Complex differential variance algorithm for optical coherence tomography angiography

Imaging peritoneal blood vessels through optical coherence tomography angiography for laparoscopic surgery

Laparoscopic surgery presents challenges in identifying blood vessels due to lack of tactile feedback. The image-guided laparoscopic surgical tool (IGLaST) integrated with optical coherence tomography (OCT) has potential for in vivo blood vessel imaging; however, distinguishing vessels from surrounding tissue remains a challenge. In this study, we propose utilizing an inter-A-line intensity differentiation-based OCT angiography (OCTA) to improve visualization of blood vessels. By evaluating a tissue phantom with varying flow speeds, we optimized the system's blood flow imaging capabilities in terms of minimum detectable flow and contrast-to-noise ratio. In vivo experiments on rat and porcine models, successfully visualized previously unidentified blood vessels and concealed blood flows beneath the 1 mm depth peritoneum. Qualitative comparison of various OCTA algorithms indicated that the intensity differentiation-based algorithm performed best for our application. We believe that implementing IGLaST with OCTA can enhance surgical outcomes and reduce procedure time in laparoscopic surgeries.

Improved in vivo optical coherence tomography imaging of animal peripheral nerves using a prism nerve holder

Significance

Modern optical volumetric imaging modalities, such as optical coherence tomography (OCT), provide enormous information about the structure, function, and physiology of living tissue. Although optical imaging achieves lateral resolution on the order of the wavelength of light used, and OCT achieves axial resolution on a similar micron scale, tissue optical properties, particularly high scattering and absorption, limit light penetration to only a few millimeters. In addition, in vivo imaging modalities are susceptible to significant motion artifacts due to cardiac and respiratory function. These effects limit access to artifact-free optical measurements during peripheral neurosurgery to only a portion of the exposed nerve without further modification to the procedure.

Aim

We aim to improve in vivo OCT imaging during peripheral neurosurgery in small and large animals by increasing the amount of visualized nerve volume as well as suppressing motion of the imaged area.

Approach

We designed a nerve holder with embedded mirror prisms for peripheral nerve volumetric imaging as well as a specific beam steering strategy to acquire prism and direct view volumes in one session with minimal motion artifacts.

Results

The axially imaged volumes from mirror prisms increased the OCT signal intensity by > 22    dB over a 1.25-mm imaging depth in tissue-mimicking phantoms. We then demonstrated the new imaging capabilities in visualizing peripheral nerves from direct and side views in living rats and minipigs using a polarization-sensitive OCT system. Prism views have shown nerve fascicles and vasculature from the bottom half of the imaged nerve which was not visible in direct view.

Conclusions

We demonstrated improved OCT imaging during neurosurgery in small and large animals by combining the use of a prism nerve holder with a specifically designed beam scanning protocol. Our strategy can be applied to existing OCT imaging systems with minimal hardware modification, increasing the nerve tissue volume visualized. Enhanced imaging depth techniques may lead to a greater adoption of structural and functional optical biomarkers in preclinical and clinical medicine.

Longitudinal monitoring of pancreatic islet damage in streptozotocin-treated mice with optical coherence microscopy

Pancreatic islets regulate glucose homeostasis in the body, and their dysfunction is closely related to diabetes. Islet transplantation into the anterior chamber of the eye (ACE) was recently developed for both in vivo islet study and diabetes treatment. Optical coherence microscopy (OCM) was previously used to monitor ACE transplanted islets in non-obese diabetic (NOD) mice for detecting autoimmune attack. In this study, OCM was applied to streptozotocin (STZ)-induced diabetic mouse models for the early detection of islet damage. A custom extended-focus OCM (xfOCM) was used to image islet grafts in the ACE longitudinally during STZ-induced beta cell destruction together with conventional bright-field (BF) imaging and invasive glucose level measurement. xfOCM detected local structural changes and vascular degradation during the islet damage which was confirmed by confocal imaging of extracted islet grafts. xfOCM detection of islet damage was more sensitive than BF imaging and glucose measurement. Longitudinal xfOCM images of islet grafts were quantitatively analyzed. All these results showed that xfOCM could be used as a non-invasive and sensitive monitoring method for the early detection of deficient islet grafts in the ACE with potential applications to human subjects.

Local vasoregulative interventions impact drug concentrations in the skin after topical laser-assisted delivery

INTRODUCTION

The ability of ablative fractional lasers (AFL) to enhance topical drug uptake is well established. After AFL delivery, however, drug clearance by local vasculature is poorly understood. Modifications in vascular clearance may enhance AFL-assisted drug concentrations and prolong drug dwell time in the skin. Aiming to assess the role and modifiability of vascular clearance after AFL-assisted delivery, this study examined the impact of vasoregulative interventions on AFL-assisted 5-fluorouracil (5-FU) concentrations in in vivo skin.

METHODS

5-FU uptake was assessed in intact and AFL-exposed skin in a live pig model. After fractional CO₂ laser exposure (15 mJ/microbeam, 5% density), vasoregulative intervention using topical brimonidine cream, epinephrine solution, or pulsed dye laser (PDL) was performed in designated treatment areas, followed by a single 5% 5-FU cream application. At 0, 1, 4, 48, and 72 h, 5-FU concentrations were measured in 500 and 1500 μm skin layers by mass spectrometry (n = 6). A supplemental assessment of blood flow following AFL ± vasoregulation was performed using optical coherence tomography (OCT) in a human volunteer.

RESULTS

Compared to intact skin, AFL facilitated a prompt peak in 5-FU delivery that remained elevated up to 4 hours (1500 μm: 1.5 vs. 31.8 ng/ml [1 hour, p = 0.002]; 5.3 vs. 14.5 ng/ml [4 hours, p = 0.039]). However, AFL's impact was transient, with 5-FU concentrations comparable to intact skin at later time points. Overall, vasoregulative intervention with brimonidine or PDL led to significantly higher peak 5-FU concentrations, prolonging the drug's dwell time in the skin versus AFL delivery alone. As such, brimonidine and PDL led to twofold higher 5-FU concentrations than AFL alone in both skin layers by 1 hour (e.g., 500 μm: 107 ng/ml [brimonidine]; 96.9 ng/ml [PDL], 46.6 ng/ml [AFL alone], p ≤ 0.024), and remained significantly elevated at 4 hours (p ≤ 0.024). A similar pattern was observed for epinephrine, although trends remained nonsignificant (p ≥ 0.09). Prolonged 5-FU delivery was provided by PDL, resulting in sustained drug deposition compared to AFL alone at both 48 and 72 hours in the superficial skin layer (p ≤ 0.024). Supporting drug delivery findings, OCT revealed that increases in local blood flow after AFL were mitigated in test areas also exposed to PDL, brimonidine, or epinephrine, with PDL providing the greatest, sustained reduction in flow over 48 hours.

CONCLUSION

Vasoregulative intervention in conjunction with AFL-assisted delivery enhances and prolongs 5-FU deposition in in vivo skin.

Extending field-of-view of retinal imaging by optical coherence tomography using convolutional Lissajous and slow scan patterns

Optical coherence tomography (OCT) is a high-speed non-invasive cross-sectional imaging technique. Although its imaging speed is high, three-dimensional high-spatial-sampling-density imaging of in vivo tissues with a wide field-of-view (FOV) is challenging. We employed convolved Lissajous and slow circular scanning patterns to extend the FOV of retinal OCT imaging with a 1-µm, 100-kHz-sweep-rate swept-source OCT prototype system. Displacements of sampling points due to eye movements are corrected by post-processing based on a Lissajous scan. Wide FOV three-dimensional retinal imaging with high sampling density and motion correction is achieved. Three-dimensional structures obtained using repeated imaging sessions of a healthy volunteer and two patients showed good agreement. The demonstrated technique will extend the FOV of simple point-scanning OCT, such as commercial ophthalmic OCT devices, without sacrificing sampling density.

Comparative study of OCTA algorithms with a high-sensitivity multi-contrast Jones matrix OCT system for human skin imaging

The multi-contrast Jones matrix OCT (JMT) system can not only improve the tissue-specific contrast but also enhance the sensitivity of detecting flow, which is gaining increasing attention. However, for the JMT system, there is currently a lack of studies evaluating and guiding the selection of suitable angiography algorithms to map the most appealing quality of angiograms for clinical use. In this paper, by a homemade high-sensitivity multi-contrast JMT system based 200 kHz swept source, the performance of two complex-signal-based OCTA methods that are insensitive to phase instability and one amplitude-signal-based OCTA method are compared for in-vivo imaging of human skin qualitatively and quantitatively. Six metrics, including vascular connectivity, image contrast-to-noise ratio, image signal-to-noise ratio, vessel diameter index, blood vessel density, and processing time, are assessed. The results show that the vascular networks processed by all OCTA methods and the texture of skin could be visualized simultaneously and markedly. Additionally, the complex-signal-based OCTA methods successfully suppress phase instabilities and even outperform the amplitude-signal-based OCTA algorithm in some indicators. This paper has a certain guiding significance for selecting an appropriate angiography algorithm and expanding the application field with this system.

Direct Visualization and Quantitative Imaging of Small Airway Anatomy In Vivo Using Deep Learning Assisted Diffractive OCT

OBJECTIVE/BACKGROUND

In vivo imaging and quantification of the microstructures of small airways in three dimensions (3D) allows a better understanding and management of airway diseases, such as asthma and chronic obstructive pulmonary disease (COPD). At present, the resolution and contrast of the currently available conventional optical coherence tomography (OCT) imaging technologies operating at 1300 nm remain challenging to directly visualize the fine microstructures of small airways in vivo.

METHODS

We developed an ultrahigh-resolution diffractive endoscopic OCT at 800 nm to afford a resolving power of 1.7 µm (in tissue) with an improved contrast and a custom deep residual learning based image segmentation framework to perform accurate and automated 3D quantification of airway anatomy.

RESULTS

The 800-nm diffractive OCT enabled the direct delineation of the structural components in the small airway wall in vivo. We further first demonstrated the 3D anatomic quantification of critical tissue compartments of small airways in sheep using the automated segmentation method.

CONCLUSION

The deep learning assisted diffractive OCT provides a unique ability to access the small airways, directly visualize and quantify the important tissue compartments, such as airway smooth muscle, in the airway wall in vivo in 3D.

SIGNIFICANCE

These pilot results suggest a potential technology for calculating volumetric measurements of small airways in patients in vivo.

Numerical calibration method for a multiple spectrometer-based OCT system

The present paper introduces a numerical calibration method for the easy and practical implementation of multiple spectrometer-based spectral-domain optical coherence tomography (SD-OCT) systems. To address the limitations of the traditional hardware-based spectrometer alignment across more than one spectrometer, we applied a numerical spectral calibration algorithm where the pixels corresponding to the same wavelength in each unit are identified through spatial- and frequency-domain interferometric signatures of a mirror sample. The utility of dual spectrometer-based SD-OCT imaging is demonstrated through in vivo retinal imaging at two different operation modes with high-speed and dual balanced acquisitions, respectively, in which the spectral alignment is critical to achieve improved retinal image data without any artifacts caused by misalignment of the spectrometers.

Multimodal evaluation of an interphotoreceptor retinoid-binding protein-induced mouse model of experimental autoimmune uveitis

We aimed to characterize the vascular phenotypes of an experimental autoimmune retinal uveitis (EAU) model induced by interphotoreceptor retinoid-binding protein (IRBP) using multimodal imaging techniques. We systemically administered IRBP or vehicle to adult C57BL/6 mice. Fundus photography, optical coherence tomography (OCT), in vivo live confocal imaging using different tracers, OCT angiography (OCTA), and electroretinography (ERG) were performed after IRBP immunization. Hematoxylin and eosin and immunofluorescence staining were performed to characterize the immune response and vascular permeability. Mice with EAU exhibited perivascular inflammation, vitritis, and superficial retinal inflammation on fundus photography and OCT. H&E revealed immune cell infiltration in the perivascular area of the retina and choroid accompanied by a significant degree of perivasculitis that subsequently damaged photoreceptors 3 weeks postimmunization. Immunofluorescence staining showed subsequent transcytosis induction after local microglial activation followed by neutrophil recruitment in the perivascular area. Transcytosis in the superficial and deep vascular areas was improved by immune cell suppression. Intravital in vivo confocal imaging showed signs of neutrophil infiltration and obstructive vasculitis with perivascular leakage 3 weeks postimmunization. OCTA revealed a significant decrease in vascular flow in the deep capillary layer of the retina. Functional analysis showed that scotopic responses were intact at 2 weeks; however, normal photopic and scotopic responses were hardly detected in mice with EAU mice at 3 weeks postimmunization. Our data suggest that inflammatory cell activation and subsequent transcytosis induction in endothelial cells might be a major pathogenic factor for vascular leakage in uveitis, providing new insights into the pathophysiology of retinal vasculitis in noninfectious uveitis.

Studying a mouse model of autoimmune uveitis, a damaging form of eye inflammation affecting the retina and choroid of the eye, reveals new cellular and molecular details of how blood vessel inflammation can damage the retina. Researchers in South Korea and Japan led by Joo Yong Lee at the University of Ulsan, Seoul, initiated autoimmune uveitis in mice by administering retinoid-binding protein, which is known to stimulate autoimmune changes which model aspects of the human disease. Their work revealed that the inflammation caused by the autoimmune response makes the blood vessels supplying the retina more permeable to a variety of large molecules. This increased permeability, due to a membrane transport process called transcytosis, was preceded by specific cellular changes. This deeper understanding of the pathology of uveitis could help research towards new treatments.

Robust and easy-to-operate stretched-pulse mode-locked wavelength-swept laser with an all-polarization-maintaining fiber cavity for 10 MHz A-line rate optical coherence tomography

We demonstrate robust and easy-to-operate stretched-pulse mode-locked laser (SPML) architectures using all-polarization-maintaining fiber laser cavities. Because of the polarization-maintaining construction, the laser performance is unaffected by mechanical perturbation on the cavity fibers. The lasers automatically initiate linear-in-wavenumber sweeps across 100 nm centered at 1290 nm with a 10 MHz repetition rate. OCT imaging with a sensitivity of 98 dB and a single-sided 6 dB coherence length of 2.5 mm is demonstrated. OCT angiography of a mouse brain that visualized three-dimensional cerebral microvasculature over a field of 1.5mm×1.5mm (398 A-lines × 380 B-scans) at a rate of 5.26 volumes per second is also presented. The robust all-PMF SPML lasers are a turnkey, high-performance source for ultrahigh-speed OCT imaging.

Phase-stabilized complex-decorrelation angiography

In this study, we developed a novel phase-stabilized complex-decorrelation (PSCD) optical coherence tomography (OCT) angiography (OCTA) method that can generate high quality OCTA images. This method has been validated using three different types of OCT systems and compared with conventional complex- and amplitude-based OCTA algorithms. Our results suggest that in combination with a pre-processing phase stabilization method, the PSCD method is insensitive to bulk motion phase shifts, less dependent on OCT reflectance than conventional complex methods and demonstrates extended dynamic range of flow signal, in contrast to other two methods.

Advances in multimodal imaging in ophthalmology

Multimodality ophthalmic imaging systems aim to enhance the contrast, resolution, and functionality of existing technologies to improve disease diagnostics and therapeutic guidance. These systems include advanced acquisition and post-processing methods using optical coherence tomography (OCT), combined scanning laser ophthalmoscopy and OCT systems, adaptive optics, surgical guidance, and photoacoustic technologies. Here, we provide an overview of these ophthalmic imaging systems and their clinical and basic science applications.

Multi-beam OCT imaging based on an integrated, free-space interferometer

While it is a common practice to increase the speed of swept-source optical coherence tomography (OCT) systems by using a high-speed source, this approach may not always be optimal. Parallelization in the form of multiple imaging beams is an alternative approach, but scalable and low-loss multi-beam OCT architectures are needed to capitalize on its advantages. In this study, we demonstrate an eight-beam OCT system using an interferometer architecture comprising planar lightwave circuits (PLC) splitters, V-groove assemblies (VGA), and optical ribbon fibers. We achieved an excess loss and heterodyne efficiency on each channel that was close to that of single-beam systems. In vivo structural imaging of a human finger and OCT angiography imaging of a mouse ear was performed to demonstrate the imaging performance of the system. This work provides further evidence supporting multi-beam architectures as a viable strategy for increasing OCT imaging speed.

Visualization of two-dimensional transverse blood flow direction using optical coherence tomography angiography

SIGNIFICANCE

Evaluation of vessel patency and blood flow direction is important in various medical situations, including diagnosis and monitoring of ischemic diseases, and image-guided vascular surgeries. While optical coherence tomography angiography (OCTA) is the most widely used functional extension of optical coherence tomography that visualizes three-dimensional vasculature, inability to provide information of blood flow direction is one of its limitations.

AIM

We demonstrate two-dimensional (2D) transverse blood flow direction imaging in en face OCTA.

APPROACH

A series of triangular beam scans for the fast axis was implemented in the horizontal direction for the first volume scan and in the vertical direction for the following volume scan, and the inter A-line OCTA was performed for the blood flow direction imaging while the stepwise pattern was used for each slow axis scan. The decorrelation differences between the forward and the backward inter A-line OCTA were calculated for the horizontal and the vertical fast axis scans, and the ratio of the horizontal and the vertical decorrelation differences was utilized to show the 2D transverse flow direction information.

RESULTS

OCTA flow direction imaging was verified using flow phantoms with various flow orientations and speeds, and we identified the flow speed range relative to the scan speed for reliable flow direction measurement. We demonstrated the visualization of 2D transverse blood flow orientations in mouse brain vascular networks in vivo.

CONCLUSIONS

The proposed OCTA imaging technique that provides information of 2D transverse flow direction can be utilized in various clinical applications and preclinical studies.

Jones matrix-based speckle-decorrelation angiography using polarization-sensitive optical coherence tomography

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.

Detection of localized pulsatile motion in cutaneous microcirculation by speckle decorrelation optical coherence tomography angiography

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 noncontact/contact scanning protocols for the visualization of pulsatility patterns in vessel-free tissue and in the microvasculature of peripheral human skin.

RESULTS

Results from five healthy subjects show distinct pulsatile patterns both in vessel-free tissue with either noncontact or contact imaging and in individual microvessels with contact imaging. Respectively, these patterns are 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. Noncontact 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.

Approaches to quantify optical coherence tomography angiography metrics

Optical coherence tomography (OCT) has revolutionized the field of ophthalmology in the last three decades. As an OCT extension, OCT angiography (OCTA) utilizes a fast OCT system to detect motion contrast in ocular tissue and provides a three-dimensional representation of the ocular vasculature in a non-invasive, dye-free manner. The first OCT machine equipped with OCTA function was approved by U.S. Food and Drug Administration in 2016 and now it is widely applied in clinics. To date, numerous methods have been developed to aid OCTA interpretation and quantification. In this review, we focused on the workflow of OCTA-based interpretation, beginning from the generation of the OCTA images using signal decorrelation, which we divided into intensity-based, phase-based and phasor-based methods. We further discussed methods used to address image artifacts that are commonly observed in clinical settings, to the algorithms for image enhancement, binarization, and OCTA metrics extraction. We believe a better grasp of these technical aspects of OCTA will enhance the understanding of the technology and its potential application in disease diagnosis and management. Moreover, future studies will also explore the use of ocular OCTA as a window to link ocular vasculature to the function of other organs such as the kidney and brain.

Cross-scanning optical coherence tomography angiography for eye motion correction

We propose a cross-scanning optical coherence tomography (CS-OCT) system to correct eye motion artifacts in OCT angiography images. This system employs a dual-illumination configuration with two orthogonally polarized beams, each of which simultaneously perform raster scanning in perpendicular direction with each other over the same area. In the reference arm, a polarization delay unit is used to acquire the two orthogonally polarized interferograms with a single photo detector by introducing different optical delay lines. The two cross-scanned volume data are affected by the same eye motion but in two orthogonal directions. We developed a motion correction algorithm, which removes artifacts in the slow axis of each angiogram using the other and merges them through a nonrigid registration algorithm. In this manner, we obtained a motion-corrected angiogram within a single volume scanning time without additional eye-tracking devices.

A Neural Network Approach to Quantify Blood Flow from Retinal OCT Intensity Time-Series Measurements

Many diseases of the eye are associated with alterations in the retinal vasculature that are possibly preceded by undetected changes in blood flow. In this work, a robust blood flow quantification framework is presented based on optical coherence tomography (OCT) angiography imaging and deep learning. The analysis used a forward signal model to simulate OCT blood flow data for training of a neural network (NN). The NN was combined with pre- and post-processing steps to create an analysis framework for measuring flow rates from individual blood vessels. The framework’s accuracy was validated using both blood flow phantoms and human subject imaging, and across flow speed, vessel angle, hematocrit levels, and signal-to-noise ratio. The reported flow rate of the calibrated NN framework was measured to be largely independent of vessel angle, hematocrit levels, and measurement signal-to-noise ratio. In vivo retinal flow rate measurements were self-consistent across vascular branch points, and approximately followed a predicted power-law dependence on the vessel diameter. The presented OCT-based NN flow rate estimation framework addresses the need for a robust, deployable, and label-free quantitative retinal blood flow mapping technique.

Real-time co-localized OCT surveillance of laser therapy using motion corrected speckle decorrelation

We present a system capable of real-time delivery and monitoring of laser therapy by imaging with optical coherence tomography (OCT) through a double-clad fiber (DCF). A double-clad fiber coupler is used to inject and collect OCT light into the core of a DCF and inject the therapy light into its larger inner cladding, allowing for both imaging and therapy to be perfectly coregistered. Monitoring of treatment depth is achieved by calculating the speckle intensity decorrelation occurring during tissue coagulation. Furthermore, an analytical noise correction was used on the correlation to extend the maximum monitoring depth. We also present a method for correcting motion-induced decorrelation using a lookup table. Using the value of the noise- and motion-corrected correlation coefficient in a novel approach, our system is capable of identifying the depth of thermal coagulation in real time and automatically shut the therapy laser off when the targeted depth is reached. The process is demonstrated ex vivo in rat tongue and abdominal muscles for depths ranging from 500 µm to 1000 µm with induced motion in real time.

Characterization of early-stage cutaneous radiation injury by using optical coherence tomography angiography

Cutaneous radiation injury (CRI) is a skin injury caused by exposure to high dose ionizing radiation (IR). Diagnosis and treatment of CRI is difficult due to its initial clinically latent period and the following inflammatory bursts. Early detection of CRI before clinical symptoms will be helpful for effective treatment, and various optical methods have been applied with limitations. Here we show that optical coherence tomography angiography (OCTA) could detect changes in the skin during the latent period in CRI mouse models non-invasively. CRI was induced on the mouse hindlimb with exposure to various IR doses and the injured skin regions were imaged longitudinally by OCTA until the onset of clinical symptoms. OCTA detected several changes in the skin including the skin thickening, the dilation of large blood vessels, and the irregularity in vessel boundaries. Some of OCTA findings were confirmed by histology. The study results showed that OCTA could be used for early CRI detection.

Real-time en-face Gabor optical coherence tomographic angiography on human skin using CUDA GPU

We recently proposed an optical coherence tomographic angiography (OCTA) algorithm, Gabor optical coherence tomographic angiography (GOCTA), which can extract microvascular signals from a spectral domain directly with lower computational complexity compared to other algorithms. In this manuscript, we combine a programmable swept source, an OCT complex signal detecting unit, and graphics process units (GPU) to achieve a real-time en-face GOCTA system for human skin microvascular imaging. The programmable swept source can balance the A-scan rate and the spectral tuning range; the polarization-modulation based complex signal detecting unit can double the imaging depth range, and the GPU can accelerate data processing. C++ and CUDA are used as the programming platform where five parallel threads are created for galvo-driving signal generation, data acquisition, data transfer, data processing, and image display, respectively. Two queues (for the raw data and en-face images, respectively) are used to improve the data exchange efficiency among different devices. In this study, the data acquisition time and data processing time for each 3D complex volume (256×304×608 pixels,) are 405.3 and 173.7 milliseconds respectively. To the best of our knowledge, this is the first time to show en-face microvascular images covering 3×3 mm² at a refresh rate of 2.5 Hz.

Scattering Angle Resolved Optical Coherence Tomography Detects Early Changes in 3xTg Alzheimer's Disease Mouse Model

Purpose

Clinical intensity-based optical coherence tomographic retinal imaging is unable to resolve some of the earliest changes to Alzheimer's disease (AD) neurons. The aim of this pilot study was to demonstrate that scattering-angle-resolved optical coherence tomography (SAR-OCT), which is sensitive to changes in light scattering angle, is a candidate retinal imaging modality for early AD detection. SAR-OCT signal data may be sensitive to changes in intracellular constituent morphology that are not detectable with conventional OCT.

Methods

In this cross-sectional study, retinas of a triple transgenic mouse model of AD (3xTg-AD) were imaged alongside age-matched control mice (C57BL/6J) using SAR-OCT. A total of 32 mice (12 control, 20 3xTg-Ad) at four ages (10, 20, 30, and 45 weeks) were included in this cross-sectional study, and three retinal feature sets (scattering, thickness, and angiography) were examined between the disease and control groups.

Results

AD mice had significantly increased scattering diversity (lower SAR-OCT C parameter) at the earliest imaging time (10 weeks). Differences in the C parameter between AD and control mice were diminished at later times when both groups showed increased scattering diversity. AD mice have reduced retinal thickness compared to controls, particularly in central regions and superficial layers. No differences in vascular density or fractional blood volume between groups were detected.

Conclusions

SAR-OCT is sensitive to scattering angle changes in a 3xTg-AD mouse model and could provide early-stage biomarkers for neurodegenerative diseases such as AD.

Translational Relevance

Clinical OCT systems may be modified to record SAR-OCT images for non-invasive retinal diagnostic imaging of patients with neurodegenerative diseases such as AD.

Noise and bias in optical coherence tomography intensity signal decorrelation

Functional optical coherence tomography (OCT) imaging based on the decorrelation of the intensity signal has been used extensively in angiography and is finding use in flowmetry and therapy monitoring. In this work, we present a rigorous analysis of the autocorrelation function, introduce the concepts of contrast bias, statistical bias and variability, and identify the optimal definition of the second-order autocorrelation function (ACF) g ⁽²⁾ to improve its estimation from limited data. We benchmark different averaging strategies in reducing statistical bias and variability. We also developed an analytical correction for the noise contributions to the decorrelation of the ACF in OCT that extends the signal-to-noise ratio range in which ACF analysis can be used. We demonstrate the use of all the tools developed in the experimental determination of the lateral speckle size depth dependence in a rotational endoscopic probe with low NA, and we show the ability to more accurately determine the rotational speed of an endoscopic probe to implement NURD detection. We finally present g ⁽²⁾-based angiography of the finger nailbed, demonstrating the improved results from noise correction and the optimal bias mitigation strategies.

Forward multiple scattering dominates speckle decorrelation in whole-blood flowmetry using optical coherence tomography

Quantitative blood flow measurements using optical coherence tomography (OCT) have a wide potential range of medical research and clinical applications. Flowmetry based on the temporal dynamics of the OCT signal may have the ability to measure three-dimensional flow profiles regardless of the flow direction. State-of-the-art models describing the OCT signal temporal statistics are based on dynamic light scattering (DLS), a model which is inherently limited to single scattering regimes. DLS methods continue to be applied to OCT despite the knowledge that red blood cells produce strong forward multiple scattering. Here, we postulate that forward multiple scattering is the primary mechanism causing the rate of speckle-decorrelation derived from data acquired in vivo to deviate from the rate of decorrelation determined in phantom experiments. We also postulate that multiple scattering contributions to decorrelation are only present when the sample exhibits velocity field inhomogeneities larger than the scale of a resolution volume and are thus absent in rigid bulk motion. To test these hypotheses, we performed a systematic study of the effects of forward multiple scattering on OCT signal decorrelation with phantom experiments under physiologically relevant flow conditions and relative bulk motion. Our experimental results confirm that the amount of forward multiple scattering affects the proportionality between lateral flow and decorrelation. We propose that multiply scattered light carries information from different locations in the sample and each location imprints scattering dynamics on the scattered light causing increased decorrelation rates. Our analysis confirms that the detection of forward scattered light inside the vessel lumen causes an increase in the rate of decorrelation which results in an overestimation of blood flow velocities at depths as shallow as 40 µm into whole blood for OCT systems with typical numerical apertures used in retinal imaging.

VASCULAR ABNORMALITIES IN DIABETIC RETINOPATHY ASSESSED WITH SWEPT-SOURCE OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY WIDEFIELD IMAGING

PURPOSE

To detect vascular abnormalities in diabetic retinopathy using swept-source optical coherence tomography angiography (SS-OCTA) widefield images, and to compare the findings with color fundus photographs (CFPs) using Early Treatment Diabetic Retinopathy Study severity grading.

METHODS

3 mm × 3 mm and 12 mm × 12 mm scans were acquired to cover 70° to 80° of the posterior pole using a 100-kHz SS-OCTA instrument. Two masked graders assessed the presence of vascular abnormalities on SS-OCTA and the Early Treatment Diabetic Retinopathy Study level on CFP. The grading results were then compared.

RESULTS

A total of 120 diabetic eyes (60 patients) were imaged with the SS-OCTA instrument. Cohort 1 (91 eyes; SS-OCTA grading only) showed microaneurysms in 91% (n = 83), intraretinal microvascular abnormalities in 79% (n = 72), and neovascularization in 21% (n = 19) of cases. Cohort 2 (52 eyes; CFP grading compared with SS-OCTA) showed microaneurysms on CFP in 90% (n = 47) and on SS-OCTA in 96% (n = 50) of cases. Agreement in intraretinal microvascular abnormality detection was fair (k = 0.2). Swept-source optical coherence tomography angiography detected 50% of intraretinal microvascular abnormality cases (n = 26), which were missed on CFP. Agreement in detecting neovascularization was moderate (k = 0.5).

CONCLUSION

Agreement in detection of diabetic retinopathy features on CFP and SS-OCTA varies depending on the vascular changes examined. Swept-source optical coherence tomography angiography shows a higher detection rate of intraretinal microvascular abnormalities (P = 0.039), compared with Early Treatment Diabetic Retinopathy Study grading.

Automatic 3D adaptive vessel segmentation based on linear relationship between intensity and complex-decorrelation in optical coherence tomography angiography

Background

Vascular quantitative metrics have been widely used in the preclinical studies and clinical applications (e.g., the diagnosis and treatment of port wine stain, PWS), which require accurate vessel segmentation. An automatic 3D adaptive vessel segmentation is in need for a reproducible and objective quantification of the optical coherence tomography angiography (OCTA) image.

Methods

Human skin imaging was performed with a lab-built optical coherence tomography (OCT) system. Rather than separately applying the conventional 2-step (intensity and binarization) thresholding in the decorrelation-contrast OCTA, we proposed a 3D adaptive threshold using the linear relationship between the local intensity and complex-decorrelation which was termed as inverse SNR-decorrelation (ID) threshold. Furthermore, the ID threshold was automatically determined by defining a binary image similarity (BISIM) index as the feedback and searching the ID threshold with the minimal BISIM value. The proposed ID-BISIM threshold was applied to the acquired OCTA skin images for further vessel quantification.

Results

The proposed ID-BISIM threshold enabled a 3D adaptive binarization and presented superior sensitivity and specificity in vessel segmentation over conventional 2-step thresholding method in the decorrelation-contrast OCTA and a 37-65% improvement of the Youden's index in human skin experiments. The 3D binarization enabled a depth-resolved vessel skeleton and enhanced the differentiation of the overlapping vessels in the depth direction. Using ID-BISIM, the quantitative OCTA image presented a significant increase of vessel diameter index (P=0.0015) and vessel area density (VAD) (P=0.0485) as well as a significant decrease of vessel complexity index (VCI) (P=0.0094) in PWS lesion skin compared with normal skin.

Conclusions

The proposed ID-BISIM method enables an automatic 3D adaptive vessel segmentation with enhanced performance in quantitative OCTA. The vascular quantitative metrics would be a useful tool for improving the diagnosis and the treatment of PWS.

Artifacts and artifact removal in optical coherence tomographic angiography

Optical coherence tomographic angiography (OCTA) enables rapid imaging of retinal vasculature in three dimensions. While the technique has provided quantification of healthy vessels as well as pathology in several diseases, it is not unusual for OCTA data to contain artifacts that may influence measurement outcomes or defy image interpretation. In this review, we discuss the sources of several OCTA artifacts-including projection, motion, and signal reduction-as well as strategies for their removal. Artifact compensation can improve the accuracy of OCTA measurements, and the most effective use of the technology will incorporate hardware and software that can perform such correction.

Widefield Optical Coherence Tomography Angiography in Diabetic Retinopathy

PURPOSE

To summarize the role of widefield optical coherence tomography angiography (WF-OCTA) in diabetic retinopathy (DR), extending from the acquisition strategies to the main clinical findings.

METHODS

A PubMed-based search was carried out using the terms "Diabetic retinopathy", "optical coherence tomography angiography", "widefield imaging", and "ultra-widefield imaging". All studies published in English up to August 2020 were reviewed.

RESULTS

WF-OCTA can be obtained with different approaches, offering advantages over traditional imaging in the study of nonperfusion areas (NPAs) and neovascularization (NV). Quantitative estimates and topographic distribution of NPA and NV are useful for treatment monitoring and artificial intelligence-based approaches. Curvature, segmentation, and motion artifacts should be assessed when using WF-OCTA.

CONCLUSIONS

WF-OCTA harbors interesting potential in DR because of its noninvasiveness and capability of objective metrics of retinal vasculature. Further studies will facilitate the migration from traditional imaging to WF-OCTA in both the research and clinical practice fields.

Optical Coherence Tomography Angiography Imaging to monitor Anti-VEGF treatment of Corneal Vascularization in a Rabbit Model

Optical coherence tomography angiography (OCTA) is a well-established non-invasive retinal vascular imaging technique. It has been recently adapted to image the anterior segment and has shown good potential to image corneal vascularisation. The purpose of the study is to evaluate the usefulness of OCTA to monitor regression of corneal vessels following anti-VEGF (vascular endothelial growth factor) treatment using a previously established corneal vascularisation rabbit model. The regression of vessels following the treatment with aflibercept and ranibizumab anti-VEGFs using both topical instillation and sub-conjunctival injection was quantified using OCTA and compared with ICGA (indocyanine green angiography). Overall vessel density measurements using OCTA showed good correlation (r = 0.988, p < 0.001) with ICGA, with no significant difference between the two treatment groups (p = 0.795). It was also shown that OCTA provided good repeatability outcomes of the quantitative measurements. Using Bland-Altman plots, vessel growth density values between anti-VEGF treatments were compared to control saline group. It was observed that aflibercept provided longer lasting effect than ranibizumab. We also observed that in both drugs, the topical route of administration topical provided longer regression outcomes compared to one-time sub-conjunctival injection. Thereby, with this pilot study, it was demonstrated that OCTA is a reliable imaging technique to follow-up and monitor corneal vascularisation and its treatment quantitatively.

Clinical multi-functional OCT for retinal imaging

A compact clinical prototype multi-functional optical coherence tomography (OCT) device for the posterior human eye has been developed. This compact Jones-matrix OCT (JM-OCT) device integrates all components into a single package. Multiple image functions, i.e., scattering intensity, OCT angiography, and the degree of polarization uniformity, are obtained. The device has the capability for measuring local birefringence. Multi-functional imaging of several eyes with age-related macular degeneration is demonstrated. The compact JM-OCT device will be useful for the in vivo non-invasive investigation of abnormal tissues.

SNR-Adaptive OCT Angiography Enabled by Statistical Characterization of Intensity and Decorrelation With Multi-Variate Time Series Model

In OCT angiography (OCTA), decorrelation computation has been widely used as a local motion index to identify dynamic flow from static tissues, but its dependence on SNR severely degrades the vascular visibility, particularly in low-SNR regions. To mathematically characterize the decorrelation-SNR dependence of OCT signals, we developed a multi-variate time series (MVTS) model. Based on the model, we derived a universal asymptotic linear relation of decorrelation to inverse SNR (iSNR), with the variance in static and noise regions determined by the average kernel size. Accordingly, with the population distribution of static and noise voxels being explicitly calculated in the iSNR and decorrelation (ID) space, a linear classifier is developed by removing static and noise voxels at all SNR, to generate a SNR-adaptive OCTA, termed as ID-OCTA. Then, flow phantom and human skin experiments were performed to validate the proposed ID-OCTA. Both qualitative and quantitative assessments demonstrated that the ID-OCTA offers a superior visibility of blood vessels, particularly in the deep layer. Finally, the implications of this work on both system design and hemodynamic quantification are further discussed.

A deep learning based pipeline for optical coherence tomography angiography

Optical coherence tomography angiography (OCTA) is a relatively new imaging modality that generates microvasculature map. Meanwhile, deep learning has been recently attracting considerable attention in image-to-image translation, such as image denoising, super-resolution and prediction. In this paper, we propose a deep learning based pipeline for OCTA. This pipeline consists of three parts: training data preparation, model learning and OCTA predicting using the trained model. To be mentioned, the datasets used in this work were automatically generated by a conventional system setup without any expert labeling. Promising results have been validated by in-vivo animal experiments, which demonstrate that deep learning is able to outperform traditional OCTA methods. The image quality is improved in not only higher signal-to-noise ratio but also better vasculature connectivity by laser speckle eliminating, showing potential in clinical use. Schematic description of the deep learning based optical coherent tomography angiography pipeline.

Optical coherence tomography angiography in preclinical neuroimaging

Preclinical neuroimaging allows for the assessment of brain anatomy, connectivity, and function in laboratory animals, such as mice and this imaging field has been a rapidly growing aimed at bridging the translation gap between animal and human research. The progress in the animal research could be accelerated by high-resolution in vivo optical imaging technologies. Optical coherence tomography-based angiography (OCTA) estimates the scattering from moving red blood cells, providing the visualization of functional micro-vessel networks within tissue beds in vivo without a need for exogenous contrast agents. Recent advancement of OCTA methods have expanded its application to neuroimaging of small animal models of brain disorders. In this paper, we overview the recent development of OCTA techniques for blood flow imaging and its preclinical applications in neuroimaging. In specific, a summary of preclinical OCTA studies for traumatic brain injury, cerebral stroke, and aging brain on mice is reviewed.

Automated segmentation of peripapillary retinal boundaries in OCT combining a convolutional neural network and a multi-weights graph search

Quantitative analysis of the peripapillary retinal layers and capillary plexuses from optical coherence tomography (OCT) and OCT angiography images depend on two segmentation tasks - delineating the boundary of the optic disc and delineating the boundaries between retinal layers. Here, we present a method combining a neural network and graph search to perform these two tasks. A comparison of this novel method's segmentation of the disc boundary showed good agreement with the ground truth, achieving an overall Dice similarity coefficient of 0.91 ± 0.04 in healthy and glaucomatous eyes. The absolute error of retinal layer boundaries segmentation in the same cases was 4.10 ± 1.25 µm.

Laser brain cancer surgery in a xenograft model guided by optical coherence tomography

Higher precision surgical devices are needed for tumor resections near critical brain structures. The goal of this study is to demonstrate feasibility of a system capable of precise and bloodless tumor ablation. An image-guided laser surgical system is presented for excision of brain tumors in vivo in a murine xenograft model. The system combines optical coherence tomography (OCT) guidance with surgical lasers for high-precision tumor ablation (Er:YAG) and microcirculation coagulation (Thulium (Tm) fiber laser).

Methods: A fluorescent human glioblastoma cell line was injected into mice and allowed to grow four weeks. Craniotomies were performed and tumors were imaged with confocal fluorescence microscopy. The mice were subsequently OCT imaged prior, during and after laser coagulation and/or ablation. The prior OCT images were used to compute three-dimensional tumor margin and angiography images, which guided the coagulation and ablation steps. Histology of the treated regions was then compared to post-treatment OCT images.

Results: Tumor sizing based on OCT margin detection matched histology to within experimental error. Although fluorescence microscopy imaging showed the tumors were collocated with OCT imaging, margin assessment using confocal microscopy failed to see the extent of the tumor beyond ~ 250 µm in depth, as verified by OCT and histology. The two-laser approach to surgery utilizing Tm wavelength for coagulation and Er:YAG for ablation yielded bloodless resection of tumor regions with minimal residual damage as seen in histology.

Conclusion: Precise and bloodless tumor resection under OCT image guidance is demonstrated in the murine xenograft brain cancer model. Tumor margins and vasculature are accurately made visible without need for exogenous contrast agents.

Differential phase standard-deviation-based optical coherence tomographic angiography for human retinal imaging in vivo

We present a differential phase standard-deviation (DPSD)-based optical coherence tomographic (OCT) angiography (OCTA) technique to calculate the angiography images of the human retina. The standard deviation was calculated along the depth direction on the differential phase image of two B-scans (from the same position, at different times) to contrast dynamic vascular signals. The performance of a DPSD was verified by both phantom and in vivo experiments. When compared to other OCTA algorithms such as phase variance OCT, speckle variance OCT, and optical microangiography, we showed that a DPSD achieved improved image contrast and higher sensitivity. Furthermore, we also found the improved signal-to-noise ratio and contrast-to-noise ratio of 1.6 dB and 0.5, respectively, in large scanning range images.

Optimization-based vessel segmentation pipeline for robust quantification of capillary networks in skin with optical coherence tomography angiography

Optical coherence tomography angiography (OCTA) provides in-vivo images of microvascular perfusion in high resolution. For its application to basic and clinical research, an automatic and robust quantification of the capillary architecture is mandatory. Only this makes it possible to reliably analyze large amounts of image data, to establish biomarkers, and to monitor disease developments. However, due to its optical properties, OCTA images of skin often suffer from a poor signal-to-noise ratio and contain imaging artifacts. Previous work on automatic vessel segmentation in OCTA mostly focuses on retinal and cerebral vasculature. Its applicability to skin and, furthermore, its robustness against imaging artifacts had not been systematically evaluated. We propose a segmentation method that improves the quality of vascular quantification in OCTA images even if corrupted by imaging artifacts. Both the combination of image processing methods and the choice of their parameters are systematically optimized to match the manual labeling of an expert for OCTA images of skin. The efficacy of this optimization-based vessel segmentation is further demonstrated on sample images as well as by a reduced error of derived quantitative vascular network characteristics.

Balloon catheter-based radiofrequency ablation monitoring in porcine esophagus using optical coherence tomography

We present a microscopic image guidance platform for radiofrequency ablation (RFA) using a clinical balloon-catheter-based optical coherence tomography (OCT) system, currently used in the surveillance of Barrett's esophagus patients. Our integrated thermal therapy delivery and monitoring platform consists of a flexible, customized bipolar RFA electrode array designed for use with a clinical balloon OCT catheter and a processing algorithm to accurately map the thermal coagulation process. Non-uniform rotation distortion was corrected using a feature tracking-based technique, which enables robust, frame-to-frame analysis of the temporal fluctuation of the complex OCT signal. With proper noise calibration, precise delineation of the thermal therapy zone was demonstrated using cumulative complex differential variance in porcine esophagus ex vivo with the integrated OCT-RFA system, as validated by nitroblue tetrazolium chloride (NBTC) histology. The ability to directly and accurately visualize the thermal coagulation process at high resolution is critical to the precise delivery of thermal energy to a wide range of epithelial lesions.

Scattering-Angle-Resolved Optical Coherence Tomography of a Hypoxic Mouse Retina Model

Several studies have noted a correlation between retinal degeneration and traumatic encephalopathy (TE) making the retina a leading candidate for detection and assessment. Scattering-angle-resolved optical coherence tomography (SAR-OCT) is a candidate imaging modality to detect sub-resolution changes in retinal microstructure. SAR-OCT images of murine retinas that experience a hypoxic insult—euthanasia by isoflurane overdose—are presented. A total of 4 SAR-OCT measurement parameters are reported in 6 longitudinal experiments: blood flow volume fraction, total retinal thickness, reflectance index, and scattering angle. As each mouse expires, blood flow volume fraction decreases, total retinal thickness increases, reflectance index decreases, and scattering angle diversity increases. Contribution of the retinal vasculature to scattering angle diversity is discussed. Results of this study suggest the utility of SAR-OCT to measure TE using scattering angle diversity contrast in the retina.

Precision analysis and optimization in phase decorrelation OCT velocimetry

Quantitative flow velocimetry in Optical Coherence Tomography is used to determine both the axial and lateral flow component at the level of individual voxels. The lateral flow is determined by analyzing the statistical properties of reflected electro-magnetic fields for repeated measurements at (nearly) the same location. The precision or statistical fluctuation of the quantitative velocity estimation depends on the number of repeated measurements and the method to determine quantitative flow velocity. In this paper, both a method to determine quantitative flow velocity and a model for the prediction of the statistical fluctuations of velocity estimations are developed to analyze and optimize the estimation precision for phase-based velocimetry methods. The method and model are validated by phantom measurements in a bulk scattering medium as well as in intralipid solution in a capillary. Based on the model, the number of repeated measurements to achieve a certain velocimetry precision is predicted.

Visualizing the vasculature of the entire human eye posterior hemisphere without a contrast agent

The platform described here combines the non-invasive measurement of the retina/choroid structure and ocular blood flow based on optical coherence tomography (OCT) and wide-field semi-quantitative global flow visualization using line-scanning Doppler flowmetry (LSDF). The combination of these two imaging modalities within the same platform enables comprehensive assessment of blood flow in the retina and choroid in animals and human subjects for diagnostic purposes. Ultra-widefield vasculature visualization is demonstrated here for the first time without injecting additional contrast agents and based only on the motion of particles within the vasculature.

Short-time series optical coherence tomography angiography and its application to cutaneous microvasculature

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.

Demonstration of Triband Multi-Focal Imaging with Optical Coherence Tomography

We demonstrate an extended depth of focus optical coherence tomography (OCT) system based on the use of chromatic aberration to create displaced focal planes in the sample. The system uses a wavelength-swept source tuning over three spectral bands and three separate interferometers, each of which interfaces to a single illumination/collection fiber. The resulting three imaged volumes are merged in post-processing to generate an image with a larger depth of focus than is obtained from each band individually. The improvements are demonstrated in structural imaging of a porous phantom and a lipid-cleared murine brain, and by angiographic imaging of human skin. By using a coaxial approach with Gaussian beams, this approach enables an extended focus with relatively simple microscope optics and data-merging algorithms.

Design Considerations for Murine Retinal Imaging Using Scattering Angle Resolved Optical Coherence Tomography

Optical coherence tomography (OCT), an optical imaging approach enabling cross-sectional analysis of turbid samples, is routinely used for retinal imaging in human and animal models of diseases affecting the retina. Scattering angle resolved (SAR-)OCT has previously been demonstrated as offering additional contrast in human studies, but no SAR-OCT system has been reported in detail for imaging the retinas of mice. An optical model of a mouse eye was designed and extended for validity at wavelengths of light around 1310 nm; this model was then utilized to develop a SAR-OCT design for murine retinal imaging. A Monte Carlo technique simulates light scattering from the retina, and the simulation results are confirmed with SAR-OCT images. Various images from the SAR-OCT system are presented and utility of the system is described. SAR-OCT is demonstrated as a viable and robust imaging platform to extend utility of retinal OCT imaging by incorporating scattering data into investigative ophthalmologic analysis.

Wide-Field Functional Microscopy of Peripheral Nerve Injury and Regeneration

Severe peripheral nerve injuries often result in partial repair and lifelong disabilities in patients. New surgical techniques and better graft tissues are being studied to accelerate regeneration and improve functional recovery. Currently, limited tools are available to provide in vivo monitoring of changes in nerve physiology such as myelination and vascularization, and this has impeded the development of new therapeutic options. We have developed a wide-field and label-free functional microscopy platform based on angiographic and vectorial birefringence methods in optical coherence tomography (OCT). By incorporating the directionality of the birefringence, which was neglected in the previously reported polarization-sensitive OCT techniques for nerve imaging, vectorial birefringence contrast reveals internal nerve microanatomy and allows for quantification of local myelination with superior sensitivity. Advanced OCT angiography is applied in parallel to image the three-dimensional vascular networks within the nerve over wide-fields. Furthermore, by combining vectorial birefringence and angiography, intraneural vessels can be discriminated from those of the surrounding tissues. The technique is used to provide longitudinal imaging of myelination and revascularization in the rodent sciatic nerve model, i.e. imaged at certain sequential time-points during regeneration. The animals were exposed to either crush or transection injuries, and in the case of transection, were repaired using an autologous nerve graft or acellular nerve allograft. Such label-free functional imaging by the platform can provide new insights into the mechanisms that limit regeneration and functional recovery, and may ultimately provide intraoperative assessment in human subjects.

The role of OCT-A in retinal disease management

Optical coherence tomography angiography (OCT-A) is a non-invasive, non-dye-based imaging modality that has the potential to enhance our understanding of retinal diseases. While this rapidly advancing imaging modality offers great potential, there is a need for community-wide understanding of the range of technologies and methods for interpreting the images, as well as a need to enhance understanding of images from disease-free eyes for reference when screening for retinal diseases. Importantly, clinical trials have been designed without OCT-A-based endpoints; therefore, caution is required when making treatment decisions based on OCT-A imaging alone. With this in mind, a full understanding of the advantages and limitations of OCT-A will be vital for effective development of the technique within the field of ophthalmology. On behalf of the Vision Academy Steering Committee (sponsored by Bayer), this publication summarizes the views of the authors on the current use of OCT-A imaging and explores its potential for future applications in research and clinical practice.

Feasibility of correlation mapping optical coherence tomography angiographic technique using a 200 kHz vertical-cavity surface-emitting laser source for in vivo microcirculation imaging applications

Optical coherence tomography (OCT) angiography is a well-established in vivo imaging technique to assess the overall vascular morphology of tissues and is an emerging field of research for the assessment of blood flow dynamics and functional parameters such as oxygen saturation. In this study, we present a modified scanning-based correlation mapping OCT using a 200 kHz high-speed swept-source OCT system operating at 1300 nm and demonstrate its wide field-imaging capability in ocular angiographic studies.

Wide dynamic range high-speed three-dimensional quantitative OCT angiography with a hybrid-beam scan

We demonstrate a novel hybrid-beam scanning-based quantitative optical coherence tomography angiography (OCTA) that provides high-speed wide dynamic range blood flow speed imaging. The hybrid-beam scanning scheme enables multiple OCTA image acquisitions with a wide range of multiple time intervals simultaneously providing wide dynamic range blood flow speed imaging independent of the blood vessel orientation, which was quantified over a speed range of 0.6∼104  mm/s through the blood flow phantom experiments. A fully automated high-speed hybrid-beam scanning-based quantitative OCTA system demonstrates visualization of blood flow speeds in various vessels from the main arteries to capillaries in a 4  mm×4  mm area (1024 A-lines × 512 B-scans) in vivo in 20 s, showing its potential as a useful imaging tool for various biomedical applications.