aperture (NA) objective lens provides the wide field of view, and the additional advantages of a long working distance, no immersion fluids, and an inexpensive, simple optical system. Following the first theoretical treatment of the technique, we present images of a microchip target derived from an annular synthetic aperture (NA = 0.61) whose area is 15 times that due to a single hologram (NA = 0.13); they exhibit a corresponding qualitative improvement. We demonstrate that a high-quality reconstruction may be obtained from a limited sub-region of Fourier space, if the object's structural information is concentrated there.
Anatomical optical coherence tomography (aOCT) is a longrange endoscopic imaging modality capable of quantifying size and shape of the human airway. A challenge to its in vivo application is motion artifact due to respiratory-related movement of the airway walls. This paper represents the first demonstration of respiratory gating of aOCT airway data, and introduces a novel error measure to guide appropriate parameter selection. Results indicate that at least four gates per respiratory cycle should be used, with only minor improvements as the number of gates is further increased. It is shown that respiratory gating can substantially improve the quality of aOCT images and reveal events and features that are otherwise obscured by blurring.
We present the first three-dimensional (3D) data sets recorded using optical coherence elastography (OCE). Uni-axial strain rate was measured on human skin in vivo using a spectral-domain optical coherence tomography (OCT) system providing >450 times higher line rate than previously reported for in vivo OCE imaging. Mechanical excitation was applied at a frequency of 125 Hz using a ring actuator sample arm with, for the first time in OCE measurements, a controlled static preload. We performed 3D-OCE, processed in 2D and displayed in 3D, on normal and hydrated skin and observed a more elastic response of the stratum corneum in the hydrated case.
OFS-19 was held in April 2008 in Perth, Australia, with Professor David Sampson (University of Western Australia) as General Chair assisted by Technical Programme Co-Chairs Professor Stephen Collins (Victoria University, Australia), Professor Kyunghwan Oh (Yonsei University, Korea) and Dr Ryozo Yamauchi (Fujikura Ltd, Japan). 'OFS-19' has once again affirmed the OFS series as the leading international conference for the optical fibre sensor community. Since its inception, in London in 1983, and under the leadership of an international steering committee independent of any learned society or professional institution, it has been held approximately every eighteen months. The venue nominally rotates from Europe, to the Americas, and thence to Asia and the Pacific. OFS-19 demonstrated the continuing vigour of the community, with some 240 papers presented, plus 8 tutorials; submissions and attendance were from 29 countries, with a little over half coming from the Asia-Pacific Region. In recent years, it has become a tradition to publish a post-conference special issue in Measurement Science and Technology, and these special issues offer a representative sample of the current status of the field. In the 25 years since OFS began, many of the early ideas and laboratory-based proof-of-principle experiments have successfully evolved into highly developed instrumentation systems and commercial products. One of the greatest success stories has been the optical fibre Bragg grating. Its exquisite intrinsic sensitivity to temperature and strain has led to an expanding niche in structural monitoring, especially in civil engineering. It has formed the 'beach-head' for penetration of optical fibre sensors into the oil and gas industry, initially in the harsh environment of down-hole monitoring. Latterly, it has paved the way for new applications of one of the earliest fibre optic sensors, the fibre hydrophone, which is now making its mark in sub-sea seismic surveying. Additionally, distributed fibre sensors, based on Raman or Brillouin scattering, are beginning to be deployed for remote and sub-sea infrastructure monitoring. Western Australia enjoys a booming oil and gas sector, and so OFS-19's Special Session entitled Oil & Gas: Current Practice–Future Opportunity was timely and locally relevant.
Multi-gigabit per second throughput is demonstrated for the first time in a photonic code-division multiplexed system. Two channels, each transmitting data at 1Gbit/s, are coherence-multiplexed, transmitted over 11 km of dispersion-shifted fibre and one channel is demultiplexed successfully. Bit error rate measurements are reported.
We present a new approach to optical coherence elastography (OCE), which probes the local elastic properties of tissue by using optical coherence tomography to measure the effect of an applied stimulus in the audio frequency range. We describe the approach, based on analysis of the Bessel frequency spectrum of the interferometric signal detected from scatterers undergoing periodic motion in response to an applied stimulus. We present quantitative results of sub-micron excitation at 820 Hz in a layered phantom and the first such measurements in human skin in vivo.
We demonstrate the use of the near-infrared attenuation coefficient, measured using optical coherence tomography (OCT), in longitudinal assessment of hypertrophic burn scars undergoing fractional laser treatment. The measurement method incorporates blood vessel detection by speckle decorrelation and masking, and a robust regression estimator to produce 2D en face parametric images of the attenuation coefficient of the dermis. Through reliable co-location of the field of view across pre- and post-treatment imaging sessions, the study was able to quantify changes in the attenuation coefficient of the dermis over a period of ~20 weeks in seven patients. Minimal variation was observed in the mean attenuation coefficient of normal skin and control (untreated) mature scars, as expected. However, a significant decrease (13 ± 5%, mean ± standard deviation) was observed in the treated mature scars, resulting in a greater distinction from normal skin in response to localized damage from the laser treatment. By contrast, we observed an increase in the mean attenuation coefficient of treated (31 ± 27%) and control (27 ± 20%) immature scars, with numerical values incrementally approaching normal skin as the healing progressed. This pilot study supports conducting a more extensive investigation of OCT attenuation imaging for quantitative longitudinal monitoring of scars. (Figure presented.) En face 2D OCT attenuation coefficient map of a treated immature scar derived from the pre-treatment (top) and the post-treatment (bottom) scans. (Vasculature (black) is masked out.) The scale bars are 0.5 mm.
We present a detailed study of the optical power spectrum and coherence properties of Fabry-Perot semiconductor lasers under gain-switched operation. We demonstrate that the distribution of longitudinal modes under gain-switched operation is described to high accuracy by a Gaussian envelope, in contrast to continuous wave (CW) lasers where the distribution is Lorentzian. We show that the minimum values of coherence under gain-switched operation are two orders of magnitude lower than under CW operation. We also demonstrate that intensity noise generated through interferometric conversion of mode partition noise is markedly different for gain-switched and CW lasers. The results are important for a host of potential applications that use low-coherence interferometry.
The ability to measure airway dimensions is important for clinicians, interventional bronchoscopists and researchers in order to accurately quantify structural abnormalities and track their changes over time or in response to treatment. Most quantitative airway measurements are based on X-ray computed tomography and, more recently, on multidetector computed tomography. Quantitative bronchoscopic techniques have also been developed, although these are less widely employed. Emerging techniques, including magnetic resonance imaging, endoscopic optical coherence tomography, endobronchial ultrasound and confocal endomicroscopy, provide new research tools with potential clinical applications. An understanding of issues related to the acquisition, processing and analysis of images, and how such issues impact on imaging the tracheobronchial tree, is essential in order to assess measurement accuracy and to make effective use of the newer methods. This article contributes to this understanding by providing a comprehensive review of current and emerging techniques for quantifying airway dimensions.
We demonstrate the first application of the recently proposed method of optical palpation to in vivo imaging of human skin. Optical palpation is a tactile imaging technique that probes the spatial variation of a sample's mechanical properties by producing an en face map of stress measured at the sample surface. This map is determined from the thickness of a translucent, compliant stress sensor placed between a loading element and the sample and is measured using optical coherence tomography. We assess the performance of optical palpation using a handheld imaging probe on skin-mimicking phantoms, and demonstrate its use on human skin lesions. Our results demonstrate the capacity of optical palpation to delineate the boundaries of lesions and to map the mechanical contrast between lesions and the surrounding normal skin.
The mechanical properties of tissue are pivotal in its function and behavior, and are often modified by disease. From the nano- to the macro-scale, many tools have been developed to measure tissue mechanical properties, both to understand the contribution of mechanics in the origin of disease and to improve diagnosis. Optical coherence elastography is applicable to the intermediate scale, between that of cells and whole organs, which is critical in the progression of many diseases and not widely studied to date. In optical coherence elastography, a mechanical load is imparted to a tissue and the resulting deformation is measured using optical coherence tomography. The deformation is used to deduce a mechanical parameter, e.g., Young's modulus, which is mapped into an image, known as an elastogram. In this chapter, we review the development of optical coherence elastography and report on the latest developments. We provide a focus on the underlying principles and assumptions, techniques to measure deformation, loading mechanisms, imaging probes and modeling, including the inverse elasticity problem.
The organization of fibrillar tissue on the micrometer scale carries direct implications for health and disease but remains difficult to assess in vivo. Polarization-sensitive optical coherence tomography measures birefringence, which relates to the microscopic arrangement of fibrillar tissue components. Here, we demonstrate a critical improvement in leveraging this contrast mechanism by employing the improved spatial resolution of focus-extended optical coherence microscopy (1.4 μm axially in air and 1.6 μm laterally, over more than 70 μm depth of field). Vectorial birefringence imaging of sheep cornea ex vivo reveals its lamellar organization into thin sections
Recent progress has enabled the reconstruction of the local (i.e., depth-resolved) optic axis (OAx) of biological tissue from measurements made with polarization-sensitive optical coherence tomography (PS-OCT). Here we demonstrate local OAx imaging in healthy human skin in vivo. The images reveal dense, weaving patterns that are imperceptible in OCT intensity tomograms or conventional PS-OCT metrics and that suggest a mesh-like tissue organization, consistent with the morphology of dermal collagen. Using co-registered polarization-sensitive optical coherence microscopy, we furthermore investigated the impact of spatial resolution on the recovered OAx patterns and confirmed their consistency. OAx orientation as a contrast mechanism merits further exploration for applications in dermatology.
We incorporate, for the first time, optical coherence elastography (OCE) into a needle probe and demonstrate its ability to measure the microscopic deformation of soft tissues located well beyond the depth limit of reports to date. Needle OCE utilizes the force imparted by the needle tip as the loading mechanism and measures tissue deformation ahead of the needle during insertion. Measurements were performed in tissue-mimicking phantoms and ex vivo porcine trachea. Results demonstrate differentiation of tissues based on mechanical properties and highlight the potential of needle OCE for in vivo tissue boundary detection.
Significance: Pulsatility is a vital characteristic of the cardiovascular system. Characterization of the pulsatility pattern locally in the peripheral microvasculature is currently not readily available and would provide an additional source of information which may prove important in understanding the pathophysiology of arterial stiffening, vascular ageing, and their linkage with cardiovascular disease development. Aim: We aim to confirm the suitability of speckle decorrelation optical coherence tomography angiography (OCTA) under various non-contact/contact scanning protocols for the visualization of pulsatility patterns in vessel-free tissue and in the microvasculature of peripheral human skin. Results: Results from 5 healthy subjects show distinct pulsatile patterns both in vessel-free tissue with either non-contact or contact imaging and in individual microvessels with contact imaging; respectively, likely caused by the pulsatile pressure and pulsatile blood flow. The pulse rates show good agreement with those from pulse oximetry, confirming that the pulsatile signatures reflect pulsatile hemodynamics. Conclusions: This study demonstrates the potential of speckle decorrelation OCTA for measuring localized peripheral cutaneous pulsatility and defines scanning protocols necessary to undertake such measurements. Non-contact imaging should be used for the study of pulsatility in vessel-free tissue and contact imaging with strong mechanical coupling in individual microvessels. Further studies of microcirculation based upon this method and protocols are warranted.
This study compared shape, size and length of the pharyngeal airway in individuals with and without obstructive sleep apnoea (OSA) using a novel endoscopic imaging technique, anatomical optical coherence tomography (aOCT). The study population comprised a preliminary study group of 20 OSA patients and a subsequent controlled study group of 10 OSA patients and 10 body mass index (BMI)-, gender- and age-matched control subjects without OSA. All subjects were scanned using aOCT while awake, supine and breathing quietly. Measurements of airway cross-sectional area (CSA) and anteroposterior (A-P) and lateral diameters were obtained from the hypo-, oro- and velopharyngeal regions. A-P : lateral diameter ratios were calculated to provide an index of regional airway shape. In all subjects, pharyngeal CSA was lowest in the velopharynx. Patients with OSA had a smaller velopharyngeal CSA than controls (maximum CSA 91 ± 40 versus 153 ± 84 mm2; P < 0.05) but comparable oro- (318 ± 80 versus 279 ± 129 mm2; P = 0.48) and hypopharyngeal CSA (250 ± 105 versus 303 ± 112 mm2; P = 0.36). In each pharyngeal region, the long axis of the airway was oriented in the lateral diameter. Airway shape was not different between the groups. Pharyngeal airway length was similar in both groups, although the OSA group had longer uvulae than the control group (16.8 ± 6.2 versus 11.2 ± 5.2 mm; P < 0.05). This study has shown that individuals with OSA have a smaller velopharyngeal CSA than BMI-, gender- and age-matched control volunteers, but comparable shape: a laterally oriented ellipse. These findings suggest that it is an abnormality in size rather than shape that is the more important anatomical predictor of OSA.
Optical coherence tomography (OCT) is a high-resolution imaging modality with the potential to provide in situ assessment to distinguish normal from cancerous tissue. However, limited image penetration depth has restricted its utility. This paper demonstrates the feasibility of an OCT needle probe to perform interstitial imaging deep below the tissue surface. The side-facing needle probe comprises miniaturized focusing optics consisting of no-core and GRIN fiber encased within either a 22- or 23-gauge needle. 3-D OCT volumetric data sets were acquired by rotating and retracting the probe during imaging. We present the first published image of a human breast cancer tumor margin, and of human axillary lymph nodes acquired with an OCT needle probe. Through accurate correlation with the histological gold standard, OCT is shown to enable a clear delineation of tumor boundary from surrounding adipose tissue, and identification of microarchitectural features.
Anatomical optical coherence tomography (aOCT) is an endoscopic imaging modality that can be used to quantify size and shape of the upper airway. We report the application of respiratory gating to aOCT images. Our results show that respiratory gating can reduce motion artefact in upper airway images. Using an error metric based on distance to the dominant reflection in each A-scan, we found notable improvements when the breath cycle was partitioned into approximately four gates, but only minor improvements as the number of gates was further increased.
This guest editorial introduces the White Papers in Biophotonics.
Study Objectives: In patients with obstructive sleep apnea (OSA), the severity and frequency of respiratory events is increased in the supine body posture compared with the lateral recumbent posture. The mechanism responsible is not clear but may relate to the effect of posture on upper airway shape and size. This study compared the effect of body posture on upper airway shape and size in individuals with OSA with control subjects matched for age, BMI, and gender. Participants: 11 males with OSA and 11 age- and BMI-matched male control subjects. Results: Anatomical optical coherence tomography was used to scan the upper airway of all subjects while awake and breathing quietly, initially when supine, and then in the lateral recumbent posture. A standard head, neck, and tongue position was maintained during scanning. Airway cross-sectional area (CSA) and anteroposterior (A-P) and lateral diameters were obtained in the oropharyngeal and velopharyngeal regions in both postures. A-P to lateral diameter ratios provided an index of regional airway shape. In equivalent postures, the ratio of A-P to lateral diameter in the velopharynx was similar in OSA and control subjects. In both groups, this ratio was significantly less for the supine than for the lateral recumbent posture. CSA was smaller in OSA subjects than in controls but was unaffected by posture. Conclusions: The upper airway changes from a more transversely oriented elliptical shape when supine to a more circular shape when in the lateral recumbent posture but without altering CSA. Increased circularity decreases propensity to tube collapse and may account for the postural dependency of OSA.
We demonstrate a highly realistic model of optical coherence tomography, based on an existing model of coherent optical microscopes, which employs a full wave description of light. A defining feature of the model is the decoupling of the key functions of an optical coherence tomography system: sample illumination, light-sample interaction and the collection of light scattered by the sample. We show how such a model can be implemented using the finite-difference time-domain method to model light propagation in general samples. The model employs vectorial focussing theory to represent the optical system and, thus, incorporates general illumination beam types and detection optics. To demonstrate its versatility, we model image formation of a stratified medium, a numerical point-spread function phantom and a numerical phantom, based upon a physical three-dimensional structured phantom employed in our laboratory. We show that simulated images compare well with experimental images of a three-dimensional structured phantom. Such a model provides a powerful means to advance all aspects of optical coherence tomography imaging.
We rigorously account for the effects of multiparticle light scattering from a fractal sphere aggregate in order to simulate the optical properties of a soft biological tissue, human skin. Using a computational method that extends Mie theory to the multisphere case, we show that multiparticle scattering significantly affects the computed optical properties, resulting in a reduction in both scattering coefficient and anisotropy for the wavelengths simulated, as well as a significantly enhanced forward peak in the simulated phase function. The model is extended to incorporate the contribution of Rayleigh scatterers, which we show is required to obtain reasonable agreement with experimentally measured optical properties of skin tissue.
We review the development of phantoms for optical coherence tomography (OCT) designed to replicate the optical, mechanical and structural properties of a range of tissues. Such phantoms are a key requirement for the continued development of OCT techniques and applications. We focus on phantoms based on silicone, fibrin and poly(vinyl alcohol) cryogels (PVA-C), as we believe these materials hold the most promise for durable and accurate replication of tissue properties.
Endoscopic treatment of lower airway pathologies requires accurate quantification of airway dimensions. We demonstrate the application of a real-time endoscopic optical coherence tomography system that can image lower airway anatomy and quantify airway lumen dimensions intra-operatively. Results demonstrate the ability to acquire 3D scans of airway anatomy and include comparison against a pre-operative X-ray CT. The paper also illustrates the capability of the system to assess the real-time dynamic changes within the airway that occur during respiration.
Multiple scattering is one of the main degrading influences in optical coherence tomography, but to date its presence in an image can only be indirectly inferred. We present a polarization-sensitive method that shows the potential to detect it more directly, based on the degree to which the detected polarization state at any given image point is correlated with the mean state over the surrounding region. We report the validation of the method in microsphere suspensions, showing a strong dependence of the degree of correlation upon the extent to which multiply scattered light is coherently detected. We demonstrate the method's utility in various tissues, including chicken breast ex vivo and human skin and nailfold in vivo.
Probing the mechanical properties of skin at high resolution could aid in the assessment of skin pathologies by, for example, detecting the extent of cancerous skin lesions and assessing pathology in burn scars. Here, we present two elastography techniques based on optical coherence tomography (OCT) to probe the local mechanical properties of skin. The first technique, optical palpation, is a high-resolution tactile imaging technique, which uses a complaint silicone layer positioned on the tissue surface to measure spatially-resolved stress imparted by compressive loading. We assess the performance of optical palpation, using a handheld imaging probe on a skin-mimicking phantom, and demonstrate its use on human skin. The second technique is a strain imaging technique, phase-sensitive compression OCE that maps depth-resolved mechanical variations within skin. We show preliminary results of in vivo phase-sensitive compression OCE on a human skin lesion.
This guest editorial introduces the Special Section on 25 Years of OCT.
Cellular-scale imaging of the mechanical properties of tissue has helped to reveal the origins of disease; however, cellular-scale resolution is not readily achievable in intact tissue volumes. Here, we demonstrate volumetric imaging of Young’s modulus using ultrahigh-resolution optical coherence elastography, and apply it to characterizing the stiffness of mouse aortas. We achieve isotropic resolution of better than 15 μm over a 1-mm lateral field of view through the entire depth of an intact aortic wall. We employ a method of quasi-static compression elastography that measures volumetric axial strain and uses a compliant, transparent layer to measure surface axial stress. This combination is used to estimate Young’s modulus throughout the volume. We demonstrate differentiation by stiffness of individual elastic lamellae and vascular smooth muscle. We observe stiffening of the aorta in regulator of G protein signaling 5-deficient mice, a model that is linked to vascular remodeling and fibrosis. We observe increased stiffness with proximity to the heart, as well as regions with micro-structural and micro-mechanical signatures characteristic of fibrous and lipid-rich tissue. High-resolution imaging of Young’s modulus with optical coherence elastography may become an important tool in vascular biology and in other fields concerned with understanding the role of mechanics within the complex three-dimensional architecture of tissue.
Objective: The anastomosis of peripheral nerves is a demanding procedure that has potential complications due to foreign body reactions elicited by sutures. In this study, the sutureless in vivo anastomosis of rat tibial nerves was successfully performed, using for the first time a chitosan-based laser-activated adhesive. The nerve thermal damage caused by the laser irradiation was quantitatively assessed. Materials and Methods: A novel adhesive composed of chitosan, indocyanine green, acetic acid, and water, was fabricated in thin sheets. Its adhesive strength was tested in vitro by bonding strips (surface area ∼20 mm2, thickness ∼20 μm) onto rat sciatic nerves and sheep intestine by laser activation with low fluence (∼50 J/cm2), using a fiber-coupled diode laser (n = 13). The tensile strength of the adhesive/tissue bonds was measured after tissue repair. The chitosan adhesive was then used to perform sutureless anastomosis of tibial nerves in vivo (n = 6). Adhesive strips were also bonded in vivo onto intact rat sciatic nerves (n = 6) in order to quantitatively assess, by counting myelinated axons, the thermal damage induced by the laser. Results: The adhesive bonded well to tissue with a tensile strength of 12.5 ± 2.6 KPa (mean ± SD; n = 13). The in vivo anastomosed nerves were in continuity 3 d after surgery. Axon counting showed the number and morphology of myelinated axons were normal proximally (∼96%) compared with intact nerves (100%). Axon demyelination was observed at the operation site (∼49%) and distally (∼27%), and was attributed to laser-induced thermal damage. Conclusions: Nerve anastomosis, performed by the laser-adhesive procedure, was successful 3 d postoperatively. Proximal myelinated axons were not significantly damaged by the low laser fluence.
We demonstrate the modification of optical coherence elastography to advance from relative strain images to quantified tissue stiffness on the micro-scale. We highlight the nonlinear dependence of tissue stiffness on the applied load and consider how nonlinearity may help characterise soft tissues.
This study tested the utility of optical coherence tomography (OCT)-based indentation to assess mechanical properties of respiratory tissues in disease. Using OCT-based indentation, the elastic modulus of mouse diaphragm was measured from changes in diaphragm thickness in response to an applied force provided by an indenter. We used a transgenic mouse model of chronic lung disease induced by the overexpression of transforming growth factor-alpha (TGF-α), established by the presence of pleural and peribronchial fibrosis and impaired lung mechanics determined by the forced oscillation technique and plethysmography. Diaphragm elastic modulus assessed by OCT-based indentation was reduced by TGF-α at both left and right lateral locations (p
High extracellular matrix (ECM) content in solid cancers impairs tumour perfusion and thus access of imaging and therapeutic agents. We have devised a new approach to degrade tumour ECM, which improves uptake of circulating compounds. We target the immune‐modulating cytokine, tumour necrosis factor alpha (TNFα), to tumours using a newly discovered peptide ligand referred to as CSG. This peptide binds to laminin–nidogen complexes in the ECM of mouse and human carcinomas with little or no peptide detected in normal tissues, and it selectively delivers a recombinant TNFα‐CSG fusion protein to tumour ECM in tumour‐bearing mice. Intravenously injected TNFα‐CSG triggered robust immune cell infiltration in mouse tumours, particularly in the ECM‐rich zones. The immune cell influx was accompanied by extensive ECM degradation, reduction in tumour stiffness, dilation of tumour blood vessels, improved perfusion and greater intratumoral uptake of the contrast agents gadoteridol and iron oxide nanoparticles. Suppressed tumour growth and prolonged survival of tumour‐bearing mice were observed. These effects were attainable without the usually severe toxic side effects of TNFα.
The formation of burn-scar tissue in human skin profoundly alters, among other things, the structure of the dermis. We present a method to characterize dermal scar tissue by the measurement of the near-infrared attenuation coefficient using optical coherence tomography (OCT). To generate accurate en face parametric images of attenuation, we found it critical to first identify (using speckle decorrelation) and mask the tissue vasculature from the three-dimensional OCT data. The resulting attenuation coefficients in the vasculature-masked regions of the dermis of human burn-scar patients are lower in hypertrophic (3.8Â±0.4 mm-1) and normotrophic (4.2Â±0.9 mm-1) scars than in contralateral or adjacent normal skin (6.3Â±0.5 mm-1). Our results suggest that the attenuation coefficient of vasculature-masked tissue could be used as an objective means to assess human burn scars.
Optical coherence elastography (OCE) is an emerging imaging technique that probes microscale mechanical contrast in tissues with the potential to differentiate healthy and malignant tissues. However, conventional OCE techniques are limited to imaging the first 1 to 2 mmof tissue in depth. We demonstrate, for the first time, OCE measurements deep within human tissues using needle OCE, extending the potential of OCE as a surgical guidance tool. We use needle OCE to detect tissue interfaces based on mechanical contrast in both normal and malignant breast tissues in freshly excised human mastectomy samples, as validated against histopathology. Further, we demonstrate the feasibility of in situ measurements 4 cm from the tissue surface using ultrasound guidance of the OCE needle probe. With further refinement, our method may potentially aid in accurate detection of the boundary of the tumor to help ensure full removal of all malignant tissues, which is critical to the success of breast-conserving surgery.
Optical coherence elastography (OCE) maps the mechanical properties of tissue microstructure and has potential applications in both fundamental investigations of biomechanics and clinical medicine. We report the first analysis of contrast in OCE, including evaluation of the accuracy with which OCE images (elastograms) represent mechanical properties and the sensitivity of OCE to mechanical contrast within a sample. Using phase-sensitive compression OCE, we generate elastograms of tissue-mimicking phantoms with known mechanical properties and identify limitations on contrast imposed by sample mechanics and the imaging system, including signal-processing parameters. We also generate simulated elastograms using finite element models to perform mechanical analysis in the absence of imaging system noise. In both experiments and simulations, we illustrate artifacts that degrade elastogram accuracy, depending on sample geometry, elasticity contrast between features, and surface conditions. We experimentally demonstrate sensitivity to features with elasticity contrast as small as 1.1:1 and calculate, based on our imaging system parameters, a theoretical maximum sensitivity to elasticity contrast of 1.002:1. The results highlight the microstrain sensitivity of compression OCE, at a spatial resolution of tens of micrometers, suggesting its potential for the detection of minute changes in elasticity within heterogeneous tissue.
Optical coherence tomography (OCT) is a medical imaging modality that opens up new opportunities for imaging in breast cancer. It provides two- and three-dimensional micro-scale images of tissue structure from bulk tissue, in vivo or freshly excised without labeling or staining, is capable of video-rate acquisition speeds, and is compatible with compact imaging probes. In this paper, the authors briefly describe OCT technology and describe in detail its capabilities for imaging breast cancer. Potential applications identified in current research are discussed, particularly in the assessment of excised breast tumors. It is concluded that OCT shows promise for margin assessment and biopsy guidance but that much more research and validation is required to confirm its level of utility.
We propose optical spectral encoding of an object's spatial frequencies as a means of transmitting, through a low-numerical-aperture optical system, spatial information with an instantaneous spatial frequency bandwidth wider than the optical system's diffraction-limited bandwidth. We validate this new superresolution approach experimentally and demonstrate one of its possible practical implementations - wide-field spectrally encoded imaging that is sensitive to nanometre-scale local variations in the microstructure of centimetre-scale samples.
We have developed an extremely miniaturized optical coherence tomography (OCT) needle probe (outer diameter 310 Î¼m) with high sensitivity (108 dB) to enable minimally invasive imaging of cellular structure deep within skeletal muscle. Three-dimensional volumetric images were acquired from ex vivo mouse tissue, examining both healthy and pathological dystrophic muscle. Individual myofibers were visualized as striations in the images. Degradation of cellular structure in necrotic regions was seen as a loss of these striations. Tendon and connective tissue were also visualized. The observed structures were validated against coregistered hematoxylin and eosin (H&E) histology sections. These images of internal cellular structure of skeletal muscle acquired with an OCT needle probe demonstrate the potential of this technique to visualize structure at the microscopic level deep in biological tissue in situ.
We present a high-resolution three-dimensional position tracking method that allows an optical coherence tomography (OCT) needle probe to be scanned laterally by hand, providing the high degree of flexibility and freedom required in clinical usage. The method is based on a magnetic tracking system, which is augmented by cross-correlation-based resampling and a two-stage moving window average algorithm to improve upon the tracker's limited intrinsic spatial resolution, achieving 18 Î¼ m RMS position accuracy. A proof-of-principle system was developed, with successful image reconstruction demonstrated on phantoms and on ex vivo human breast tissue validated against histology. This freehand scanning method could contribute toward clinical implementation of OCT needle imaging.
We present a theoretical framework for strain estimation in optical coherence elastography (OCE), based on a statistical analysis of displacement measurements obtained from a mechanically loaded sample. We define strain sensitivity, signal-to-noise ratio and dynamic range, and derive estimates of strain using three methods: finite difference, ordinary least squares and weighted least squares, the latter implemented for the first time in OCE. We compare theoretical predictions with experimental results and demonstrate a ~12 dB improvement in strain sensitivity using weighted least squares compared to finite difference strain estimation and a ~4 dB improvement over ordinary least squares strain estimation. We present strain images (i.e., elastograms) of tissue-mimicking phantoms and excised porcine airway, demonstrating in each case clear contrast based on the sample's elasticity.
We have designed high-performance imaging optics on the tip of single-mode optical fibers. Embedding such probes within a rigid needle provides access to deep tissues impossible with surface-based imaging and, as well, a natural actuator for optical elastography.
Anatomical optical coherence tomography (aOCT) is an endoscopic optical technique that enables continuous, quantitative assessment of hollow organ size and shape in three dimensions. It is a powerful alternative to X-ray computed tomography, magnetic resonance imaging, and video endoscopy for the assessment of gross hollow-organ anatomy. This paper reviews our instrument and its application to the upper and lower airway, and includes a number of new results.
In this paper, we report on anatomical optical coherence tomography, a catheter-based optical modality designed to provide quantitative sectional images of internal hollow organ anatomy over extended observational periods. We consider the design and performance of an instrument and its initial intended application in the human upper airway for the characterization of obstructive sleep apnea (OSA). Compared with current modalities, the technique uniquely combines quantitative imaging, bedside operation, and safety for use over extended periods of time with no cumulative dose limit. Our experiments show that the instrument is capable of imaging subjects during sleep, and that it can record dynamic changes in airway size and shape.
We report single-measurement, full-range imaging of local polarization properties in the human anterior segment in vivo with polarization-sensitive optical coherence tomography (PS-OCT). Off-pivot galvanometer-mirror phase shifting used to extend the system's axial imaging range sufficiently to reconstruct local polarization properties of the anterior segment.
This paper presents results of in vivo studies on the effect of refractive index-matching media on image artifacts in optical coherence tomography (OCT) images of human skin. These artifacts present as streaks of artificially low backscatter and displacement or distortion of features. They are primarily caused by refraction and scattering of the OCT light beam at the skin surface. The impact of the application of glycerol and ultrasound gel is assessed on both novel skin-mimicking phantoms and in vivo human skin, including assessment of the epidermal thickening caused by the media. Based on our findings, recommendations are given for optimal OCT imaging of skin in vivo.
In situ imaging of alveoli and small airways with optical coherence tomography (OCT) needle probes has significant potential in the study and clinical assessment of lung disease. We present the smallest reported OCT needle probe capable of acquiring 3D volumetric data. The side-facing needle probe comprises miniaturized focusing optics consisting of no-core and GRIN fibre, terminated with a reflection-coated, fibre tip beam deflector. The optics are encased within a 30-gauge (outer diameter 310 m) needle, and interfaced to a spectral-domain OCT scanner. Multiple 3D-OCT data sets were acquired on preterm lamb lungs (excised) filled with amniotic fluid and saline. Results demonstrated the ability of such a probe to image individual alveoli and bronchioles, and enabled the rendering of 3D volumetric visualisations of the data. We observed notably less tissue distortion than in earlier work with larger 23 gauge needle probes.
We incorporate for the first time optical coherence elastography (OCE) into a needle probe and demonstrate its ability to provide depth-resolved information about the mechanical properties of soft tissues. This allows analysis of tissues located much deeper than has previously been possible with other forms of OCE. OCE exploits the microscopic resolution of optical coherence tomography (OCT) to produce high-resolution maps of tissue mechanical properties. While OCE has potential to delineate diseased and healthy tissues (e.g., stiff tumor in soft tissue), standard techniques are limited by the penetration depth of OCT in tissue (2-3 mm). Our OCE needle probe overcomes this limitation, as it may be inserted deep within the body to perform measurements. We tested needle-based OCE in tissue-mimicking phantoms and ex vivo porcine airway tissue comprising layers of varying stiffness. Results demonstrate mechanical differentiation of tissues and identification of tissue interfaces. The proof-of-principle results presented here pave the way for future measurements in human breast tissue that will aim to establish needle-based OCE as a viable technique for intraoperative guidance of breast cancer surgery.
We propose a passive optical network for customer access based on a hybrid coherence multiplexing/coarse wavelength-division multiplexing (WDM) technique. Coherence multiplexing provides asynchronous multichannel transmission, and coarse WDM provides bidirectional transmission. A costshared superfluorescent source is used for downstream transmission, and inexpensive light-emitting diodes are employed for upstream transmission. Asynchronous two channel transmission at 40 Mb/s per channel is demonstrated for both upstream and downstream. Our experiment indicates that upstream and downstream aggregate bit rates of 640 Mb/s are feasible based on current commercially available components.
Photonic code-division multiple access schemes have been proposed since the 1970s. Although there are many published proposals for new coding schemes, there are many less experimental verifications of these schemes, even fewer reports of successful data transmission, and no commercial systems. We attempt to explain the key factors that have led to the current state-of-the-art. In so doing, we describe the fundamental principles of matched filtering and noise in photonic CDMA schemes. We survey important developments and show how various schemes are related. We review recent experimental advances and compare the published experimental and theoretical performance for different schemes. We discuss the current major issues and like future directions.
We report on a novel scheme for extending the depth of focus (DOF) of ultrathin (125 Î¼m diameter) fiber probes for optical coherence tomography (OCT) using a simple phase mask consisting of graded-index (GRIN) fiber. The technique is compatible with existing all-in-fiber probe fabrication techniques, and our simulations show that it can provide a DOF gain of ~2 at a modest ~5 dB reduction of peak sensitivity. In a prototype device using commercially available GRIN fiber, a DOF gain of 1.55 is obtained, validated by beam profiling and OCT imaging.
We present the smallest reported side-viewing needle probe for optical coherence tomography (OCT). Design, fabrication, optical characterization, and initial application of a 30-gauge (outer diameter 0.31mm) needle probe are demonstrated. Extreme miniaturization is achieved by using a simple all-fiber probe design incorporating an anglepolished and reflection-coated fiber-tip beam deflector. When inserted into biological tissue, aqueous interstitial fluids reduce the probe's inherent astigmatism ratio to 1.8, resulting in a working distance of 300 μm and a depth-of-field of 550 μm with beam diameters below 30 μm. The needle probe was interfaced with an 840nm spectral-domain OCT system and the measured sensitivity was shown to be only 7 dB lower than that of a comparable galvo-scanning sample arm configuration. 3D OCT images of lamb lungs were acquired over a depth range of ∼600 μm, showing individual alveoli and bronchioles.
We investigate wideband reduction of excess intensity noise in incoherent light for application to spectrum-sliced WDM systems. The noise reduction scheme is based on optoelectronic feedforward compensation. We derive expressions for the probability density function of noise-reduced incoherent light and present measurements that are in good agreement with theory. We evaluate the significant levels of improvement obtainable in the capacity of spectrum-sliced WDM channels. For example, to obtain a signal-to-noise ratio of 50, a noise-reduced channel requires six times less optical bandwidth than a spectral slice without noise reduction.
The excess noise in modulated amplified spontaneous emission (ASE) that is transmitted over a dispersive fiber was investigated both theoretically and experimentally. The signal-to-excess noise ratio (SNR ex) of the detected signal varied across the waveform and SNR ex was modified by transmission over the dispersive fiber. These effects must be accounted for in high bit-rate optical communication systems that use sources of ASE.
We demonstrate a multiwavelength source of amplified spontaneous emission suitable for spectrum-sliced, wavelength-division multiplexed access and local-area networks. The source, based on flat-gain optical amplification of a Fabry-Perot laser biased below threshold, provides up to 30-wavelength channels spaced by 138 GHz. We show the potential of the source in network applications by demonstrating wavelength-stabilized, low (<10-9) error rate operation of all 30 wavelength channels at 622 Mb/s.
An all-optical customer access network based on a hybrid approach combining coherence multiplexing and a coarse wavelength-division multiplexing (WDM) were demonstrated. Low bit error rates were measured for asynchronous transmission of two 40-Mbit/s channels both upstream and downstream over 10 km of standard fiber.
Synthesis of nanocrystals that exhibit strong upconversion (UC) luminescence upon infrared excitation has been challenging due to the stringent control needed over experimental variables. Herein, we report a method to synthesize nanocrystals demonstrating high UC at room temperature in aqueous solution on graphene.
We present measurements, in good agreement with theory, of the noise power spectral density at the outputs of a coherence-multiplexed system employing a low-coherence source. We investigate correlations in the noise between outputs and confirm that differential detection can be used to improve the signal-to-noise ratio of sensor and communication systems based on coherence multiplexing.
We present a three-dimensional structured tissue-mimicking phantom for use in optical coherence tomography (OCT). The phantom was fabricated from a silicone matrix and titanium dioxide additive using a lithographic casting method capable of producing a wide range of well-defined geometries with optical contrast and mesoscopic feature sizes relevant to OCT. We describe the fabrication, characterization and OCT imaging of two phantoms and demonstrate their utility in assessing the performance of a spatial-diversity speckle reduction technique. Such phantoms will be important in the development of standards in OCT, as well as in enabling quantitative performance assessment.
We investigate, both theoretically and experimentally, the signal-to-noise ratio (SNR) of modulated amplified spontaneous emission (ASE) transmitted over dispersive fiber. We observe two significant effects; firstly, the signal-to-excess-noise ratio (SNR ex) varies across the pulse reaching its maximum value near the peak of the detected signal; and secondly, this maximum value decreases with increasing fiber dispersion-induced pulse broadening. Accurate calculation of transmission performance of high bit-rate optical communication systems employing ASE sources, such as spectrum slicing, requires inclusion of these effects.
We report the use of optical coherence tomography (OCT) to determine spatially localized optical attenuation coefficients of human axillary lymph nodes and their use to generate parametric images of lymphoid tissue. 3D-OCT images were obtained from excised lymph nodes and optical attenuation coefficients were extracted assuming a single scattering model of OCT. We present the measured attenuation coefficients for several tissue regions in benign and reactive lymph nodes, as identified by histopathology. We show parametric images of the measured attenuation coefficients as well as segmented images of tissue type based on thresholding of the attenuation coefficient values. Comparison to histology demonstrates the enhancement of contrast in parametric images relative to OCT images. This enhancement is a step towards the use of OCT for in situ assessment of lymph nodes.
Coherent recombination by matched filtering of a series of pulses derived from a single laser pulse, using optical ladder networks is shown to result in a highly single-peaked correlation function. When this coherent correlation is generated using a low coherence source, the technique may be used in a local area network based on a star topology. This provides a more practical but equally powerful alternative to other current proposals.
We determine the capacity and numbers of users that can be supported by CDMA networks that use coherence multiplexing. The analysis takes into account reduced optical beat noise due to differential detection of the decoder outputs. The analysis also includes the effect of fibre dispersion and shows that there is an optimum source line width for a given link length and user bit rate. We confirm that the capacity is severely restricted by optical beat noise. However, our calculations show that performances achievable may be adequate for certain local area and access network type applications. For example, at 40 Mb/s per user a maximum of 50 users can be supported over 12 km. We compare our calculations with the measured performance of our recent network demonstration and find good agreement.
The most collapsible part of the upper airway in the majority of individuals is the velopharynx which is the segment positioned behind the soft palate. As such it is an important morphological region for consideration in elucidating the pathogenesis of obstructive sleep apnea (OSA). This study compared steady flow properties during inspiration in the pharynges of nine male subjects with OSA and nine body-mass index (BMI)- and age-matched control male subjects without OSA. The k– SST turbulence model was used to simulate the flow field in subject-specific pharyngeal geometric models reconstructed from anatomical optical coherence tomography (aOCT) data. While analysis of the geometry of reconstructed pharynges revealed narrowing at velopharyngeal level in subjects with OSA, it was not possible to clearly distinguish them from subjects without OSA on the basis of pharyngeal size and shape alone. By contrast, flow simulations demonstrated that pressure fields within the narrowed airway segments were sensitive to small differences in geometry and could lead to significantly different intraluminal pressure characteristics between subjects. The ratio between velopharyngeal and total pharyngeal pressure drops emerged as a relevant flow-based criterion by which subjects with OSA could be differentiated from those without.
Angular diversity is a successful speckle reduction technique in optical coherence tomography. We employ the same technique for a different purpose: discriminating between the singly backscattered and multiply scattered signal components.
We demonstrate tomographic imaging of the refractive index of turbid media using bifocal optical coherence refractometry (BOCR). The technique, which is a variant of optical coherence tomography, is based on the measurement of the optical pathlength difference between two foci simultaneously present in a medium of interest. We describe a new method to axially shift the bifocal optical pathlength that avoids the need to physically relocate the objective lens or the sample during an axial scan, and present an experimental realization based on an adaptive liquid-crystal lens. We present experimental results, including video clips, which demonstrate refractive index tomography of a range of turbid liquid phantoms, as well as of human skin in vivo.
Characterization of the size of lung structures can aid in the assessment of a range of respiratory diseases. In this paper, we present a fully automated segmentation and quantification algorithm for the delineation of large numbers of lung structures in optical coherence tomography images, and the characterization of their size using the stereological measure of median chord length. We demonstrate this algorithm on scans acquired with OCT needle probes in fresh, ex vivo tissues from two healthy animal models: pig and rat. Automatically computed estimates of lung structure size were validated against manual measures. In addition, we present 3D visualizations of the lung structures using the segmentation calculated for each data set. This method has the potential to provide an in vivo indicator of structural remodeling caused by a range of respiratory diseases, including chronic obstructive pulmonary disease and pulmonary fibrosis.
We examined the impact of axial length on superficial retinal vessel density (SRVD) and foveal avascular zone area (FAZA) measurement using optical coherence tomography angiography. The SRVD and FAZA were quantified before and after correction for magnification error associated with axial length variation. Although SRVD did not differ before and after correction for magnification error in the parafoveal region, change in foveal SRVD and FAZA were significant. This has implications for clinical trials outcome in diseased eyes where significant capillary dropout may occur in the parafovea.
Fourier-holographic light scattering spectroscopy is applied to record complex angular scattering spectra of two- and three-dimensional samples over a wide field of view. We introduce a computational depth sectioning technique and, for the first time, demonstrate that a single-exposure hologram can generate a quantitative, three-dimensional map of particle sizes and locations over several cubic millimeters with micrometer resolution. Such spatially resolved maps of particle sizes are generated by Mie-inversion and could not be ascertained from the directly reconstructed intensity-distribution images. We also demonstrate synthesis of multiple angular scattering intensity spectra to increase the angular range and improve size detection sensitivity.
There are limited imaging technologies available that can accurately assess or provide surrogate markers of the in vivo cutaneous microvessel network in humans. In this study, we establish the use of optical coherence tomography (OCT) as a novel imaging technique to assess acute changes in cutaneous microvessel area density and diameter in humans. OCT speckle decorrelation images of the skin on the ventral side of the forearm up to a depth of 500 μm were obtained prior to and following 20-25 min of lower limb heating in eight healthy men [30.3 ± 7.6 (SD) yr]. Skin red blood cell flux was also collected using laser Doppler flowmetry probes immediately adjacent to the OCT skin sites, along with skin temperature. OCT speckle decorrelation images were obtained at both baseline and heating time points. Forearm skin flux increased significantly (0.20 ± 0.15 to 1.75 ± 0.38 cutaneous vascular conductance, P < 0.01), along with forearm skin temperature (32.0 ± 1.2 to 34.3 ± 1.0°C, P < 0.01). Quantitative differences in the automated calculation of vascular area densities (26 ± 9 to 49 ± 19%, P < 0.01) and individual microvessel diameters (68 ± 17 to 105 ± 25 μm, P < 0.01) were evident following the heating session. This is the first in vivo within-subject assessment of acute changes in the cutaneous microvasculature in response to heating in humans and highlights the use of OCT as an exciting new imaging approach for skin physiology and clinical research.
Background Evaluation of lymph node involvement is an important factor in detecting metastasis and deciding whether to perform axillary lymph node dissection (ALND) in breast cancer surgery. As ALND is associated with potentially severe long term morbidity, the accuracy of lymph node assessment is imperative in avoiding unnecessary ALND. The mechanical properties of malignant lymph nodes are often distinct from those of normal nodes. A method to image the micro-scale mechanical properties of lymph nodes could, thus, provide diagnostic information to aid in the assessment of lymph node involvement in metastatic cancer. In this study, we scan axillary lymph nodes, freshly excised from breast cancer patients, with optical coherence micro-elastography (OCME), a method of imaging micro-scale mechanical strain, to assess its potential for the intraoperative assessment of lymph node involvement. Methods Twenty-six fresh, unstained lymph nodes were imaged from 15 patients undergoing mastectomy or breast-conserving surgery with axillary clearance. Lymph node specimens were bisected to allow imaging of the internal face of each node. Co-located OCME and optical coherence tomography (OCT) scans were taken of each sample, and the results compared to standard post-operative hematoxylin-and-eosin-stained histology. Results The optical backscattering signal provided by OCT alone may not provide reliable differentiation by inspection between benign and malignant lymphoid tissue. Alternatively, OCME highlights local changes in tissue strain that correspond to malignancy and are distinct from strain patterns in benign lymphoid tissue. The mechanical contrast provided by OCME complements the optical contrast provided by OCT and aids in the differentiation of malignant tumor from uninvolved lymphoid tissue. Conclusion The combination of OCME and OCT images represents a promising method for the identification of malignant lymphoid tissue. This method shows potential to provide intraoperative assessment of lymph node involvement, thus, preventing unnecessary removal of uninvolved tissues and improving patient outcomes.
The guest editors introduce a feature issue containing papers based on research presented at the BIOMED 2016 Congress, held in Fort Lauderdale, FL, 24–28 April, 2016.
We demonstrate an inexpensive and easily fabricated, high-performance system for the generation of single and multiple optical pulse bursts. The system, which is comprised of commercially available components, provides single pulses as short as 190 ps full width half maximum. Used as an active, very low jitter, delay line for application to fast coincidence timing experiments, delays of much less than 1 ns to in excess of 1 Î¼s are possible using 1300 nm standard telecommunications fiber. Optical fiber ladder networks containing differential fiber delays, connected together using single-mode fiber connectors have been used to construct flexible and reconfigurable optical burst generators. Precise delay time determination (Â±5 psecs) allows very accurate, discretely variable burst frequencies demonstrated at up to 3 GHz. Instruments capable of 4, 8, and 16 pulse trains are also demonstrated. A conservative estimate indicates pulse trains consisting of 128 pulses at 8 Î¼W per pulse should be possible with a simple extension to the present apparatus.
We have developed a quasi-distributed temperature sensor consisting of an array of fibre Bragg gratings (FBGs), illuminated by a superluminescent diode (SLD) and interrogated by a fibre Fabry-Perot (FFP) tunable filter. This sensor has been previously tested both on agar-gel tissue phantoms and in vivo on tumours, in rabbit livers that were treated by hyperthermia. The FFP filter is controlled by a piezoelectric transducer operating in an open-loop configuration, and this introduces repeatability and long-term stability issues. Here we report the further development of this system in order to account for the FFP filter issues, and we reduce the noise levels to less than 0.035Â°C rms and the long-term temperature drift below 0.1Â°C/hr.
Minimally invasive, highresolution imaging of muscle necrosis has the potential to aid in the assessment of diseases such as Duchenne muscular dystrophy. Undamaged muscle tissue possesses high levels of optical birefringence due to its anisotropic ultrastructure, and this birefringence decreases when the tissue undergoes necrosis. In this study, we present a novel technique to image muscle necrosis using polarization-sensitive optical coherence tomography (PS-OCT). From PS-OCT scans, our technique is able to quantify the birefringence in muscle tissue, generating an image indicative of the tissue ultrastructure, with areas of abnormally low birefringence indicating necrosis. The technique is demonstrated on excised skeletal muscles from exercised dystrophic mdx mice and control C57BL/10ScSn mice with the resulting images validated against colocated histological sections. The technique additionally gives a measure of the proportion (volume fraction) of necrotic tissue within the three-dimensional imaging field of view. The percentage necrosis assessed by this technique is compared against the percentage necrosis obtained from manual assessment of histological sections, and the difference between the two methods is found to be comparable to the interobserver variability of the histological assessment. This is the first published demonstration of PSOCT to provide automated assessment of muscle necrosis.
This study reports two types of imaging artifacts in 3D optical coherence tomography images of human skin intensity deficits and geometrical distortion, and describes their reduction through the application of refractive index matching media.
High-power ultra-broadband sources such as a supercontinuum are very attractive in optical coherence tomography (OCT) and optical coherence-domain reflectometry (OCDR) due to their very high resolution potential. However, the large and extensive coherence-function sidelobes typical of such sources preclude their use in conventional OCDR and OCT systems. In addition, device or sample dispersion over such broad band-widths may also significantly limit the achievable performance. Here we describe a novel experiment using a supercontinuum source with a static Michelson interferometer to perform OCDR at 1.55 Î¼m. Quadrature spectral detection is used to maximise the scanning range and to allow direct compensation for both the undesirable spectral shape of the source and for the dispersion in the system. Such a non-scanning-interferometer approach is an interesting possible alternative for very broadband, ultra-high resolution OCT systems. We demonstrate that an otherwise obscured small reflection next to a large reflection can be revealed by appropriately weighting the data to reshape the supercontinuum spectrum and compensate for dispersion. Significant reduction of the supercontinuum coherence function sidelobes is achieved, and a resolution in air of 7 Î¼m (FWHM) is obtained, or less than 5 Î¼m in media of refractive index 1.45.
Fiber-optic probes are a key component in a range of emerging clinical applications of optical coherence tomography (OCT). These miniaturized probes offer new possibilities to image diseased tissue deep within the body. This paper presents an overview of the design and use of fiber-optic probes for OCT. Three different deployment scenarios are identified: endoscopic, intravascular and needle-based probes, and specific case studies are presented for both endoscopic and intravascular probes.
Many bioimaging studies, including those in engineered tissue constructs, intravital microscopy in animal models, and medical imaging in humans, require cellular-resolution imaging of structures deep within a sample. Yet, many of the current approaches are limited in terms of resolution, but also in invasiveness, repeatable imaging of the same location, and accessible imaging depth. We coin the term micro-endomicroscope to describe the emerging class of small, cellular-resolution endoscopic imaging systems designed to image cells in situ while minimizing perturbation of the sample. In this Perspective, we motivate the need for further development of micro-endomicroscopes, highlighting applications that would greatly benefit, reviewing progress, and considering how photonics might contribute. We identify areas ripe for technological development, such as micro-scanners and small lens systems, that would advance micro-endomicroscope performance. With the right developments in photonics, many possibilities exist for new minimally invasive translatable imaging tools across the scientific, pre-clinical, and clinical spectrum: from longitudinal studies of engineered tissue constructs, to tracking disease progression in animal models, to expanding the ability to diagnose and develop treatments for diseases without the need for invasive medical procedures.
An all-optical CDMA network is presented which employs optical phase information to encode and decode transmitted data from a low-coherence source, using simple, compact, reconfigurable optical delay networks. The use of a master encoding network to provide a reference for the system is demonstrated experimentally, and examples of phase codes ensuring minimum interference between users are given.
We describe a long-range optical coherence tomography system for size and shape measurement of large hollow organs in the human body. The system employs a frequency-domain optical delay line of a configuration that enables the combination of high-speed operation with long scan range. We compare the achievable maximum delay of several delay line configurations, and identify the configurations with the greatest delay range. We demonstrate the use of one such long-range delay line in a catheter-based optical coherence tomography system and present profiles of the human upper airway and esophagus in vivo with a radial scan range of 26 millimeters. Such quantitative upper airway profiling should prove valuable in investigating the pathophysiology of airway collapse during sleep (obstructive sleep apnea).
We present a parametric optical coherence tomography (OCT) technique to improve contrast between malignant and healthy non-neoplastic tissue. The technique incorporates a fully automated method to extract tissue attenuation characteristics. Results are represented visually as a parametric en face image, where the parameter used for contrast is indicative of the relative optical attenuation coefficient of the tissue. We present the first parametric OCT images of human lymph nodes containing malignant cells, and demonstrate improved tissue contrast over en face OCT images.
Quantitative assessment of upper airway geometry using optical coherence tomography in burns patients could provide physicians with the information needed to make critical decisions. We have developed a high speed catheter based OCT system capable of real time imaging in airways up to 3cm in diameter. Preliminary scans of inhalation injured airways are presented to demonstrate the feasibility of aOCT as a diagnostic tool for assessing burns patients.
An achromatic optical phase shifter based on frequency-domain delay line was proposed. A Michelson interferometer illuminated with a broadband source was used to perform theoretical and experimental study on different regimes of operation of delay lines. The delay line incorporated unique properties of phase decoupling and group delays. The analysis showed that the phase delay was generated by the variation of optical length difference between two paths in an interferometer.
We present an automated, label-free method for lymphangiography of cutaneous lymphatic vessels in humans in vivo using optical coherence tomography (OCT). This method corrects for the variation in OCT signal due to the confocal function and sensitivity fall-off of a spectral-domain OCT system and utilizes a single-scattering model to compensate for A-scan signal attenuation to enable reliable thresholding of lymphatic vessels. A segment-joining algorithm is then incorporated into the method to mitigate partial-volume effects with small vessels. The lymphatic vessel images are augmented with images of the blood vessel network, acquired from the speckle decorrelation with additional weighting to differentiate blood vessels from the observed high decorrelation in lymphatic vessels. We demonstrate the method with longitudinal scans of human burn scar patients undergoing ablative fractional laser treatment, showing the visualization of the cutaneous lymphatic and blood vessel networks.
The suitability for low-coherence interferometry of a high-power, semiconductor laser line source operated at a forward bias current below threshold is demonstrated. Measurements of the important characteristics of the source are presented. For example, the source produces an output power of 1.3mW and a spatially uniform coherence length of 16 mmat a bias current of 86% of threshold (250 mA) at 20Â°C. The usefulness of the source is verified by measurement of the line profile of a contact lens.
The reflective frequency-domain optical delay line employing a diffraction grating, a lens, and a tiltable mirror has emerged as a device particularly suitable for interferometry and optical coherence tomography. The device is comprehensively described, both theoretically and experimentally, in the context of interferometry. The variations of phase and group delay produced by the device as well as its dispersive properties are described and demonstrated experimentally.
Optical microscope-in-a-needle technology for 3D tissue micro-imaging will open up new avenues in diagnosis and treatment of disease. We describe innovations in guided-wave optical design for needle probes and demonstrate applications in tissues.
In scars arising from burns, objective assessment of vascularity is important in the early identification of pathological scarring, and in the assessment of progression and treatment response. We demonstrate the first clinical assessment and automated quantification of vascularity in cutaneous burn scars of human patients in vivo that uses optical coherence tomography (OCT). Scar microvasculature was delineated in three-dimensional OCT images using speckle decorrelation. The diameter and area density of blood vessels were automatically quantified. A substantial increase was observed in the measured density of vasculature in hypertrophic scar tissues (38%) when compared against normal, unscarred skin (22%). A proliferation of larger vessels (diameter≥100 μm) was revealed in hypertrophic scarring, which was absent from normal scars and normal skin over the investigated physical depth range of 600 μm. This study establishes the feasibility of this methodology as a means of clinical monitoring of scar progression.
A quasi-distributed temperature sensor consisting of an array of fibre Bragg gratings (FBG) illuminated by a superluminescent diode (SLD) was described. The sensor combined in vivo operation, quasi-distributed sensing and the stability at 0.1Â° centigrade required for providing diagnostic measurements of tumor margins for human hyperthermia techniques. The good thermal isolation between gratings, low thermal mass and loose adhesion between fibre and liver tissue helped in making accurate measurements.
Optical coherence elastography (OCE) provides images of tissue elasticity and has potential for several clinical applications, including guidance of tumor resection. However, advancement toward clinical implementation of OCE is currently limited by the technique?s small imaging depth in tissue (1-2 mm), as well as a lack of validation of the elastic contrast generated in OCE. We have overcome the depth limitation of current OCE techniques by developing a method for performing OCE via a needle probe. Our technique, needle OCE, uses an OCT needle probe to perform axial measurements of tissue deformation during needle insertion, and has demonstrated potential for subsurface detection of the boundaries of diseased tissue. In this paper, we demonstrate how elastic contrast is generated in needle OCE by performing measurements in tissue phantoms and porcine airway wall. In addition, we have developed a finite element model of tissue deformation in compression OCE as a first step toward better understanding of the generation and interpretation of contrast in OCE images. We show initial results demonstrating excellent agreement between measured and simulated deformation in a tissue phantom.
A technique for generating en face parametric images of tissue birefringence from scans acquired using a fiber-based polarization-sensitive optical coherence tomography (PS-OCT) system utilizing only a single-incident polarization state is presented. The value of birefringence is calculated for each A-scan in the PS-OCT volume using a quadrature demodulation and phase unwrapping algorithm. The algorithm additionally uses weighted spatial averaging and weighted least squares regression to account for the variation in phase accuracies due to varying OCT signal-to-noise-ratio. The utility of this technique is demonstrated using a model of thermally induced damage in porcine tendon and validated against histology. The resulting en face images of tissue birefringence are more useful than conventional PS-OCT B-scans in assessing the severity of tissue damage and in localizing the spatial extent of damage.
Repetitive closure of the upper airway characterizes obstructive sleep apnea. It disrupts sleep causing excessive daytime drowsiness and is linked to hypertension and cardiovascular disease. Previous studies simulating the underlying fluid mechanics are based upon geometries, time-averaged over the respiratory cycle, obtained usually via MRI or CT scans. Here, we generate an anatomically correct geometry from data captured in vivo by an endoscopic optical technique. This allows quantitative real-time imaging of the internal cross section with minimal invasiveness. The steady inhalation flow field is computed using a k- shear-stress transport (SST) turbulence model. Simulations reveal flow mechanisms that produce low-pressure regions on the sidewalls of the pharynx and on the soft palate within the pharyngeal section of minimum area. Soft-palate displacement and side-wall deformations further reduce the pressures in these regions, thus creating forces that would tend to narrow the airway. These phenomena suggest a mechanism for airway closure in the lateral direction as clinically observed. Correlations between pressure and airway deformation indicate that quantitative prediction of the low-pressure regions for an individual are possible. The present predictions warrant and can guide clinical investigation to confirm the phenomenology and its quantification, while the overall approach represents an advancement toward patient-specific modeling.
We present an automated technique to detect and quantify damage to biological tissue by sensing changes in the tissue's optical birefringence. Birefringence is a property of many types of tissue, which decreases with damage. Using a polarisation-sensitive optical coherence tomography scanner, the method first acquires a 3D scan of the area of tissue under analysis. By calculating the birefringence at each location on the surface of the tissue, we build a 2D image indicative of the biological microstructure, with areas of abnormally low birefringence indicating tissue damage. The technique is demonstrated using a model of localised thermal damage on porcine tendon. The resulting birefringence images are validated against a histological gold standard, showing strong correspondence between areas of low and high birefringence, and areas of damaged and undamaged tissue respectively.
Fiber-optic probes for sensing and biomedical imaging applications such as optical coherence tomography (OCT) frequently employ sections of graded-index (GRIN) fiber to re-focus the diverging light from the delivery fiber. Such GRIN fiber microlenses often possess aberrations that cause significant distortions of the focused output beam. Current design methods based on ABCD matrix transformations of Gaussian beams cannot model such effects and are therefore inadequate for the analysis and design of high-performance probes that require diffraction-limited output beams. We demonstrate use of the beam propagation method (BPM) to analyze beam distortion in GRIN-lensed fibers resulting from index profiles that exhibit a deviation from the ideal parabolic shape or artifacts such as ripples or a central dip. Furthermore, we demonstrate the power of this method for exploring novel probe designs that incorporate GRIN phase masks to generate wavefront-shaped output beams with extended depth-of-focus (DOF). We present results using our method that are in good agreement with experimental data. The BPM enables accurate simulation of fiber probes using non-ideal or custom-engineered GRIN fibers with arbitrary refractive index profiles, which is important in the design of high-performance fiber-based micro-imaging systems for biomedical applications.
We present a speckle reduction technique for optical coherence tomography based on strain compounding. Decorrelation is introduced between B-scans by altering the sample's strain. A theoretical description of the technique, based on a transfer-function formalism, and experimental results on silicone phantoms are presented. Nearly complete decorrelation between successive B-scan speckle patterns was observed for a variation in strain of 0.045. Strain compounding by averaging nine B-scans, with 0.003 strain increments between them, resulted in a 1.5-fold reduction in speckle contrast ratio.
Multifunctional materials exhibiting photon upconversion show promising applications for biological imaging and sensing. In this study, we examine the solid-state upconversion emission of NaYF4:Yb,Er nanoparticles in the presence of iron oxide nanoparticles. Fe3O4 nanoparticles (6 nm) were mixed with NaYF4:Yb,Er nanoparticles (either 10 or 50 nm) in varying proportions by drying chloroform solutions of nanoparticles onto glass slides. Upconversion spectra were acquired, and a laser power-dependent emission was observed and correlated with the iron oxide content in the mixture. Changes in the lattice temperature of the upconverting particles were monitored by careful observation of the relative intensities of the 2H11/2 and 4S3/2 → 4I15/2 transitions. The emission characteristics observed are consistent with an iron oxide-induced thermal effect that is dependent on both the laser power and the proportion of iron oxide. The results highlight that the thermal effects of mixed nanoparticle systems should be considered in the design of luminescent upconverting hybrid materials.
A Supercontinuum source is used with a static Michelson interferometer to perform high-resolution optical coherence-domain reflectometry (OCDR) at 1.551Î¼m. Quadrature spectral detection enables compensation for both the undesirable spectral shape of the source and for the dispersion in the system. A resolution of less than 5Î¼m in fibre (full width at half maximum) at 1.55Î¼m is obtained.
Interventional bronchoscopists manage central airway obstruction (CAO) through dilation, tumor ablation, and/or stent insertion. Anatomical optical coherence tomography (aOCT), a validated light-based imaging technique, has the unique capacity of providing bronchoscopists with intraprocedural central airway measurements. This study aims to describe the potential role of real-time aOCT in guiding interventions during CAO procedures. Methods: Prospective case series were recruited from patients referred for bronchoscopic management of symptomatic CAO. Preprocedure chest computed tomography (CT) scans were analyzed for relevant airway dimensions, such as stenosis caliber and length, and aided procedure planning. During bronchoscopy, an aOCT fiberoptic probe was inserted through the working channel of the bronchoscope to image the airway stenosis. From these aOCT images, stenosis dimensions were measured and compared with the preprocedure CT measurements. Preprocedure and postprocedure spirometry, Medical Research Council dyspnea score, and Eastern Cooperative Oncology Group performance status were collected to assess intervention efficacy. Results: Fourteen patients were studied. CT and aOCT-based measurements of airway caliber and length correlated closely (r2=0.87, P
We propose and demonstrate a novel technique, which we term bifocal optical coherence refractometry, for the rapid determination of the refractive index of a turbid medium. The technique is based on the simultaneous creation of two closely spaced confocal gates in a sample. The optical path-length difference between the gates is measured by means of low-coherence interferometry and used to determine the refractive index. We present experimental results for the refractive indices of milk solutions and of human skin in vivo. As the axial scan rate determines the acquisition time, which is potentially of the order of tens of milliseconds, the technique has potential for in vivo refractive-index measurements of turbid biological media under dynamic conditions.
We have developed a high-resolution optical coherence elastography system capable of estimating Young’s modulus in tissue volumes with an isotropic resolution of 15 μm over a 1 mm lateral field of view and a 100 μm axial depth of field. We demonstrate our technique on healthy and hypertensive, freshly excised and intact mouse aortas. Our technique has the capacity to delineate the individual mechanics of elastic lamellae and vascular smooth muscle. Further, we observe global and regional vascular stiffening in hypertensive aortas, and note the presence of local micro-mechanical signatures, characteristic of fibrous and lipid-rich regions.
Significance: To advance our understanding of the contrast observed when imaging with polarization-sensitive optical coherence tomography (PS-OCT) and its correlation with oral cancerous pathologies, a detailed comparison with histology provided via ex vivo fixed tissue is required. The effects of tissue fixation, however, on such polarization-based contrast have not yet been investigated. Aim: A study was performed to assess the impact of tissue fixation on depth-resolved (i.e., local) birefringence measured with PS-OCT. Approach: A PS-OCT system based on depth-encoded polarization multiplexing and polarization-diverse detection was used to measure the Jones matrix of a sample. A wide variety of ex vivo samples were measured freshly after excision and 24 h after fixation, consistent with standard pathology. Some samples were also measured 48 h after fixation. Results: The tissue fixation does not diminish the birefringence contrast. Statistically significant changes were observed in 11 out of 12 samples; these changes represented an increase in contrast, overall, by 11% on average. Conclusions: We conclude that the fixed samples are suitable for studies seeking a deeper understanding of birefringence contrast in oral tissue pathology. The enhancement of contrast removes the need to image immediately postexcision and will facilitate future investigations with PS-OCT and other advanced polarization-sensitive microscopy methods, such as mapping of the local optic axis with PS-OCT and PS-optical coherence microscopy.
We demonstrate superresolved, wide-field, reflectance imaging using synthetic-aperture Fourier holography. By applying a correlation scheme applicable to scattering samples, we show coherent synthesis of large numbers of holograms into reflectance images of, for the first time, tissue sections.
The real-time dispersion compensation in scanning interferometry was discussed. The static grating tilt in a scanning frequency-domain optical delay line was shown to produce dispersion that was linearly proportional to scan position. It was found that this device has application in scanning interferometry in general and in optical coherence tomography (OCT) in particular.
Angular diversity is a successful speckle-reduction technique in optical coherence tomography (OCT). We employ angle-dependent detection for a different purpose: to distinguish the singly backscattered and multiply scattered signal components. Single backscattering is highly correlated over a large range of detection angles; multiple scattering rapidly decorrelates as the angle is varied. Theoretical justification is provided using a linear-systems description of the OCT imaging process; detection of multiple scattering is corroborated experimentally. Â
After more than a decade of research, optical coherence tomography is in the early phases of establishing a niche as a medical imaging technology for routine clinical use. Far from tailing off, however, research activity is, if anything, on the increase. In this paper, we briefly and selectively review the current state-of-the-art in the more prominent areas of activity in the technology and its application. One is inevitably drawn to the conclusion that optical coherence tomography has much more to offer clinical practice than has yet been transferred.
We examine the effects of dispersion and absorption in ultrahigh-resolution optical coherence tomography (OCT), particularly the necessity to compensate for high dispersion orders in order to narrow the axial point-spread function envelope. We present a numerical expansion in which the impact of the various dispersion orders is quantified; absorption effects are evaluated numerically. Assuming a Gaussian source spectrum (in the optical frequency domain), we focus on imaging through water as a first approximation to biological materials. Both dispersion and absorption are found to be most significant for wavelengths above ~1µm, so that optimizing the system effective resolution (ER) requires choosing an operating wavelength below this limit. As an example, for 1-µm source resolution (FWHM), and propagation through a 1-mm water cell, if up to third-order dispersion compensation is applied, then the optimal center wavelength is 0.8µm, which generates an ER of 1.5µm (in air). The incorporation of additional bandwidth yields no ER improvement, due to uncompensated high-order dispersion and long-wavelength absorption.
Realistic simulation of image formation in optical coherence tomography, based on Maxwell’s equations, has recently been demonstrated for sample volumes of practical significance. Yet, there remains a limitation whereby reducing the size of cells used to construct a computational grid, thus allowing for more realistic representation of scatterer microstructure, necessarily reduces the overall sample size that can be modelled. This is a significant problem since, as is well known, the microstructure of a scatterer significantly influences its scattering properties. Here we demonstrate that optimized scatterer design can overcome this problem resulting in good agreement between simulated and experimental images for a structured phantom. This approach to OCT image simulation allows for image formation for biological tissues to be simulated with unprecedented realism.
We describe tomographic imaging of the refractive index of turbid media based on optical coherence tomography (OCT). We describe a variant OCT technique, bifocal optical coherence refractometry (BOCR), in which the optical pathlength difference between two foci simultaneously present in a medium of interest is measured. This technique is potentially suitable for dynamic measurements of the refractive index of biological tissues. We describe different schemes for realization of BOCR including one based on an adaptive liquid-crystal lens. We present experimental results from a range of tissue phantoms and from human skin in vivo that demonstrate the unique possibilities of BOCR for refractive index tomography, including its intrinsic immunity to motion artefacts and suitability for dynamic measurements.
We describe and experimentally demonstrate a novel (to our knowledge) surface profiling technique, for which we propose the term closed-loop optical coherence topography. This technique is a scanning beam, servo-locked variation of low-coherence interferometry. It allows for the sub-wavelengthresolution tracking of a weakly scattering macroscopic-scale surface, with the surface profile being directly output by the controlling electronics. The absence of significant real-time computational overhead makes the technique well suited to high-speed tracking. The use of a micrometer-scale coherence gate efficiently suppresses signals arising from structures not associated with the surface. These features make the technique particularly well suited to real-time surface profiling of in vivo, macroscopic biological surfaces.
We investigate the application of optical clearing agents to improve the image contrast in two-photon microscopy of human dermis. Results obtained with glycerol, propylene glycol and glucose in aqueous solution are presented.
The here presented work describes a surface profiling technique,for which the term closed-loop oprical coherence topography (CLOCT) was proposed . This technique i s a scanning beam, servo-locked Variation of lowcoherence interferometry. It allowsfor the sub-wavelengthresolution tracking of a weakly scattering macroscopicscale surface with the absence ofsignificant real-time computational overhead and is thus particularly well suited to real-time surface profiling of in vivo, macroscopic biological surfaces.
We investigate the application of hyperosmotic optical clearing agents to improve the image contrast and penetration depth in two-photon microscopy of human dermis ex vivo. We show that the agents glycerol, propylene glycol, and glucose all convey significant improvements and we provide results on their dynamic behaviour and the reversibility of the effect. At suitable concentrations, such agents have the potential to be compatible with living tissue and may possibly enhance in-vivo deep-tissue imaging.
Optical coherence tomography is a rapidly maturing optical imaging technology, enabling study of the in vivo structure of lung tissue at a scale of tens of micrometers. It has been used to assess the layered structure of airway walls, quantify both airway lumen caliber and compliance, and image individual alveoli. This article provides an overview of the technology and reviews its capability to provide new insights into respiratory disease.
We demonstrate tomographic imaging of the refractive index of turbid media using bifocal optical coherence refractometry (BOCR). The technique, which is a variant of optical coherence tomography, is based on the measurement of the optical pathlength difference between two foci simultaneously present in a medium of interest. We describe a new method to axially shift the bifocal optical pathlength that avoids the need to physically relocate the objective lens or the sample during an axial scan, and present an experimental realization based on an adaptive liquid-crystal lens. We present experimental results, including video clips, which demonstrate refractive index tomography of a range of turbid liquid phantoms, as well as of human skin in vivo.
Hull thickness is an important component of seed quality, which effects dehulling ability, feed or food nutritional aspects and cooking times. A breeding objective in Lupinus angustifolius crop improvement is to reduce hull thickness and a rapid screening method is needed to efficiently screen genotypes. Optical coherence tomography (OCT) imaging using infrared illumination at 980 nm was used to compare hull thickness of genotypes of four lupin species. OCT-derived hull layer thickness correlated highly with actual hull thickness determined by environmental scanning electron microscopy (r = 0.90) and allowed reliable distinction between mutant (thin-hulled) and parent genotypes of L. angustifolius. The imaging could clearly penetrate lupin seed to a depth of approximately 200 μm. The use of OCT to measure hull thickness has the advantage that it is rapid and non-destructive and should be very useful in selecting thin hull lines of lupins and other species on a single seed basis in germplasm or progeny from crosses.
In isolated airway smooth muscle (ASM) strips, an increase or decrease in ASM length away from its current optimum length causes an immediate reduction in force production followed by a gradual time-dependent recovery in force, a phenomenon termed length adaptation. In situ, length adaptation may be initiated by a change in transmural pressure (Ptm), which is a primary physiological determinant of ASM length. The present study sought to determine the effect of sustained changes in Ptm and therefore, ASM perimeter, on airway function. We measured contractile responses in whole porcine bronchial segments in vitro before and after a sustained inflation from a baseline Ptm of 5 cmH2O to 25 cmH2O, or deflation to −5 cmH2O, for ∼50 min in each case. In one group of airways, lumen narrowing and stiffening in response to electrical field stimulation (EFS) were assessed from volume and pressure signals using a servo-controlled syringe pump with pressure feedback. In a second group of airways, lumen narrowing and the perimeter of the ASM in situ were determined by anatomical optical coherence tomography. In a third group of airways, active tension was determined under isovolumic conditions. Both inflation and deflation reduced the contractile response to EFS. Sustained Ptm change resulted in a further decrease in contractile response, which returned to baseline levels upon return to the baseline Ptm. These findings reaffirm the importance of Ptm in regulating airway narrowing. However, they do not support a role for ASM length adaptation in situ under physiological levels of ASM lengthening and shortening.
We demonstrate the in vivo assessment of human scars by parametric imaging of birefringence using polarization-sensitive optical coherence tomography (PS-OCT). Such in vivo assessment is subject to artifacts in the detected birefringence caused by scattering from blood vessels. To reduce these artifacts, we preprocessed the PS-OCT data using a vascular masking technique. The birefringence of the remaining tissue regions was then automatically quantified. Results from the scars and contralateral or adjacent normal skin of 13 patients show a correspondence of birefringence with scar type: the ratio of birefringence of hypertrophic scars to corresponding normal skin is 2.2 Â± 0.2 (mean Â± standard deviation), while the ratio of birefringence of normotrophic scars to normal skin is 1.1 Â± 0.4. This method represents a new clinically applicable means for objective, quantitative human scar assessment.
We describe the use of Fourier holography for recording the spatially resolved complex angular scattering spectrum from scattering samples over wide fields of view in a single or few image captures. Without resolving individual scatterers, we are able to differentiate between spherical scatterers of different sizes in solutions containing mixtures by correspondence with Mie theory. In this paper, we describe the theory behind Fourier holographic light scattering angular spectroscopy and demonstrate its performance experimentally. Such methods represent potentially efficient alternatives to the time consuming and laborious conventional procedure of light microscopy, image tiling and inspection for the characterization of morphology over wide fields of view.
We report the synthesis, characterisation and evaluation of the in vitro biocompatibility of polymeric nanoparticles with both magnetic and upconverting fluorescent properties. The particles consist of superparamagnetic iron oxide nanoparticles and upconverting NaYF4:Yb,Er nanoparticles co-encapsulated within a poly(glycidyl methacrylate) sphere. Two different upconverting nanoparticles (10 nm Î±-NaYF4:Yb,Er and 50 nm Î²-NaYF4:Yb,Er) were synthesised and the optical and magnetic properties of the composite polymeric nanoparticle systems assessed by near infra-red laser spectroscopy, SQUID magnetometry and proton relaxometry. A live-dead assay was used to assess the viability of PC-12 neural cells incubated with varying concentrations of the nanoparticles. The composite nanoparticles produced no observed impact on cellular viability even at concentrations as high as 1000 Î¼g mL-1. Confocal microscopy revealed uptake of nanoparticles by PC-12 cells and peri-nuclear cytoplasmic localisation. Both particle systems show favourable magnetic properties. However, only the nanospheres containing 50 nm Î²-NaYF4:Yb,Er were suitable for optical tracking because the presence of iron oxide within the composites imparts a significant quenching of the upconversion emission. This study demonstrates the size and phase of the upconverting nanoparticles are important parameters that have to be taken into account in the design of multimodal nanoparticles using co-encapsulation strategies.
Fiber-optic probes for sensing and imaging applications often employ sections of graded-index (GRIN) fiber to refocus the diverging light from the delivery fiber. Such GRIN fiber microlenses possess aberrations which can cause significant distortions of the focused output beam. Using a numerical beam propagation method, we analyze the output beams resulting from index profiles that exhibit a central dip or a deviation from the ideal parabolic shape. Our method is in good agreement with experimental data and it enables the accurate simulation of fiber probes for biomedical applications using non-ideal or custom-engineered GRIN fibers with arbitrary refractive index profiles.
Optical coherence elastography (OCE) is a strain imaging technique that characterizes the elastic properties of tissues with microscopic resolution in three dimensions. In OCE, the displacement introduced to tissue by mechanical excitation is measured using optical coherence tomography. The local strain is calculated from the spatial derivative of displacement to generate strain images, known as elastograms. To validate elastograms, we compare them to a finite element analysis model of sample deformation. We also present preliminary OCE measurements performed on excised human breast tissue, and demonstrate discrimination of tissue types based on their elastic properties.
We show that polarization-sensitive optical coherence tomography angiography (PS-OCTA) based on full Jones matrix assessment of speckle decorrelation offers improved contrast and depth of vessel imaging over conventional OCTA. We determine how best to combine the individual Jones matrix elements and compare the resulting image quality to that of a conventional OCT scanner by co-locating and imaging the same skin locations with closely matched scanning setups. Vessel projection images from finger and forearm skin demonstrate the benefits of Jones matrix-based PS-OCTA. Our study provides a promising starting point and a useful reference for future pre-clinical and clinical applications of Jones matrix-based PS-OCTA.
To the best of our knowledge, we present the first needle probe for combined optical coherence tomography (OCT), and fluorescence imaging. The probe uses double-clad fiber (DCF) that guides the OCT signal and fluorescence excitation light in the core and collects and guides the returning fluorescence in the large-diameter multimode inner cladding. It is interfaced to a 1310 nm swept-source OCT system that has been modified to enable simultaneous 488 nm fluorescence excitation and >500 nm emission detection by using a DCF coupler to extract the returning fluorescence signal in the inner cladding with high efficiency. We present imaging results from an excised sheep lung with fluorescein solution infused through the vasculature. We were able to identify alveoli, bronchioles, and blood vessels. The results demonstrate that the combined OCT plus fluorescence needle images provide improved tissue differentiation over OCT alone.
We have developed an anatomical optical coherence tomography system for imaging the human upper airway in vivo. We describe an example of clinical research currently underway with this system; the ability to measure the change in airway dimensions due to anatomical position. Although the system imaging range is well-matched to a typical airway, we have observed a range of conditions which preclude the capture of full airway profiles in a small number of cases. Here, we demonstrate an improvement in the system range which allows us to successfully measure a larger percentage of subjects.
The development and deployment of OCT needle-probe technologies are reviewed. Their use through several different clinical applications, including demarcation of breast cancer tumor margins and lung imaging, is demonstrated.
We survey attempts to accurately diagnose skin cancer in vivo and in real time through the use of optics and automatic computation without the intervention of the clinician. Although no system has yet been shown to have such a capability, commercialization of a variety of diagnostic aids has taken place. We describe our own research into the diagnosis of malignant melanoma, the most lethal form of skin cancer, based on the spectroscopy of diffuse white light reflectance collected with a single multimode fiber after delivery with a multimode fiber bundle. Our clinical study, which collected 82 melanomas and 277 lesions in total, highlights the complexity of this challenging application.
We utilize Fourier-holographic light scattering angular spectroscopy to record the spatially resolved complex angular scattering spectra of samples over wide fields of view in a single or few image captures. Without resolving individual scatterers, we are able to generate spatially-resolved particle size maps for samples composed of spherical scatterers, by comparing generated spectra with Mie-theory predictions. We present a theoretical discussion of the fundamental principles of our technique and, in addition to the sphere samples, apply it experimentally to a biological sample which comprises red blood cells. Our method could possibly represent an efficient alternative to the time-consuming and laborious conventional procedure in light microscopy of image tiling and inspection, for the characterization of microscopic morphology over wide fields of view.
We present an acquisition method for optical coherence elastography (OCE) that enables acquisition of three-dimensional elastograms in 5 s, an order of magnitude faster than previously reported. In this method, based on compression elastography, the mechanical load applied to the sample is altered between acquisitions of consecutive optical coherence tomography volume scans (C-scans). The voxel-by-voxel phase difference between the volumes is used to determine the axial displacement and determining the gradient of the axial displacement versus depth gives the local axial strain. We demonstrate sub-100-microstrain sensitivity and high contrast in elastograms, acquired in 5 s, of structured phantoms and freshly excised rat muscle tissue that are comparable in strain sensitivity and dynamic range to our previously reported B-scan-based method. The much higher acquisition speed may expedite the translation of OCE to clinical and in vivo applications
Fiber-optic needle probes are highly miniaturized imaging devices that enable imaging deep inside the body. Utilizing optical coherence tomography (OCT), these devices replace the standard scanning mechanism of an OCT scanner with all-fiber focusing optics small enough to be encased within a hypodermic needle. We describe recent innovation in the design of these probes, including novel fiber-optic configurations to achieve extended depth of focus, and the use of double-clad fiber to enable the first dual-modality fiber-optic needle probe, simultaneously acquiring both OCT and fluorescence images.
Bessel beams feature a very large depth-of-focus (DOF) compared to conventional focusing schemes, but their central lobe carries only a small fraction of the total beam power, leading to a strongly reduced peak irradiance. This is problematic for power-limited applications, such as optical coherence tomography (OCT) or optical coherence microscopy, as it can result in a prohibitive reduction of the signal-to-noise ratio (SNR). Using scalar diffraction theory, we show that the trade-off between DOF and peak irradiance of Bessel beams depends solely on the Fresnel number N. We demonstrate the existence of a low-Fresnel-number regime,N < 10, in which axicons with Gaussian illumination can generate energy-efficient Bessel beams with a small number of sidelobes. In the context of OCT, this translates into DOF enhancements of up to 13X for a SNR penalty below 20 dB, which is confirmed by our experiments. We expect that these findings will enable improved performance of optical systems with extended DOF.
The speckle contrast ratio in optical coherence tomography images has been shown to depend on scatterer density when the detected signal is dominated by single backscattering. Here we investigate the influence of multiple scattering on the speckle contrast ratio, and also on the parallel and perpendicular polarization channels in polarization-sensitive optical coherence tomography images, including the correlation between them. Conditions under which the contrast ratio and polarization sensitive detection can be used to discriminate regions of OCT images affected by multiple scattering are discussed. The contrast ratio and the correlation between polarization channels were both found to markedly decrease as the ratio of multiple to single scattering increased. A high correlation between polarization channels, indicating that imaging is being performed in the single-scattering regime, provides greater confidence in interpreting the value of scatterer density obtained from the contrast ratio.
In this paper, an overview of author's research is presented, commencing at the University of Kent under Prof. David A. Jackson. Early research in short optical pulses and fiber-optic delay-line digital correlators led to optical communications research in code-division multiple access networking. This research was based on broadband incoherent light, and this theme continued with research into spectrum-sliced wavelength-division multiplexing. In shifting from photonics research to biomedical optics and biophotonics in the late 1990s, the emphasis on exploiting broadband light continued with research in optical coherence tomography, amongst other topics. In addition to the research outcomes, how these outcomes were attained is described, including mention of the exceptional contributions of many of my colleagues.
We present a high-optical-quality imaging needle for optical coherence tomography (OCT) that achieves sensitivity and resolution comparable to conventional free-space OCT sample arms. The side-viewing needle design utilizes total internal reflection from an angle-polished fiber tip, encased in a glass microcapillary. Fusion of the capillary to the fiber provides a robust, optical-quality output window. The needle's focusing optics are based on an astigmatism-free design, which exploits the "focal shift" phenomenon for focused Gaussian beams to achieve equal working distances (WDs) for both axes. We present a fabricated needle with a WD ratio of 0.98 for imaging in an aqueous environment. Our needle achieves the highest sensitivity of currently reported OCT imaging needles (112 dB), and we demonstrate its performance by superficial imaging of human skin and 3D volumetric imaging within a biological sample.
We show for what is the first time to our knowledge that digital Fourier holography can be used to record spatially resolved angular light scattering spectra from microscopically structured samples. This is achieved in one or a few digital image captures over large millimeter-scale fields of view. Such spectra are a sensitive measure of microscopic morphology, with wide applications in biological and medical imaging. We demonstrate good agreement between results of experiment and Mie theory for the angular scattering spectra of microspheres in water extracted from local regions within reconstructed 2 x 1 millimeter image sets.
We describe a study of the discrimination of early melanoma from common and dysplastic nevus using fiber optic diffuse reflectance spectroscopy. Diffuse reflectance spectra in the wavelength range 550 to 1000 nm are obtained using 400-µm core multimode fibers arranged in a six-illumination-around-one- collection geometry with a single fiber-fiber spacing of 470 µm. Spectra are collected at specific locations on 120 pigmented lesions selected by clinicians as possible melanoma, including 64 histopathologically diagnosed as melanoma. These locations are carried through to the histopathological diagnosis, permitting a spatially localized comparison with the corresponding spectrum. The variations in spectra between groups of lesions with different diagnoses are examined and reduced to features suitable for discriminant analysis. A classifier distinguishing between benign and malignant lesions performs with sensitivity/ specificity of between 64/69% and 72/78%. Classifiers between pairs of the group common nevus, dysplastic nevus, in situ melanoma, and invasive melanoma show better or similar performance than the benign/malignant classifier, and analysis provides evidence that different spectral features are needed for each pair of groups. This indicates that multiple discriminant systems are likely to be required to distinguish between melanoma and similar lesions.
We investigate the effects arising from the topical application of optical clearing agents (OCA) in two photon microscopy imaging of human ex-vivo dermis tissue. We demonstrate that hyperosmotic agents such glycerol, propylene glycol and glucose in aqueous solution are effective in improving penetration depth and enhancing image contrast. We examine the results provided by the three agents, as well as the dynamical behaviour of the clarifying effect. Our results show that propylene glycol and anhydrous glycerol are more effective than glucose in enhancing contrast, also if glucose diffuses three and live times faster, respectively. All results are in agrement with a diffusive model of the agent into the tissue. At suitable concentrations, such agents have the potential to be compatible with living tissue and may possibly enhance in-vivo deep-tissue imaging.
Miniaturised optical coherence tomography (OCT) fibre-optic probes have enabled high-resolution cross-sectional imaging deep within the body. However, existing OCT fibre-optic probe fabrication methods cannot generate miniaturised freeform optics, which limits our ability to fabricate probes with both complex optical function and dimensions comparable to the optical fibre diameter. Recently, major advances in two-photon direct laser writing have enabled 3D printing of arbitrary three-dimensional micro/nanostructures with a surface roughness acceptable for optical applications. Here, we demonstrate the feasibility of 3D printing of OCT probes. We evaluate the capability of this method based on a series of characterisation experiments. We report fabrication of a micro-optic containing an off-axis paraboloidal total internal reflecting surface, its integration as part of a common-path OCT probe, and demonstrate proof-of-principle imaging of biological samples.
Background: Measurements of upper airway size and shape are important in investigating the pathophysiology of obstructive sleep apnea (OSA) and in devising, applying, and determining the effectiveness of treatment modalities. We describe an endoscopic optical technique (anatomic optical coherence tomography, aOCT) that provides quantitative real-time imaging of the internal anatomy of the human upper airway. Methods: Validation studies were performed by comparing aOCT- and computed tomography (CT)-derived measurements of cross-sectional area (CSA) in (1) conduits in a wax phantom and (2) the velo-, oro-, and hypopharynx during wakefulness in five volunteers. aOCT scanning was performed during sleep in one subject with OSA. Results: aOCT generated images of pharyngeal shape and measurements of CSA and internal dimensions that were comparable to radiographic CT images. The mean difference between aOCT- and CT-derived measurements of CSA in (1) the wax phantom was 2.1 mm2 with limits of agreement (2 SD) from -13.2 to 17.4 mm2 and intraclass correlation coefficient of 0.99 (p < 0.001) and (2) the pharyngeal airway was 14.1 mm2 with limits of agreement from -43.7 to 57.8 mm 2 and intraclass correlation coefficient of 0.89 (p < 0.001). aOCT generated quantitative images of changes in upper airway size and shape before, during, and after an apneic event in an individual with OSA. Conclusions: aOCT generates quantitative, real-time measurements of upper airway size and shape with minimal invasiveness, allowing study over lengthy periods during both sleep and wakefulness. These features should make it useful for study of upper airway behavior to investigate OSA pathophysiology and aid clinical management.
We demonstrate imaging of soft tissue viscoelasticity using optical coherence elastography. Viscoelastic creep deformation is induced in tissue using step-like compressive loading and the resulting time-varying deformation is measured using phase-sensitive optical coherence tomography. From a series of co-located B-scans, we estimate the local strain rate as a function of time, and parameterize it using a four-parameter Kelvin-Voigt model of viscoelastic creep. The estimated viscoelastic strain and time constant are used to visualize viscoelastic creep in 2D, dual-parameter viscoelastograms. We demonstrate our technique on six silicone tissue-simulating phantoms spanning a range of viscoelastic parameters. As an example in soft tissue, we report viscoelastic contrast between muscle and connective tissue in fresh, ex vivo rat gastrocnemius muscle and mouse abdominal transection. Imaging viscoelastic creep deformation has the potential to provide complementary contrast to existing imaging modalities, and may provide greater insight into disease pathology.
Molecular imaging using optical techniques provides insight into disease at the cellular level. In this paper, we report on a novel dualmodality probe capable of performing molecular imaging by combining simultaneous three-dimensional optical coherence tomography (OCT) and two-dimensional fluorescence imaging in a hypodermic needle. The probe, referred to as a molecular imaging (MI) needle, may be inserted tens of millimeters into tissue. The MI needle utilizes double-clad fiber to carry both imaging modalities, and is interfaced to a 1310-nm OCT system and a fluorescence imaging subsystem using an asymmetrical double-clad fiber coupler customized to achieve high fluorescence collection efficiency. We present, to the best of our knowledge, the first dual-modality OCT and fluorescence needle probe with sufficient sensitivity to image fluorescently labeled antibodies. Such probes enable high-resolution molecular imaging deep within tissue.
An accurate intraoperative identification of malignant tissue is a challenge in the surgical management of breast cancer. Imaging techniques that help address this challenge could contribute to more complete and accurate tumor excision, and thereby help reduce the current high reexcision rates without resorting to the removal of excess healthy tissue. Optical coherence microelastography (OCME) is a three-dimensional, high-resolution imaging technique that is sensitive to microscale variations of the mechanical properties of tissue. As the tumor modifies the mechanical properties of breast tissue, OCME has the potential to identify, on the microscale, involved regions of fresh, unstained tissue. OCME is based on the use of optical coherence tomography (OCT) to measure tissue deformation in response to applied mechanical compression. In this feasibility study on 58 ex vivo samples from patients undergoing mastectomy or wide local excision, we demonstrate the performance of OCME as a means to visualize tissue microarchitecture in benign and malignant human breast tissues. Through a comparison with corresponding histology and OCT images, OCME is shown to enable ready visualization of features such as ducts, lobules, microcysts, blood vessels, and arterioles and to identify invasive tumor through distinctive patterns in OCME images, often with enhanced contrast compared with OCT. These results lay the foundation for future intraoperative studies.
We used combined simultaneous two-photon excitation fluorescence microscopy (TPE) and second harmonic generation microscopy (SHG) on human skin tissue slices. We studied the effect caused by topical application of optical clearing agents (OCAs). We demonstrated that hyperosmotic agents as glycerol, propylene glycol and glucose in aqueous solution, are all effective in improving excitation light penetration depth and in enhancing image contrast. The effect caused on acquired images by sample immersion in OCAs or in their aqueous dilution, was studied. We observed a similar clearing effect with TPE and SHG acquisitions, with different effectiveness and rising time for each agent. The TPE acquired data are in good agreement with a simple diffusion model developed. From the SHG acquisition some different behaviour was observed. All three agents are potentially bio-compatible and effective in reducing scattering, improving light penetration depth and image contrast. Use of OCA can be suitable for in vivo application in two-photon microscopy, as well as in other techniques performing optical biopsy of human skin tissue.
We present a new optical coherence tomography (OCT) angiography method for imaging tissue microvasculature in vivo based on the characteristic frequency-domain flow signature in a short time series of a single voxel. The angiography signal is generated by Fourier transforming the OCT signal time series from a given voxel in multiple acquisitions and computing the average magnitude of non-zero (high-pass) frequency components. Larger temporal variations of the OCT signal caused by blood flow result in higher values of the average magnitude in the frequency domain compared to those from static tissue. Weighting of the signal by the inverse of the zero-frequency component (i.e., the sum of the OCT signal time series) improves vessel contrast in flow regions of low OCT signal. The method is demonstrated on a fabricated flow phantom and on human skin in vivo and, at only 5 time points per voxel, shows enhanced vessel contrast in comparison to conventional correlation mapping/speckle decorrelation and speckle variance methods.
We present a graphics processing unit (GPU)-accelerated optical coherence elastography (OCE) system capable of generating strain images (elastograms) of soft tissue at near video-rates. The system implements phase-sensitive compression OCE using a pipeline of GPU kernel functions to enable a highly parallel implementation of OCE processing using the OpenCL framework. Developed on a commercial-grade GPU and desktop computer, the system achieves a processing rate of 21 elastograms per second at an image size of 960 X 400 pixels, enabling high-rate visualization during acquisition. The system is demonstrated on both tissue-simulating phantoms and fresh ex vivo mouse muscle. To the best of our knowledge, this is the first implementation of near video-rate OCE and the fastest reported OCE processing rate, enabling, for the first time, a system capable of computing and displaying OCE elastograms interactively during acquisition. This advance provides new opportunities for medical imaging of soft tissue stiffness using optical methods.
Light scattered by turbid tissue is known to degrade optical coherence tomography (OCT) image contrast progressively with depth. Bessel beams have been proposed as an alternative to Gaussian beams to image deeper into turbid tissue. However, studies of turbid tissue comparing the image quality for different beam types are lacking. We present such a study, using numerically simulated beams and experimental OCT images formed by Bessel or Gaussian beams illuminating phantoms with optical properties spanning a range typical of soft tissue. We demonstrate that, for a given scattering parameter, the higher the scattering anisotropy the lower the OCT contrast, regardless of the beam type. When focusing both beams at the same depth in the sample, we show that, at focus and for equal input power and resolution, imaging with the Gaussian beam suffers less reduction of contrast. This suggests that, whilst Bessel beams offer extended depth of field in a single depth scan, for low numerical aperture (NA 0.95), superior contrast (by up to ~40%) may be obtained over an extended depth range by a Gaussian beam combined with dynamic focusing.
OCE provides 3-D maps of tissue stiffness on the micrometer to millimeter scale, bridging a crucial gap in probing the mechanical properties of diseased tissues
We present theoretical calculations, based on a random phasor sum model, which show that the optical coherence tomography speckle contrast ratio is dependent on the local density of scattering particles in a sample, provided that the effective number of scatterers in the probed volume is less than about five. We confirm these theoretical predictions experimentally, using suspensions of microspheres in water. The observed contrast ratios vary in value from the Rayleigh limit of 0.52 to in excess of 2, suggesting that the contrast ratio could be useful in optical coherence tomography, particularly when imaging in ultrahigh-resolution regimes.
Synthetic multifunctional electrospun composites are a new class of hybrid materials with many potential applications. However, the lack of an efficient, reactive large-area substrate has been one of the major limitations in the development of these materials as advanced functional platforms. Herein, we demonstrate the utility of electrospun poly(glycidyl methacrylate) films as a highly versatile platform for the development of functional nanostructured materials anchored to a surface. The utility of this platform as a reactive substrate is demonstrated by grafting poly(N-isopropylacrylamide) to incorporate stimuli–responsive properties. Additionally, we demonstrate that functional nanocomposites can be fabricated using this platform with properties for sensing, fluorescence imaging, and magneto-responsiveness.
This PDF file contains the front matter associated with SPIE Proceedings Volume 9710, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.
A fibre-based full-range polarisation-sensitive optical coherence tomography system is developed to enable complete capture of the structural and birefringence properties of the anterior segment of the human eye in a single acquisition. The system uses a wavelength swept source centered at 1.3 μm, passively depth-encoded, orthogonal polarisation states in the illumination path and polarisation-diversity detection. Off-pivot galvanometer scanning is used to extend the imaging range and compensate for sensitivity drop-off. A Mueller matrix-based method is used to analyse data. We demonstrate the performance of the system and discuss issues relating to its optimisation.
The transmission of a spectrum-sliced WDM channel at 622 Mbit/s over 60 km of nondispersion-shifted fibre using an optical bandwidth of only 0.23 nm is reported. This is the highest single channel bit rate-length product (40Gbit/s-km) and smallest channel bandwidth reported to date for spectrum-sliced WDM systems. The bit error rate performance is theoretically predicted and experimentally confirmed and limits on the bit rate-length products of spectrum-sliced WDM channels using nondispersion-shifted fibre in the 1550nm window are given.
Elastography images can be difficult to interpret for an untrained observer. We introduce a new method of facilitating the interpretation of optical coherence elastography, by measuring mechanical heterogeneity, and mapping it onto an image.
Polarization-sensitive OCT (PS-OCT) has proven useful in determining the stress-induced birefringence of non-biological materials, but such utility in biological tissues subjected to stress has not been well studied yet. To study stress-induced birefringence of biological tissues, we use a swept-source PS-OCT system with passively depth-encoded, orthogonal polarization states in the illumination path and polarization-diversity detection and a Mueller formalism in post-processing. We present measurements of stress-induced changes in the birefringence of non-biological and biological samples that provide useful benchmarks in further assessing the utility of this approach.
A > 100mW source of broadband ASE with exceptionally flat spectral density and 22nm bandwidth is presented. The source is ideally suited for spectrum-sliced WDM systems. Data transmission at 622Mbit/s using a spectraily-sliced channel tuned over a 15nm range is demonstrated.
We present optical coherence micro-elastography, an improved form of compression optical coherence elastography. We demonstrate the capacity of this technique to produce en face images, closely corresponding with histology, that reveal micro-scale mechanical contrast in human breast and lymph node tissues. We use phase-sensitive, three-dimensional optical coherence tomography (OCT) to probe the nanometer-to-micrometer-scale axial displacements in tissues induced by compressive loading. Optical coherence micro- elastography incorporates common-path interferometry, weighted averaging of the complex OCT signal and weighted least-squares regression. Using three-dimensional phase unwrapping, we have increased the maximum detectable strain eleven-fold over no unwrapping and the minimum detectable strain is 2.6 Î¼Îµ. We demonstrate the potential of mechanical over optical contrast for visualizing micro-scale tissue structures in human breast cancer pathology and lymph node morphology.
We present optical palpation, a tactile imaging technique for mapping micrometer- to millimeter-scale mechanical variations in soft tissue. In optical palpation, a stress sensor consisting of translucent, compliant silicone with known stress-strain behavior is placed on the tissue surface and a compressive load is applied. Optical coherence tomography (OCT) is used to measure the local strain in the sensor, from which the local stress at the sample surface is calculated and mapped onto an image. We present results in tissue-mimicking phantoms, demonstrating the detection of a feature embedded 4.7 mm below the sample surface, well beyond the depth range of OCT. We demonstrate the use of optical palpation to delineate the boundary of a region of tumor in freshly excised human breast tissue, validated against histopathology.
The capacity of optical coherence tomography to characterize biological tissues can be augmented by extensions to detect motion (blood and lymph flow), response to load (stiffness) and birefringence (stress and sub-structure). This talk will review these extensions and describe example applications in cancer, the eye and skin.
This PDF file contains the front matter associated with SPIE Proceedings Volume 10340, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.
We present an optofluidic optical coherence tomography (OCT) needle probe capable of modifying the local optical properties of tissue to improve needle-probe imaging performance. The side-viewing probe comprises an all-fiberoptic design encased in a hypodermic needle (outer diameter 720 Î¼m) and integrates a coaxial fluid-filled channel, terminated by an outlet adjacent to the imaging window, allowing focal injection of fluid to a target tissue. This is the first fully integrated OCT needle probe design to incorporate fluid injection into the imaging mechanism. The utility of this probe is demonstrated in air-filled sheep lungs, where injection of small quantities of saline is shown, by local refractive index matching, to greatly improve image penetration through multiple layers of alveoli. 3D OCT images are correlated against histology, showing improvement in the capability to image lung structures such as bronchioles and blood vessels.
Label-free imaging of the blood and lymphatic vessel networks of the conjunctiva of the eye is important in assessing the drainage pathways affected by glaucoma. We utilize the characteristically low signal in optical coherence tomography (OCT) provided by such vessels in ex vivo tissue to characterize their morphology in two and three dimensions. We demonstrate this method on conjunctiva from six porcine eyes, showing the ready visualization of both vessel networks. Such ex vivo characterization is a necessary precursor for future in vivo studies directed towards improving glaucoma surgery.
The pseudospectral time-domain (PSTD) method greatly extends the physical volume of biological tissue in which light scattering can be calculated, relative to the finite-difference time-domain (FDTD) method. We have developed an analogue of the total-field scattered-field source condition, as employed in FDTD, for introducing focussed illuminations into PSTD simulations. This new source condition requires knowledge of the incident field, and applies update equations, at a single plane in the PSTD grid. Numerical artifacts, usually associated with compact PSTD source conditions, are minimized by using a staggered grid. This source condition's similarity with that used by the FDTD suggests a way in which existing FDTD codes can be easily adapted to PSTD codes.
In many muscle pathologies, impairment of skeletal muscle function is closely linked to changes in the mechanical properties of the muscle constituents. Optical coherence micro-elastography (OCME) uses optical coherence tomography (OCT) imaging of tissue under a quasi-static, compressive mechanical load to map variations in tissue mechanical properties on the micro-scale. We present the first study of OCME on skeletal muscle tissue. We show that this technique can resolve features of muscle tissue including fibers, fascicles and tendon, and can also detect necrotic lesions in skeletal muscle from the mdx mouse model of Duchenne muscular dystrophy. In many instances, OCME provides better or additional contrast complementary to that provided by OCT. These results suggest that OCME could provide new understanding and opportunity for assessment of skeletal muscle pathologies.
It is widely accepted that accurate mechanical properties of three-dimensional soft tissues and cellular samples are not available on the microscale. Current methods based on optical coherence elastography can measure displacements at the necessary resolution, and over the volumes required for this task. However, in converting this data to maps of elastic properties, they often impose assumptions regarding homogeneity in stress or elastic properties that are violated in most realistic scenarios. Here, we introduce novel, rigorous, and computationally efficient inverse problem techniques that do not make these assumptions, to realize quantitative volumetric elasticity imaging on the microscale. Specifically, we iteratively solve the three-dimensional elasticity inverse problem using displacement maps obtained from compression optical coherence elastography. This is made computationally feasible with adaptive mesh refinement and domain decomposition methods. By employing a transparent, compliant surface layer with known shear modulus as a reference for the measurement, absolute shear modulus values are produced within a millimeter-scale sample volume. We demonstrate the method on phantoms, on a breast cancer sample ex vivo, and on human skin in vivo. Quantitative elastography on this length scale will find wide application in cell biology, tissue engineering and medicine.
We propose a method to image cutaneous lymphatic vessels in vivo with optical coherence tomography (OCT). Our method segments the transparent lymphatic vessels by thresholding the OCT signal after calibration for systematic variations due to the confocal effect and the sensitivity drop-off, and subsequent compensation for light attenuation with depth. We describe the method and present a pilot demonstration on two human burn scar patients undergoing ablative laser fractionation treatment. The results show visualization of the lymphatic vessels (diameter: ~30-150 μm) separate from the blood microvasculature network.
The scattering contrast probed by optical coherence tomography is augmented by extensions to motion detection (imaging blood/lymph vessels and tissue mechanics), and to birefringence measurement (imaging tissue stress and sub-structure), with wide application including in cancer and burn treatments.