Optical coherence tomography based angiography [Invited]

A feasibility study of OCT for anatomical and vascular phenotyping of mouse embryo

The embryo phenotyping of genetic murine model is invaluable when investigating functions of genes underlying embryonic development and birth defect. Although traditional imaging technologies such as ultrasound are very useful for evaluating phenotype of murine embryos, the use of advanced techniques for phenotyping is desirable to obtain more information from genetic research. This letter tests the feasibility of optical coherence tomography (OCT) as a high-throughput phenotyping tool for murine embryos. Three-dimensional OCT imaging is performed for live and cleared mouse embryos in the late developmental stage (embryonic day 17.5). By using a dynamic focusing method and OCT angiography (OCTA) approach, our OCT imaging of the embryo exhibits rapid and clean visualization of organ structures deeper than 5 mm and complex microvasculature of perfused blood vessels in the murine embryonic body. This demonstration suggests that OCT imaging can be useful for comprehensively assessing embryo anatomy and angiography of genetically engineered mice.

Mean-Subtraction Method for De-shadowing of Tail Artifacts in Cerebral OCTA Images: A Proof of Concept

When imaging brain vasculature with optical coherence tomography angiography (OCTA), volumetric analysis of cortical vascular networks in OCTA datasets is frequently challenging due to the presence of artifacts, which appear as multiple-scattering tails beneath superficial large vessels in OCTA images. These tails shadow underlying small vessels, making the assessment of vascular morphology in the deep cortex difficult. In this work, we introduce an image processing technique based on mean subtraction of the depth profile that can effectively reduce these tails to better reveal small hidden vessels compared to the current tail removal approach. With the improved vascular image quality, we demonstrate that this simple method can provide better visualization of three-dimensional vascular network topology for quantitative cerebrovascular studies.

Penetration-enhanced optical coherence tomography angiography with optical clearing agent for clinical evaluation of human skin

BACKGROUND

Optical coherence tomography angiography (OCTA) is an emerging imaging technique which shows its advantages over visualizing microcirculation with free label. However, its shortcomings in imaging depth limit its development in dermatological field. Nowadays, the newly optical clearing agent (OCA) designed for skin optical imaging demonstrates its potential. In our study, whether this OCA can improve the imaging ability of OCTA in healthy human skin and whether the combination of them is beneficial to compare the lesions and the contralateral normal skins in the patients with port wine stains (PWS) have been investigated.

METHODS

Five healthy volunteers and 3 PWS patients were recruited in this study. In terms of healthy people, the opisthenar area which has same structure information as facial skin was taken for investigating the OCA's ability of enhancing OCTA imaging depth on healthy human skin, besides, in order to verifying whether the exists of skin corneum interfere OCA's function, we compared the effect of only using OCA with that of comprehensive using pre-processing skin and OCA. There are one physical removing corneum method by using medical tape to strip opisthenar skin for over 20-time and one chemical way through applying exfoliating cream. For PWS patient, the combining using OCA and OCTA was applied at the lesion area and the contralateral normal area for the purpose of verifying their ability to provide the information of vessels.

RESULTS

This novel OCA had excellent efficacy to increase the penetration depth of human opisthenar skin for the OCTA imaging by approximately 0.16 ± 0.03 mm. Pre-processing of stratum corneum with an exfoliating cream or medical tape stripping did not further benefit the penetrating efficacy of the OCA. Moreover, according to a comprehensive analysis of the OCTA images enhanced by the OCA, the PWS lesions usually have larger density and diameter of the vessels which located in deep layers (beyond 0.21 mm) than the contralateral normal skin.

CONCLUSIONS

The OCTA imaging depth and contrast were significantly improved by the OCA. The OCA application is a simple and efficient clinical procedure for OCTA enhancement. Moreover, it demonstrated great clinical value to compare the normal skin and the PWS lesions in the patients by the enhanced OCTA imaging.

Label-free optical imaging in developmental biology [Invited]

Application of optical imaging in developmental biology marks an exciting frontier in biomedical optics. Optical resolution and imaging depth allow for investigation of growing embryos at subcellular, cellular, and whole organism levels, while the complexity and variety of embryonic processes set multiple challenges stimulating the development of various live dynamic embryonic imaging approaches. Among other optical methods, label-free optical techniques attract an increasing interest as they allow investigation of developmental mechanisms without application of exogenous markers or fluorescent reporters. There has been a boost in development of label-free optical imaging techniques for studying embryonic development in animal models over the last decade, which revealed new information about early development and created new areas for investigation. Here, we review the recent progress in label-free optical embryonic imaging, discuss specific applications, and comment on future developments at the interface of photonics, engineering, and developmental biology.

Analysis of longitudinal sections of retinal vessels using Doppler OCT

We present a new method for imaging retinal vessels that provides both structural and hemodynamic information. Our technique is based on a single beam OCT system with an integrated retinal tracker that enables recording of arbitrary scan patterns. We record longitudinal sections along the traces of retinal vessels. The tracker function enables the acquisition of multiple longitudinal sections along the same trace to provide high-quality averaged OCT scans as well as temporal changes of flow dynamics. The vessel walls are clearly identified as narrow, bright lines from which the vessel diameter can be retrieved as a function of position along the vessel. Furthermore, the Doppler angle can be obtained at each position along the vessel trace, enabling measurement of absolute blood flow by Doppler OCT analysis. The method is demonstrated in flow phantoms and in-vivo on retinal vessel bifurcations in healthy volunteers. In 7 of 9 imaged bifurcations, measured in- and outflow deviate by less than 11%, demonstrating the consistency of the method.

Automated morphometric measurement of the retinal pigment epithelium complex and choriocapillaris using swept source OCT

We report the development of an automated method to measure morphological features of the retinal pigment epithelium (RPE), Bruch's membrane (BM) and choriocapillaris (CC) using a commercially available swept source OCT (SS-OCT) system. The locations of the inner segment/outer segment (IS/OS), RPE and CC were determined by optical coherence tomography (OCT) and OCT angiography (OCTA) A-scan intensity profiles, which were used to calculate the mean IS/OS-to-RPE distance, mean RPE-to-CC distance, mean RPE-to-CC outer boundary distance, mean RPE thickness and mean CC thickness across the entire scan volume. The automated method was tested on two groups of normal subjects: younger age group (n=20, 30.3 ± 5.72 years, axial length = 24.2 ± 0.96 mm) and older age group (n=20, 80.8 ± 4.12 years, axial length = 23.5 ± 0.93 mm). The 6×6 mm macular scans were acquired from one eye of each subject. Repeatability of the measurements showed a coefficient of variance < 5% for all the cases. CC locations were confirmed qualitatively with pixel-by-pixel moving of the en face OCT/OCTA images. Relative distance and thickness maps of the RPE-BM-CC complex were generated for visualization of regional changes. We observed thinner CC, thinner RPE and increased RPE-to-CC distance in the older age group. Correlation between CC thickness and choroid thickness suggests that the CC thins with the overall thinning of the choroid. These metrics should be useful to reveal more morphological details of RPE-BM-CC complex, provide a better understanding of the CC in three dimensions, and further investigate potential functional relationships between RPE, BM and CC, and their involvement in age-related ocular diseases.

Optical coherence tomography for the investigation of skin adaptation to mechanical stress

BACKGROUND

Skin breakdown due to limb-socket interface stress is a significant problem for lower limb prosthesis users. While it is known that skin can adapt to stress to become more resistant to breakdown, little is understood about skin adaptation and few methods exist to noninvasively investigate it. In this study, we present novel, noninvasive imaging methods using Optical Coherence Tomography (OCT) to assess key features of the cutaneous microvasculature that may be involved in skin adaptation.

MATERIALS AND METHODS

Eight able-bodied participants wore a modified below-knee prosthetic socket for two weeks to stress the skin of their lower limb. Two OCT-based imaging tests were used to assess the function and structure, respectively, of the cutaneous microvasculature at multiple time points throughout the socket wear protocol.

RESULTS

A measurable reactive hyperemia response was reliably induced in the skin of study participants in the vascular function assessment test. The vascular structure assessment demonstrated excellent field-of-view repeatability, providing rich data sets of vessel structure. No statistically significant differences were found in any of the measurements when compared between time points of the adaptation protocol. The participants' limbs were likely not stressed enough by the able-bodied socket to induce measurable skin adaptation.

CONCLUSION

This study introduced new techniques to investigate skin adaptation to mechanical stress. If the key limitations are addressed, these methods have the potential to provide insight into the function and structure of the cutaneous microvasculature that previously could not be attained noninvasively.

Optics Based Label-Free Techniques and Applications in Brain Monitoring

Functional near-infrared spectroscopy (fNIRS) has been utilized already around three decades for monitoring the brain, in particular, oxygenation changes in the cerebral cortex. In addition, other optical techniques are currently developed for in vivo imaging and in the near future can be potentially used more in human brain research. This paper reviews the most common label-free optical technologies exploited in brain monitoring and their current and potential clinical applications. Label-free tissue monitoring techniques do not require the addition of dyes or molecular contrast agents. The following optical techniques are considered: fNIRS, diffuse correlations spectroscopy (DCS), photoacoustic imaging (PAI) and optical coherence tomography (OCT). Furthermore, wearable optical brain monitoring with the most common applications is discussed.

Tracking of cutaneous vascular structural changes post-UV irradiation using optical coherence tomography angiography

BACKGROUND

Ultraviolet (UV) irradiation to skin induces biological responses to protect and heal the wounded tissue. Cutaneous blood vessels play an important role in maintaining skin homeostasis, by inducing angiogenesis and vasodilation. However, the vascular dynamics in vivo, such as morphological changes over time and their depth dependency, are not fully understood.

METHODS

Ten Asian males were enrolled in this study and received UV (UVA + UVB) irradiation at two minimal erythema dose (MED) to the inner upper arm. Changes in epidermal thickness and vascular structures associated with UV irradiation were evaluated over time for 28 days by optical coherence tomography angiography (OCTA). This technique enables non-invasive visualization of three-dimensional vascular networks in human skin based on OCT assessment of skin structures with near-infrared light.

RESULTS

Notable dilation of vascular structures and increases in epidermal thickness were observed after UV irradiation. Vessel density was markedly increased from the papillary dermis to the upper reticular dermis at a depth of 200 µm. These increases in vascular density showed significant persistence even at 28 days after UV irradiation.

CONCLUSION

We visualized the vascular structural changes caused by UV irradiation and revealed that the effects of a single UV irradiation at 2 MED persisted for up to 28 days after exposure. The OCTA technique allows not only the in situ assessment of micro-vasculature in human skin but also its monitoring of vascular dynamics over time.

Vascular Density of the Anterior Segment of the Eye Determined by Optical Coherence Tomography Angiography and Slit-Lamp Photography

<b><i>Objective:</i></b> To determine the vascular density of the anterior segment (AS) of the eye from optical coherence tomography angiography (OCTA) images and slit-lamp photographs. <b><i>Methods:</i></b> A swept-source OCTA (Plex Elite 9000; Carl Zeiss) device modified with a +10-diopter lens was used to record the vasculature of the AS. Twenty eyes, including 4 eyes of 4 healthy subjects and 16 eyes of 12 patients scheduled for cataract surgery or combined vitrectomy and cataract surgery, were studied. The slit-lamp photographs of the AS were acquired concurrently with the AS-OCTA images. The vascular density was measured preoperatively and postoperatively in the nasal, temporal, superior, and inferior quadrants after binarization with ImageJ software. <b><i>Results:</i></b> Acceptable AS-OCTA images were obtained of 65% (superior), 80% (nasal), 70% (inferior), and 80% (temporal) of the eyes. The percentage of acceptable images was significantly lower in the superior quadrant among the AS-OCTA images than among the AS photographs (100%; <i>p</i> = 0.004). The vascular density determined by AS-OCTA was higher than that determined in the AS photographs in all quadrants (<i>p</i> = 0.011 to &#x3c;0.001). The AS-OCTA B-mode images showed that vascular flow was identified mainly between the conjunctiva and sclera but not in the ciliary body. The vascular density increased significantly after cataract surgery in the superior quadrant, which was significantly correlated with the location of the surgical incision (<i>p</i> = 0.03). <b><i>Conclusion:</i></b> AS-OCTA can obtain images with higher vascular density of the conjunctiva and sclera than slit-lamp photographs, and AS-OCTA images can show a postoperative increase in vascular density.

Optical coherence tomography angiography distortion correction in widefield montage images

Background

Optical coherence tomography (OCT) imaging is inherently susceptible to distortion artifacts due to the natural curvature of the eye. This study proposes a novel model for widefield OCT angiography (OCTA) distortion correction and analyzes the effects of this correction on quantification metrics.

Methods

Widefield OCTA images were obtained on normal subjects at five fixation spatial positions. Radial and field distortion correction were applied and images stitched together to form a corrected widefield montage image. Vessel area density (VAD), vessel complexity index (VCI), and flow impairment area were quantified on the original and corrected montage images.

Results

This model allows for distortion correction and montaging of widefield images. There were either statistically insignificant or small magnitude changes in vessel density and vessel complexity between uncorrected and corrected widefield images. There was a significant and large difference in flow impairment area, both in the macular (+8.2%, P=0.049) and peripheral areas (+17.2%, P=0.011), following correction. The relationship between pre- and post-correction flow impairment area was non-linear.

Conclusions

Distortion correction of widefield OCTA images can result in clinically and statistically significant differences in important quantification metrics. This effect appears to be most pronounced in the periphery.

Optical coherence tomography angiography measures blood pulsatile waveforms at variable tissue depths

Background

Photoplethysmography (PPG) is routinely used to detect the blood pulse signal from skin tissue beds in clinics. However, the origin of the PPG signal remains controversial. The purpose of this study is to explore optical coherence tomography angiography (OCTA) to indicate pulsatile waveforms in the papillary plexus and dermal plexus separately under different hand elevations.

Method

Optical microangiography (OMAG) algorithm was used to obtain a 3D OCTA signals, from which the depth-resolved pulsatile blood flow signals were extracted from different skin vascular plexus. The systolic amplitude, crest time, and delta T were measured from the OCTA pulsatile signals when the hand was placed at the positions of 50 cm below, 0 cm, and 50 cm above the heart level.

Results

The pulse signal integrated from all the depths has a similar waveform to that of the PPG and showed the same morphological change at different hand elevations. The pulsatile patterns from the papillary plexus and dermal plexus showed distinct morphological changes at different local blood pressure. Less amplitude difference was found from papillary plexus comparing to that of the dermal plexus. Crest time was found in an increasing trend in the OCTA pulsatile waveform from both plexuses when the arm was raised from the position below to above the heart level. In contrast, a decreasing trend of Delta T was detected in the dermal pulsatile but was not observed from that of the papillary plexus, indicating that vascular resistance associated with the arm elevations does not necessarily have the same effect on the two plexuses.

Conclusions

OCTA can provide depth-resolved pulsatile waveforms within different microvascular plexus within tissue skin beds. This technique could open doors to understanding the mechanisms of how blood flow changes at different skin circulatory plexus.

Impact of ocular magnification on retinal and choriocapillaris blood flow quantification in myopia with swept-source optical coherence tomography angiography

Background

To evaluate the impact of ocular magnification on retinal and choriocapillaris (CC) blood flow quantification in myopic eyes using swept-source optical coherence tomography angiography (SS-OCTA).

Methods

Subjects with myopia were recruited for comprehensive ophthalmic examination and SS-OCTA imaging with 6×6 mm² scanning protocol. Retinal vessel area density (RVAD), retinal vessel skeleton density (RVSD), and percentage of CC flow deficits (CC FD%) were quantified within a 5-mm-diameter circle centered on the fovea before and after magnification correction using the Littman and the modified Bennett formulae.

Results

Images from 28 myopic eyes were qualified for quantitative analyses including 12 eyes with non-high myopia (43%) and 16 eyes with high myopia (57%). The mean spherical equivalent (SE) refractive error was -8.18±4.58 diopters (D) and the mean axial length was 27.9±2.5 mm. The mean corrected RVAD was significantly lower than the uncorrected RVAD in all myopic eyes (0.51±0.02 vs. 0.52±0.02, P<0.001). The mean corrected RVSD was also significantly lower than the uncorrected RVSD in myopic eyes (0.13±0.01 vs. 0.15±0.00, P<0.001). In highly myopic eyes, the mean corrected CC FD% was significantly higher than the uncorrected CC FD% (14.9%±4.9% vs. 14.2%±4.5%, P=0.009). In non-highly myopic eyes, no statistically significant difference was observed between the corrected and uncorrected CC FD% measurements (11.7%±5.8% vs. 11.5%±5.8%, P=0.133).

Conclusions

Ocular magnification significantly affects the results of retinal and CC blood flow quantification with OCTA in myopic eyes. For accurate determination of the OCTA derived parameters in myopia, magnification correction should be taken into consideration.

Quantitative research on the interaction between cerebral edema and peripheral cerebral blood perfusion using swept-source optical coherence tomography

Background

Ischemic cerebral edema (CE) is a major leading cause of death in patients with ischemic stroke. The CE progression is closely related to the local cerebral blood perfusion (LCBP) level surrounding the edema area. Quantitative studying the interaction between the CE and peripheral LCBP may provide new inspiration for control and even treatment of CE.

Methods

Photothrombosis ischemia mouse model was established and observed for 9 hours using swept-source optical coherence tomography (SS-OCT). OCT-based angiography and OCT-based attenuation imaging techniques were used to reconstruct the angiograms reflecting the cerebral blood perfusion (CBP) level and optical attenuation coefficient (OAC) maps reflecting the edema state. The influence of edema on LCBP was analyzed by quantifying the blood perfusion in different spatial locations around the edema tissue, and the influence of LCBP on CE progression was revealed by comparing the changes of the edema area and LCBP level over time.

Results

Preliminary studies show that the effect of edema tissue on LCBP is very significant, which shows a clear spatial dependence. LCBP near the edema tissue is 15-20% lower than that far away from the edema tissue. When the LCBP drops to around 60% of the initial value, the edema area increases sharply. In addition, the level of CBP in the contralateral hemisphere also decreases with time. When the contralateral CBP drops to around 60%, there is a certain probability that contralateral edema will occur.

Conclusions

CE progression is not only related to the LCBP around the edema tissue but also related to the CBP of non-edematous regions. Controlling the CBP level of non-edematous regions may play a positive role in the treatment of CE. This work provides a new method and inspiration for exploring the mechanism of ischemic CE progression.

Real-time colorectal cancer diagnosis using PR-OCT with deep learning

Prior reports have shown optical coherence tomography (OCT) can differentiate normal colonic mucosa from neoplasia, potentially offering an alternative technique to endoscopic biopsy - the current gold-standard colorectal cancer screening and surveillance modality. To help clinical translation limited by processing the large volume of generated data, we designed a deep learning-based pattern recognition (PR) OCT system that automates image processing and provides accurate diagnosis potentially in real-time. Method: OCT is an emerging imaging technique to obtain 3-dimensional (3D) "optical biopsies" of biological samples with high resolution. We designed a convolutional neural network to capture the structure patterns in human colon OCT images. The network is trained and tested using around 26,000 OCT images acquired from 20 tumor areas, 16 benign areas, and 6 other abnormal areas. Results: The trained network successfully detected patterns that identify normal and neoplastic colorectal tissue. Experimental diagnoses predicted by the PR-OCT system were compared to the known histologic findings and quantitatively evaluated. A sensitivity of 100% and specificity of 99.7% can be reached. Further, the area under the receiver operating characteristic (ROC) curves (AUC) of 0.998 is achieved. Conclusions: Our results demonstrate that PR-OCT can be used to give an accurate real-time computer-aided diagnosis of colonic neoplastic mucosa. Future development of this system as an "optical biopsy" tool to assist doctors in real-time for early mucosal neoplasms screening and treatment evaluation following initial oncologic therapy is planned.

Simultaneous detection of cerebral blood perfusion and cerebral edema using swept-source optical coherence tomography

The progression of ischemic cerebral edema (CE) is closely related to the level of cerebral blood perfusion (CBP) and affects each other. Simultaneous detection of CBP and CE is helpful in understanding the mechanisms of ischemic CE development. In this article, a wide field of view swept-source optical coherence tomography system was used to detect CE status and CBP levels simultaneously in middle cerebral artery occlusion rats. Images reflecting these two physiological states can be reconstructed with only one C-scan. We quantify these two physiological states into four parameters, which contain two vascular parameters (vascular displacement distance and vascular perfusion density) and two edema parameters (optical attenuation coefficient and edema area). The association between the two vascular parameters and the two edema parameters was analyzed. The results show that there is a strong linear relationship between blood flow parameters and edema parameters. This work provides a new option for CE in vivo detection, and is very likely to play an important role in the development of relevant drugs or in selection of treatment options.

Measurement and visualization of stimulus-evoked tissue dynamics in mouse barrel cortex using phase-sensitive optical coherence tomography

We describe a method to measure tissue dynamics in mouse barrel cortex during functional activation via phase-sensitive optical coherence tomography (PhS-OCT). The method measures the phase changes in OCT signals, which are induced by the tissue volume change, upon which to localize the activated tissue region. Phase unwrapping, compensation and normalization are applied to increase the dynamic range of the OCT phase detection. To guide the OCT scanning, intrinsic optical signal imaging (IOSI) system equipped with a green light laser source (532 nm) is integrated with the PhS-OCT system to provide a full field time-lapsed images of the reflectance that is used to identify the transversal 2D localized tissue response in the mouse brain. The OCT results show a localized decrease in the OCT phase signal in the activated region of the mouse brain tissue. The decrease in the phase signal may be originated from the brain tissue compression caused by the vasodilatation in the activated region. The activated region revealed in the cross-sectional OCT image is consistent with that identified by the IOSI imaging, indicating the phase change in the OCT signals may associate with the changes in the corresponding hemodynamics. In vivo localized tissue dynamics in the barrel cortex at depth during whisker stimulation is observed and monitored in this study.

Optical coherence tomography angiography for marginal corneal vascular remodelling after pterygium surgery with limbal-conjunctival autograft

PURPOSE

To demonstrate the marginal corneal vascular remodelling using optical coherence tomography angiography (OCTA) after pterygium surgery.

METHODS

Twenty-two eyes of 19 patients (8 males, 11 females; age, 58.68 ± 0.34 years) with primary grade-T3 nasal pterygium were enroled in this study. The eyes underwent excision of the pterygium followed by a free limbal-conjunctival autograft. OCTA was performed in the nasal limbal area before surgery and at 10 days, 1 month, and 3 months after surgery. The scans were analyzed in terms of postoperative vascular remodelling of the autograft and marginal corneal vascular arcades (MCAs).

RESULTS

Preoperatively, the pterygium presented as abnormal centripetal vascular growth in OCTA scans. The conjunctival vessel density in the nasal quadrant was 29.26% ± 1.00%, 15.80% ± 0.83%, 19.80% ± 0.88%, and 20.26% ± 0.89% before and 10 days, 1 month, and 3 months, respectively, after surgery (F = 1.55, P < 0.01). The vessel density of MCAs was 28.33% ± 0.88%, 42.09% ± 0.41%, and 42.46% ± 0.31% 10 days, 1 month, and 3 months, respectively, after surgery (F = 188.2, P < 0.01).

CONCLUSIONS

We describe a new application of OCTA for MCA vasculature imaging. Vascular remodelling of the graft and MCAs appeared at 1 month and continued for 3 months after surgery.

Suite of methods for assessing inner retinal temporal dynamics across spatial and temporal scales in the living human eye

Significance: There are no label-free imaging descriptors related to physiological activity of inner retinal cells in the living human eye. A major reason is that inner retinal neurons are highly transparent and reflect little light, making them extremely difficult to visualize and quantify. Aim: To measure physiologically-induced optical changes of inner retinal cells despite their challenging optical properties. Approach: We developed an imaging method based on adaptive optics and optical coherence tomography (AO-OCT) and a suite of postprocessing algorithms, most notably a new temporal correlation method. Results: We captured the temporal dynamics of entire inner retinal layers, of specific tissue types, and of individual cells across three different timescales from fast (seconds) to extremely slow (one year). Time correlation analysis revealed significant differences in time constant (up to 0.4 s) between the principal layers of the inner retina with the ganglion cell layer (GCL) being the most dynamic. At the cellular level, significant differences were found between individual GCL somas. The mean time constant of the GCL somas ( 0.69 ± 0.17    s ) was ∼ 30 % smaller than that of nerve fiber bundles and inner plexiform layer synapses and processes. Across longer durations, temporal speckle contrast and time-lapse imaging revealed motion of macrophage-like cells (over minutes) and GCL neuron loss and remodeling (over one year). Conclusions: Physiological activity of inner retinal cells is now measurable in the living human eye.

In vivo brain imaging with multimodal optical coherence microscopy in a mouse model of thromboembolic photochemical stroke

We used a new multimodal imaging system that combines optical coherence microscopy and brightfield microscopy. Using this in vivo brain monitoring approach and cranial window implantation, we three-dimensionally visualized the vascular network during thrombosis, with high temporal (18 s) and spatial (axial, 2.5    μ m ; lateral, 2.2    μ m ) resolution. We used a modified mouse model of photochemical thromboembolic stroke in order to more accurately parallel human stroke. Specifically, we applied green laser illumination to focally occlude a branch of the middle cerebral artery. Despite the recanalization of the superficial arteries at 24 h after stroke, no blood flow was detected in the small vessels within deeper regions. Moreover, after 24 h of stroke progression, scattering signal enhancement was observed within the stroke region. We also evaluated the infarct extent and shape histologically. In summary, we present a novel approach for real-time mouse brain monitoring and ischemic variability analysis. This multimodal imaging method permits the analysis of thrombosis progression and reperfusion. Additionally and importantly, the system could be used to study the effect of poststroke drug treatments on blood flow in small arteries and capillaries of the brain.

Retinal analysis of a mouse model of Alzheimer's disease with multicontrast optical coherence tomography

Significance. Recent Alzheimer's disease (AD) patient studies have focused on retinal analysis, as the retina is the only part of the central nervous system that can be imaged noninvasively by optical methods. However, as this is a relatively new approach, the occurrence and role of retinal pathological features are still debated. Aim. The retina of an APP/PS1 mouse model was investigated using multicontrast optical coherence tomography (OCT) in order to provide a documentation of what was observed in both transgenic and wild-type mice. Approach. Both eyes of 24 APP/PS1 transgenic mice (age: 45 to 104 weeks) and 15 age-matched wild-type littermates were imaged by the custom-built OCT system. At the end of the experiment, retinas and brains were harvested from a subset of the mice (14 transgenic, 7 age-matched control) in order to compare the in vivo results to histological analysis and to quantify the cortical amyloid beta plaque load. Results. The system provided a combination of standard reflectivity data, polarization-sensitive data, and OCT angiograms. Qualitative and quantitative information from the resultant OCT images was extracted on retinal layer thickness and structure, presence of hyper-reflective foci, phase retardation abnormalities, and retinal vasculature. Conclusions. Although multicontrast OCT revealed abnormal structural properties and phase retardation signals in the retina of this APP/PS1 mouse model, the observations were very similar in transgenic and control mice.

Age-Related Decline of Retinal Oxygen Extraction in Healthy Subjects

Purpose

To investigate the age-dependence of total retinal blood flow and total retinal oxygen extraction in healthy subjects and determine their possible correlations with structural optical coherence tomography (OCT) parameters.

Methods

This observational cross-sectional study consisted of 68 healthy subjects (mean ± SD age, 45.6 ± 16.3 years; 47% female). Total retinal oxygen extraction was calculated based on measurement of total retinal blood flow using bi-directional Doppler OCT and measurement of oxygen saturation using spectroscopic reflectometry. Retinal nerve fiber layer thickness was measured using OCT, and the total number of retinal ganglion cells was estimated based on a previous published model. Correlation of these parameters with age was studied and the association between structural OCT parameters and hemodynamic vascular parameters was calculated.

Results

Both structural and vascular parameters showed a significant decline with increasing age. The correlation coefficients were between r = -0.25 and r = -0.41. Furthermore, structural and vascular parameters were significantly correlated with each other. The strongest association was found between the level of total retinal oxygen extraction and the number of retinal ganglion cells (r = 0.75, P < 0.001).

Conclusions

We showed that there was an age-related decline of retinal oxygen extraction. Levels of retinal oxygen extraction are correlated to retinal nerve fiber layer thickness and number of retinal ganglion cells. Our data partially explain the wide inter-individual variability in retinal blood flow values in healthy subjects. Longitudinal studies are required to study the time course of vascular and neuronal loss in humans.

Quantification of Choriocapillaris with Optical Coherence Tomography Angiography: A Comparison Study

PURPOSE

To demonstrate the variation in quantitative choriocapillaris (CC) metrics with various binarization approaches using optical coherence tomography angiography (OCTA).

DESIGN

Retrospective, observational, cross-sectional case series.

METHODS

Macular OCTA scans, 3- × 3-mm and 6- × 6-mm, were obtained from normal eyes and from eyes with drusen secondary to age-related macular degeneration (AMD). The CC slab was extracted, and the CC flow deficits (FDs) were segmented with 2 previously published algorithms: the fuzzy C-means approach (FCM method) and Phansalkar's local thresholding (Phansalkar method). Four different values for the radius were used in order to investigate the effect on the FD segmentation when using the Phansalkar method. FD density (FDD), mean FD size (MFDS), FD number (FDN), FD area (FDA) and intercapillary distance (ICD) were calculated for comparison. Repeatability was assessed as coefficient of variation (CV), and Pearson's correlation analysis was conducted.

RESULTS

Six eyes from 6 subjects with normal eyes and 6 eyes from 6 subjects with drusen secondary to AMD were scanned. The 3- × 3-mm scans resulted in higher repeatability than the 6- × 6-mm scans. For the Phansalkar method, larger values of the radius resulted in higher repeatability. ANOVA tests resulted in significant differences (P < 0.001) among the FCM method and the Phansalkar method with different radius options for all CC metrics and scan sizes investigated. In 3- × 3-mm scans, significant correlation was found between the FCM method and the Phansalkar method for all quantitative CC metrics other than FDN (all P < 0.001; 0.90 < r <0.99).

CONCLUSIONS

Quantitative CC analysis with commercially available OCTA is complicated and researchers need to pay close attention to how they conduct such analyses.

Advances in Doppler optical coherence tomography and angiography

Since the first demonstration of Doppler optical coherence tomography (OCT) in 1997, several functional extensions of Doppler OCT have been developed, including velocimetry, angiogram, and optical coherence elastography. These functional techniques have been widely used in research and clinical applications, particularly in ophthalmology. Here, we review the principles, representative methods, and applications of different Doppler OCT techniques, followed by discussion on the innovations, limitations, and future directions of each of these techniques.

Ultra-Widefield Protocol Enhances Automated Classification of Diabetic Retinopathy Severity with OCT Angiography

PURPOSE

To assess the diagnostic usefulness of retinal nonperfusion to classify eyes based on diabetic retinopathy (DR) severity on OCT angiography (OCTA) and determine whether wider field of view (FOV) OCTA protocols enhance the diagnostic usefulness of retinal nonperfusion in the classification of DR severity.

DESIGN

Retrospective cross-sectional study.

PARTICIPANTS

Diabetic patients undergoing ultra-widefield (UWF) OCTA imaging at 1 academic retina practice.

METHODS

Ultra-widefield OCTA images with 100° FOV were obtained from 60 eyes. Eyes were grouped as those with diabetes without retinopathy (DWR), those with nonproliferative diabetic retinopathy (NPDR), or those with proliferative diabetic retinopathy (PDR). The ratio of nonperfusion (RNP) was expressed as the percent area of capillary nonperfusion within the FOV. The RNP was obtained in the FOV 100° image and concentric sectors encompassing 10°, 10° to 30°, 30° to 50°, and 50° to 100°.

MAIN OUTCOME MEASURES

Mean RNP among DR groups, mean RNP measured among FOV sectors, and area under the curve (AUC) of the receiver operating characteristics when using RNP as a cutoff value to distinguish between DR groups.

RESULTS

Mean RNP from the FOV 50° to 100° sector was different among all groups: DWR, 14.6±5.1%; NPDR, 27.5±7.5%; and PDR, 41.5±19.1% (P < 0.01). Within each DR group, field of view from 50° to 100° measured higher RNP than all other sectors (P < 0.01). Field of view from 50° to 100° showed the highest optimal sensitivity and specificity to distinguish NPDR from DWR with an RNP cutoff value of 21.2% (89.5% and 88.2%; AUC, 0.944) and PDR from NPDR with an RNP cutoff value of 31.6% (79.2% and 78.9%; AUC, 0.752).

CONCLUSIONS

Ratio of nonperfusion on average is higher in more severe DR. The most peripheral sector of the widefield OCTA (FOV 50°-100°) showed on average higher RNP and showed more diagnostic usefulness in determining DR severity compared with more central sectors and the FOV 100 image as a whole.

Optical coherence tomography velocimetry based on decorrelation estimation of phasor pair ratios (DEPPAIR)

Quantitative velocity estimations in optical coherence tomography requires the estimation of the axial and lateral flow components. Optical coherence tomography measures the depth resolved complex field reflected from a sample. While the axial velocity component can be determined from the Doppler shift or phase shift between a pair of consecutive measurements at the same location, the estimation of the lateral component for in vivo applications is still challenging. One approach to determine lateral velocity is multiple simultaneous measurements at different angles. In another approach the lateral component can be retrieved through repeated measurements at (nearly) the same location by an analysis of the decorrelation over time. In this paper we follow the latter approach. We describe a model for the complex field changes between consecutive measurements and use it to predict the uncertainties for amplitude-based, phase-based and complex algorithms. The uncertainty of the flow estimations follows from a statistical analysis and is determined by the number of available measurements and the applied analysis method. The model is verified in phantom measurements and the dynamic range of velocity estimations is investigated. We demonstrate that phase-based and complex (phasor) based lateral flow estimation methods are superior to amplitude-based algorithms.

Automated quantification of superficial retinal capillaries and large vessels for diabetic retinopathy on optical coherence tomographic angiography

Optical coherence tomography angiography (OCTA) is a relatively new technique with capillary-level resolution, which has shown great potential for the diagnosis of diabetic retinopathy (DR). A fully automatic algorithm for the quantitative measurement of microcirculatory changes in sight-threatening DR is presented. The foveal avascular zone (FAZ) segmentation was improved with a graph-theoretic method and the large vessels and capillaries were separately identified and analyzed. The method was evaluated in healthy and diabetic eyes with various stages of retinopathy. Results showed that, compared with the healthy group, the diabetic group showed a significantly larger large vessel density, but a significantly smaller capillary density (P < .001). Circularity of FAZ was significantly smaller while nonperfusion area was significantly larger in the diabetic group. The combined variable of all image metrics reached an area under the ROC of 0.853 (95% CI, 0.784-0.923) for mild to moderate nonproliferative DR and 0.950 (95% CI, 0.922-0.979) for proliferative DR. Microvascular and FAZ changes with various DR stages can be accurately delineated using the developed automatic program. Quantitative metrics on OCTA serve as potential biomarkers for the staging of DR.

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.

Age-Related Changes in Choroidal Thickness and the Volume of Vessels and Stroma Using Swept-Source OCT and Fully Automated Algorithms

PURPOSE

To determine age-related changes in choroidal thickness and the volume of choroidal vessels and stroma using automated algorithms based on structural swept-source OCT (SS-OCT) scans.

DESIGN

Prospective and observational study.

PARTICIPANTS

The study included 144 normal participants with ages ranging from 20 to 88 years.

METHODS

A previously reported strategy was used to automatically segment the choroid using SS-OCT structural images. Attenuation correction was applied on B-scans to enhance the choroidal contrast and facilitate more accurate automatic segmentation of the 3-dimensional choroidal vessel and stroma. The parameters that we investigated included mean choroidal thickness (MCT), choroidal vessel volume (CVV), choroidal stroma volume (CSV), choroid vascularity index (CVI), and the choroidal stroma-to-vessel volume ratio (CSVR). Correlations between MCT and choroidal vessel metrics of CVV, CSV, CVI, and CSVR were studied. Regional distributions of MCT and CVI were analyzed using a grid centered on the fovea. Age-related changes in MCT, CVV, CSV, and CVI were studied in the entire scanning region, as well as in the subregions of the grids.

MAIN OUTCOME MEASURES

Age-related changes in MCT, CVV, CSV, and CVI using 6×6-mm and 12×12-mm SS-OCT scans.

RESULTS

The automated choroid segmentations were validated against manual segmentations, and MCT measurements were shown to be in good agreement (P < 0.0001). Choroidal vessel volume and CSV showed significant correlations with MCT (all P < 0.0001). Interestingly, CVI and CSVR were constant, with little variation among all participants regardless of age and MCT (61.1±1.8% and 0.64±0.05, respectively). Measurements on 12×12-mm and 6×6-mm scans showed excellent agreement in all scan regions (all P < 0.0001). While choroidal thickness and choroidal volume, which includes both choroidal vessels and stroma, decrease with age (all P < 0.0001), the CVI and CSVR vary little among all ages in all regions.

CONCLUSIONS

Whereas MCT, CVV, and CSV decrease with age, the CVI and CSVR remain constant in all regions with age. Ongoing studies are using these automated algorithms on SS-OCT structural datasets to investigate the diagnostic usefulness of these choroidal parameters in a myriad of ocular and systemic diseases.

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.

Handheld swept-source optical coherence tomography with angiography in awake premature neonates

Background

Retinopathy of prematurity (ROP) can lead to retinal detachment and severe vision loss and is a common cause of childhood blindness. Optical coherence tomography angiography (OCTA) is a non-invasive imaging modality that can be used to detect potential abnormalities in the microvasculature in this population. The objective of this study is to assess the feasibility of a newly developed handheld swept source OCT (SS-OCT) device to successfully acquire structural vitreoretinal and retinal microvascular images in awake premature infants.

Methods

OCT and OCTA images were acquired at the time of routine ROP examinations from awake, unsedated preterm infants in the Neonatal Intensive Care Unit using a clinical research prototype handheld probe integrated with an SS-OCT system working at 1,060 nm wavelength and an imaging speed of 200,000 A-scans per second (200 kHz), enabling volume OCT and OCTA scans. Each volume was acquired with approximately 36˚ field of view (~6.3×6.3 mm in infants) in 4.8 s. Quality of acquired OCT and OCTA volume images, microvascular information, and vitreoretinal features were determined by 3-masked grader consensus.

Results

Twelve infants (5 females, mean gestational age 28.3 weeks, median birth weight 901 g, stages 0 to 3 ROP) underwent a total of 73 individual eye imaging sessions. High-quality OCT images of the fovea and the optic nerve were present in 69/73 (94.5%) and 56/73 (76.7%) scans, respectively. Vitreous bands were observed in 10/73 (13.7%); punctate hyperreflective vitreous opacities in 47/73 (64.4%); epiretinal membrane (ERM) in 6/73 (8.2%); and cystoid macular edema (CME) in 12/73 (16.4%) scans. Mild vessel elevation was noted in 3/73 (4.1%) images, and severe vessel elevation in 4/73 (5.5%) scans. OCTA images obtained in 8 awake infants revealed good quality images of the foveal microvasculature in 11/19 (58%) eye imaging sessions for 6/8 (75%) infants; and peripapillary microvasculature in 14/19 (74%) eye imaging sessions for 5/8 (63%) infants.

Conclusions

The SS-OCTA handheld device can capture important vitreoretinal characteristics such as peripapillary and foveal microvasculature, as well as hyperreflective punctate vitreous opacities and tractional vitreous bands, which may predict ROP severity. These images were captured in awake, premature infants without the use of direct ocular contact, an eyelid speculum, or sedation.

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.

Phase-sensitive interferometry of decorrelated speckle patterns

Phase-sensitive coherent imaging exploits changes in the phases of backscattered light to observe tiny alterations of scattering structures or variations of the refractive index. But moving scatterers or a fluctuating refractive index decorrelate the phases and speckle patterns in the images. It is generally believed that once the speckle pattern has changed, the phases are scrambled and any meaningful phase difference to the original pattern is removed. As a consequence, diffusion and tissue motion that cannot be resolved, prevent phase-sensitive imaging of biological specimens. Here, we show that a phase comparison between decorrelated speckle patterns is still possible by utilizing a series of images acquired during decorrelation. The resulting evaluation scheme is mathematically equivalent to methods for astronomic imaging through the turbulent sky by speckle interferometry. We thus adopt the idea of speckle interferometry to phase-sensitive imaging in biological tissues and demonstrate its efficacy for simulated data and imaging of photoreceptor activity with phase-sensitive optical coherence tomography. We believe the described methods can be applied to many imaging modalities that use phase values for interferometry.

Optical coherence tomography angiography and photoacoustic imaging in dermatology

Optical coherence tomography angiography (OCTA) is a relatively novel functional extension of the widely accepted ophthalmic imaging tool named optical coherence tomography (OCT). Since OCTA's debut in ophthalmology, researchers have also been trying to expand its translational application in dermatology. The ability of OCTA to resolve microvasculature has shown promising results in imaging skin diseases. Meanwhile, photoacoustic imaging (PAI), which uses laser pulse induced ultrasound waves as the signal, has been studied to differentiate human skin layers and to help in skin disease diagnosis. This perspective article gives a short review of OCTA and PAI in the field of photodermatology. After an introduction to the principles of OCTA and PAI, we describe the most updated results of skin disease imaging using these two optical imaging modalities. We also place emphasis on dual modality imaging combining OCTA and photoacoustic tomography (PAT) for dermatological applications. In the end, the challenges and prospects of these two imaging modalities in dermatology are discussed.

Longitudinal in vivo intrinsic optical imaging of cortical blood perfusion and tissue damage in focal photothrombosis stroke model

A thorough understanding of the spatiotemporal dynamics of blood supply and tissue viability is of great importance in stroke researches. In the current study, vascular and cellular responses to focal ischemia were monitored with optical coherence tomography on chronic rat photothrombotic stroke model. The 3D mapping of blood perfusion and cellular scattering were achieved by analyzing the temporal dynamics and depth attenuation of intrinsic backscattered light respectively. Optical coherence tomography revealed that vessels of different types presented various spatial and temporal dynamics during the photothrombotic occlusion and the later recovery period. The large distal middle cerebral arteries presented a spontaneous recanalization and the small pial microvessels presented a reperfusion along with newly appeared vessels from the peripheral into the core area. The cortical capillary perfusion presented a weak recovery. Compared to the male group, the female rats showed a faster vascular recovery after photothrombotic. Moreover, the dynamic changes of the cellular scattering signal showed a high spatial and temporal correlation with the cortical capillary perfusion. Combined with well-designed photothrombotic stroke model and chronic optical window, optical coherence tomography imaging offers a unique approach to improve the understanding of stroke procedure and evaluate the treatment outcomes.

Contrast-enhanced serial optical coherence scanner with deep learning network reveals vasculature and white matter organization of mouse brain

Optical coherence tomography provides volumetric reconstruction of brain structure with micrometer resolution. Gray matter and white matter can be highlighted using conventional and polarization-based contrasts; however, vasculature in ex-vivo fixed brain has not been investigated at large scale due to lack of intrinsic contrast. We present contrast enhancement to visualize the vasculature by perfusing titanium dioxide particles transcardially into the mouse vascular system. The brain, after dissection and fixation, is imaged by a serial optical coherence scanner. Accumulation of particles in blood vessels generates distinguishable optical signals. Among these, the cross-polarization images reveal the vasculature organization remarkably well. The conventional and polarization-based contrasts are still available for probing the gray matter and white matter structures. The segmentation and reconstruction of the vasculature are presented by using a deep learning algorithm. Axonal fiber pathways in the mouse brain are delineated by utilizing the retardance and optic axis orientation contrasts. This is a low-cost method that can be further developed to study neurovascular diseases and brain injury in animal models.

Three-dimensional structural and angiographic evaluation of foveal ischemia in diabetic retinopathy: method and validation

Optical coherence tomography angiography (OCTA) allows us to noninvasively investigate foveal ischemia, a key feature of diabetic retinopathy (DR). However, the sizes of the foveal avascular zone (FAZ) have a significant variation in normal population, preventing the objective assessment of pathological enlargement of FAZ due to capillary dropout. Based on the relationship between FAZ and ganglion cell complex (GCC) thickness in normal eyes, we defined a theoretical baseline FAZ (tbFAZ) on structural OCT and measured 2D and 3D vessel density in its vicinity on the simultaneously acquired OCTA in normal and diabetic eyes. We found that the structure-based tbFAZ was a reliable reference to identify foveal ischemia and that the 3D vessel density demonstrated ischemia more effectively than the 2D method. The proposed 3D para-FAZ vessel density correlates well with DR severity and potentially is a useful diagnostic biomarker, especially in the early stages of DR.

Changes in Choroidal Thickness and Choroidal Blood Perfusion in Guinea Pig Myopia

Purpose

The purpose of this study was to study changes in choroidal thickness (ChT) and choroidal blood perfusion (ChBP), and the correlation between them, in guinea pig myopia.

Methods

The reliability of optical coherence tomography angiography (OCTA) for measuring ChT and ChBP was verified in guinea pigs, after cervical dislocation (n = 7) or temporal ciliary artery transection (n = 6). Changes in refraction, axial length, ChT, and ChBP were measured during spontaneous myopia (n = 9), monocular form-deprivation myopia (FDM, n = 13), or lens-induced myopia (LIM, n = 14), and after 4 days of recovery from FDM and LIM.

Results

The abolition (by cervical dislocation) or reduction (by temporal ciliary artery transection) of ChBP, and of the associated changes in ChT, were verified by OCTA, thus validating the method of measurement. In the spontaneous myopia group, ChT and ChBP were reduced by 25.2% and 31.9%, respectively. In FDM eyes, mean ± SD ChT and ChBP decreased significantly compared with the untreated fellow eyes (ChT fellow: 76.13 ± 9.34 μm versus 64.76 ± 11.15 μm for FDM; ChBP fellow: 37.87 ± 6.37 × 103 versus 30.27 ± 6.06 × 103 for FDM) and increased after 4 days of recovery (ChT: 77.94 ± 12.57 μm; ChBP: 37.41 ± 6.11 × 103). Effects of LIM were similar to those of FDM. Interocular differences in ChT and ChBP were significantly correlated in each group (FDM: R = 0.71, P < 0.001; LIM: R = 0.53, P < 0.001).

Conclusions

ChT and ChBP were significantly decreased in all three models of guinea pig myopia, and they both increased during recovery. Changes in ChT were positively correlated with changes in ChBP. Therefore, it is possible that the changes of ChT are responsible for the changes of ChBP or vice versa.

A Review of OCT Angiography in Glaucoma

There is growing evidence that vascular dysfunction plays a role in the pathogenesis of glaucoma. The details of this relationship have remained elusive partially due to limitations in our ability to assess blood flow in the optic nerve. Optical coherence tomography angiography (OCTA) has emerged as a promising new technology well positioned to become the first clinically suitable test of optic nerve perfusion. OCTA uses the motion of red blood cells as an intrinsic contrast agent to create reproducible images of microvascular networks rapidly and non-invasively. A significant body of research regarding the use of OCTA in glaucoma has emerged in recent years. This review aims to provide an overview of the basic principles underlying OCTA technology, summarize the current literature regarding the application of OCTA in the management of glaucoma, and address the role of OCTA in explicating the vascular pathogenesis of glaucoma.

Visualizing choriocapillaris using swept-source optical coherence tomography angiography with various probe beam sizes

Imaging choriocapillaris (CC) is a long-term challenge for commercial OCT angiography (OCTA) systems due to limited transverse resolution. Effects of transverse resolution on the visualization of a CC microvascular network are explored and demonstrated in this paper. We use three probe beams with sizes of ~1.12 mm, ~2.51 mm and ~3.50 mm at the pupil plane, which deliver an estimated transverse resolution at the retina of 17.5 µm, 8.8 µm and 7.0 µm, respectively, to investigate the ability of OCTA to resolve the CC capillary vessels. The complex optical microangiography algorithm is applied to extract blood flow in the CC slab. Mean retinal pigment epithelium (RPE) to CC (RPE-CC) distance, mean CC inter-vascular spacing and the magnitude in the radially-averaged power spectrum are quantified. We demonstrate that a clearer CC lobular capillary network is resolved in the angiograms provided by a larger beam size. The image contrast of the CC angiogram with a large beam size of 3.50 mm is 114% higher than that with a small beam size of 1.12 mm. While the measurements of the mean RPE-CC distance and CC inter-vascular spacing are almost consistent regardless of the beam sizes, they are more reliable and stable with the larger beam size of 3.50 mm. We conclude that the beam size is a key parameter for CC angiography if the purpose of the investigation is to visualize the individual CC capillaries.

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.

OCT Angiography and Cone Photoreceptor Imaging in Geographic Atrophy

Purpose

To compare cone spacing and choriocapillaris (CC) perfusion adjacent to geographic atrophy (GA) in patients with age-related macular degeneration (AMD) and age-similar normal eyes.

Methods

Subjects were imaged using adaptive optics scanning laser ophthalmoscopy (AOSLO), fundus autofluorescence (FAF), and swept-source optical coherence tomography angiography. The GA border was identified using FAF images; CC flow void was analyzed in 1° regions extending from the GA border. A grader masked to CC perfusion selected regions of interest (ROIs) with unambiguous cone mosaics in AOSLO images. At each ROI, cone spacing and CC flow void were converted to Z-scores (standard deviations from the mean of 12 normal eyes aged 50 to 81 years for cone spacing, and 60 normal eyes age 51 to 88 years for CC flow void).

Results

Excluding regions of GA and drusen, CC flow void in eight eyes of six patients with AMD was significantly greater than in four age-similar normal eyes (exact permutation test, P = 0.024). CC flow void was negatively correlated with distance from the GA margin (r = -0.35; 95% confidence interval [CI], -0.53 to -0.12). Increased cone spacing was significantly correlated with CC flow void (r = 0.33; 95% CI, 0.12 to 0.59). Cone spacing was increased in 39% of ROIs, while CC flow void was increased in 96% of ROIs.

Conclusions

In eyes with GA due to AMD, CC hypoperfusion was significantly correlated with, and more extensive than, cone photoreceptor loss. The results suggest that reduced CC perfusion contributes to the development of GA.

Pericyte constriction underlies capillary derecruitment during hyperemia in the setting of arterial stenosis

Capillary derecruitment distal to a coronary stenosis is implicated as the mechanism of reversible perfusion defect and potential myocardial ischemia during coronary hyperemia; however, the underlying mechanisms are not defined. We tested whether pericyte constriction underlies capillary derecruitment during hyperemia under conditions of stenosis. In vivo two-photon microscopy (2PM) and optical microangiography (OMAG) were used to measure hyperemia-induced changes in capillary diameter and perfusion in wild-type and pericyte-depleted mice with femoral artery stenosis. OMAG demonstrated that hyperemic challenge under stenosis produced capillary derecruitment associated with decreased RBC flux. 2PM demonstrated that hyperemia under control conditions induces 26 ± 5% of capillaries to dilate and 19 ± 3% to constrict. After stenosis, the proportion of capillaries dilating to hyperemia decreased to 14 ± 4% (P = 0.05), whereas proportion of constricting capillaries increased to 32 ± 4% (P = 0.05). Hyperemia-induced changes in capillary diameter occurred preferentially in capillary segments invested with pericytes. In a transgenic mouse model featuring partial pericyte depletion, only 14 ± 3% of capillaries constricted to hyperemic challenge after stenosis, a significant reduction from 33 ± 4% in wild-type littermate controls (P = 0.04). These results provide for the first time direct visualization of hyperemia-induced capillary derecruitment distal to arterial stenosis and demonstrate that pericyte constriction underlies this phenomenon in vivo. These results could have important therapeutic implications in the treatment of exercise-induced ischemia. NEW & NOTEWORTHY In the setting of coronary arterial stenosis, hyperemia produces a reversible perfusion defect resulting from capillary derecruitment that is believed to underlie cardiac ischemia under hyperemic conditions. We use optical microangiography and in vivo two-photon microscopy to visualize capillary derecruitment distal to a femoral arterial stenosis with cellular resolution. We demonstrate that capillary constriction in response to hyperemia in the setting of stenosis is dependent on pericytes, contractile mural cells investing the microcirculation.

Revealing the morphology and function of the cochlea and middle ear with optical coherence tomography

Optical coherence tomography (OCT) has revolutionized physiological studies of the hearing organ, the vibration and morphology of which can now be measured without opening the surrounding bone. In this review, we provide an overview of OCT as used in the otological research, describing advances and different techniques in vibrometry, angiography, and structural imaging.

Optical coherence tomography correlates multiple measures of tissue damage following acute burn injury

Background

The visual assessment of burned skin is inherently subjective, and whilst a number of imaging modalities have identified quantifiable parameters to characterize vascular and structural changes following burn damage, none have become common place in the assessment protocol. Here, we use optical coherence tomography (OCT)-based angiography (OCTA) to introduce novel correlations between vessel depth, i.e., the depth of functional blood vessels beneath the tissue surface, edema depth, i.e., the depth of interstitial fluid buildup beneath the tissue surface, and tissue injury depth, i.e., the depth of collagen denaturation beneath the tissue surface, following burn injury.

Methods

A clinical prototype OCT system was used to collect OCT images from various sites of burned skin in patients. Optical microangiography (OMAG) algorithm was used to derive OCTA information from the acquired OCT images, from which the presence of blood vessels and edema were detected. The optical attenuation mapping of structural OCT information was used to detect tissue injury depth. The depths of vessel, edema and tissue injury were measured using a semi-automatic segmentation algorithm. Correlation analysis was performed using a Pearson correlation coefficient using one-tailed analysis with significance being established by a P value ≤0.05.

Results

Four burn patients were recruited and scanned at multiple sites using the prototype system within 3-6 days of injury. Approximate measurements include a vessel depth range of 320-1,360 µm, an edema depth range of 0-400 µm, and a tissue injury depth range of 130-420 µm. Correlations were subsequently observed between vessel depth and edema depth (r=0.8521, P=0.0001), and vessel depth and tissue injury depth (r=0.6296, P=0.0106).

Conclusions

OCT is feasible to provide the critical information of vessel depth, edema depth, and tissue injury depth of skin burns, which may represent viable assessment criteria for the characterization of cutaneous burns in future.

En face optical coherence tomography: a technology review [Invited]

A review on the technological development of en face optical coherence tomography (OCT) and optical coherence microscopy (OCM) is provided. The terminology originally referred to time domain OCT, where the preferential scanning was performed in the en face plane. Potentially the fastest realization of en face image recording is full-field OCT, where the full en face plane is illuminated and recorded simultaneously. The term has nowadays been adopted for high-speed Fourier domain approaches, where the en face image is reconstructed from full 3D volumes either by direct slicing or through axial projection in post processing. The success of modern en face OCT lies in its immediate and easy image interpretation, which is in particular of advantage for OCM or OCT angiography. Applications of en face OCT with a focus on ophthalmology are presented. The review concludes by outlining exciting technological prospects of en face OCT based both on time as well as on Fourier domain OCT.

Development of a clinical prototype of a miniature hand-held optical coherence tomography probe for prematurity and pediatric ophthalmic imaging

We report a novel design and operation of a highly integrated miniature handheld OCT probe, with high-speed angiography function that can be used in clinical settings for young children and infants, providing rapid, non-invasive structural and angiographic imaging of the retina and choroid. The imaging system is operated at 200 kHz, with 3D OCT and OCTA scan time of 0.8 and 3.2 seconds, respectively, and the scanning angle on the pupil is ± 36°, covering the full perifoveal region. Operator assisting features of the direct-view iris camera and on-probe display are integrated into the hand-held probe, and the fixation target can display animations to attract the attention of young subjects. Compared to conventional OCT systems, the high-speed hand-held OCT system significantly improves the operator's experience and scanning efficiency, which is important for imaging infants. Imaging results indicate a significant reduction in total time consumption in pediatric ophthalmic imaging sessions, as well as the image quality of OCT angiography.

[Optical coherence tomography angiography in ophthalmic oncology]

The article reviews literature on various methods of optical coherence tomography (OCT). The review also describes clinical aspects of OCT angiography and its applications in ophthalmic oncology.