The fundus photo has met its match: optical coherence tomography and adaptive optics ophthalmoscopy are here to stay

A Prospective, Observational, Non-interventional Clinical Study of Participants With Choroideremia: The NIGHT Study

PURPOSE

The NIGHT study aimed to assess the natural history of choroideremia (CHM), an X-linked inherited chorioretinal degenerative disease leading to blindness, and determine which outcomes would be the most sensitive for monitoring disease progression.

DESIGN

A prospective, observational, multicenter cohort study.

METHODS

Males aged ≥18 years with genetically confirmed CHM, visible active disease within the macular region, and best-corrected visual acuity (BCVA) ≥34 Early Treatment Diabetic Retinopathy Study (ETDRS) letters at baseline were assessed for 20 months. The primary outcome was the change in BCVA over time at Months 4, 8, 12, 16, and 20. A range of functional and anatomical secondary outcome measures were assessed up to Month 12, including retinal sensitivity, central ellipsoid zone (EZ) area, and total area of fundus autofluorescence (FAF). Additional ocular assessments for safety were performed.

RESULTS

A total of 220 participants completed the study. The mean BCVA was stable over 20 months. Most participants (81.4% in the worse eye and 77.8% in the better eye) had change from baseline > -5 ETDRS letters at Month 20. Interocular symmetry was low overall. Reductions from baseline to Month 12 were observed (worse eye, better eye) for retinal sensitivity (functional outcome; -0.68 dB, -0.48 dB), central EZ area (anatomical outcome; -0.276 mm2, -0.290 mm2), and total area of FAF (anatomical outcome; -0.605 mm2, -0.533 mm2). No assessment-related serious adverse events occurred.

CONCLUSIONS

Retinal sensitivity, central EZ area, and total area of FAF are more sensitive than BCVA in measuring the natural progression of CHM.

Robotic Optical Coherence Tomography Retinal Imaging for Emergency Department Patients: A Pilot Study for Emergency Physicians' Diagnostic Performance

STUDY OBJECTIVE

To evaluate the diagnostic performance of emergency physicians' interpretation of robotically acquired retinal optical coherence tomography images for detecting posterior eye abnormalities in patients seen in the emergency department (ED).

METHODS

Adult patients presenting to Duke University Hospital emergency department from November 2020 through October 2021 with acute visual changes, headache, or focal neurologic deficit(s) who received an ophthalmology consultation were enrolled in this pilot study. Emergency physicians provided standard clinical care, including direct ophthalmoscopy, at their discretion. Retinal optical coherence tomography images of these patients were obtained with a robotic, semi-autonomous optical coherence tomography system. We compared the detection of abnormalities in optical coherence tomography images by emergency physicians with a reference standard, a combination of ophthalmology consultation diagnosis and retina specialist optical coherence tomography review.

RESULTS

Nine emergency physicians reviewed the optical coherence tomography images of 72 eyes from 38 patients. Based on the reference standard, 33 (46%) eyes were normal, 16 (22%) had at least 1 urgent/emergency abnormality, and the remaining 23 (32%) had at least 1 nonurgent abnormality. Emergency physicians' optical coherence tomography interpretation had 69% (95% confidence interval [CI], 49% to 89%) sensitivity for any abnormality, 100% (95% CI, 79% to 100%) sensitivity for urgent/emergency abnormalities, 48% (95% CI, 28% to 68%) sensitivity for nonurgent abnormalities, and 64% (95% CI, 44% to 84%) overall specificity. In contrast, emergency physicians providing standard clinical care did not detect any abnormality with direct ophthalmoscopy.

CONCLUSION

Robotic, semi-autonomous optical coherence tomography enabled ocular imaging of emergency department patients with a broad range of posterior eye abnormalities. In addition, emergency provider optical coherence tomography interpretation was more sensitive than direct ophthalmoscopy for any abnormalities, urgent/emergency abnormalities, and nonurgent abnormalities in this pilot study with a small sample of patients and emergency physicians.

Twenty-five years of clinical applications using adaptive optics ophthalmoscopy [Invited]

Twenty-five years ago, adaptive optics (AO) was combined with fundus photography, thereby initiating a new era in the field of ophthalmic imaging. Since that time, clinical applications of AO ophthalmoscopy to investigate visual system structure and function in both health and disease abound. To date, AO ophthalmoscopy has enabled visualization of most cell types in the retina, offered insight into retinal and systemic disease pathogenesis, and been integrated into clinical trials. This article reviews clinical applications of AO ophthalmoscopy and addresses remaining challenges for AO ophthalmoscopy to become fully integrated into standard ophthalmic care.

Insights into Sickle Cell Disease through the Retinal Microvasculature: Adaptive Optics Scanning Light Ophthalmoscopy Correlates of Clinical OCT Angiography

PURPOSE

Clinical OCT angiography (OCTA) of the retinal microvasculature offers a quantitative correlate to systemic disease burden and treatment efficacy in sickle cell disease (SCD). The purpose of this study was to use the higher resolution of adaptive optics scanning light ophthalmoscopy (AOSLO) to elucidate OCTA features of parafoveal microvascular compromise identified in SCD patients.

DESIGN

Case series of 11 SCD patients and 1 unaffected control.

PARTICIPANTS

A total of 11 eyes of 11 SCD patients (mean age, 33 years; range, 23-44; 8 female, 3 male) and 1 eye of a 34-year-old unaffected control.

METHODS

Ten sequential 3 × 3 mm parafoveal OCTA full vascular slab scans were obtained per eye using a commercial spectral domain OCT system (Avanti RTVue-XR; Optovue). These were used to identify areas of compromised perfusion near the foveal avascular zone (FAZ), designated as regions of interest (ROIs). Immediately thereafter, AOSLO imaging was performed on these ROIs to examine the cellular details of abnormal perfusion. Each participant was imaged at a single cross-sectional time point. Additionally, 2 of the SCD patients were imaged prospectively 2 months after initial imaging to study compromised capillary segments across time and with treatment.

MAIN OUTCOME MEASURES

Detection and characterization of parafoveal perfusion abnormalities identified using OCTA and resolved using AOSLO imaging.

RESULTS

We found evidence of abnormal blood flow on OCTA and AOSLO imaging among all 11 SCD patients with diverse systemic and ocular histories. Adaptive optics scanning light ophthalmoscopy imaging revealed a spectrum of phenomena, including capillaries with intermittent blood flow, blood cell stasis, and sites of thrombus formation. Adaptive optics scanning light ophthalmoscopy imaging was able to resolve single sickled red blood cells, rouleaux formations, and blood cell-vessel wall interactions. OCT angiography and AOSLO imaging were sensitive enough to document improved retinal perfusion in an SCD patient 2 months after initiation of oral hydroxyurea therapy.

CONCLUSIONS

Adaptive optics scanning light ophthalmoscopy imaging was able to reveal the cellular details of perfusion abnormalities detected using clinical OCTA. The synergy between these clinical and laboratory imaging modalities presents a promising avenue in the management of SCD through the development of noninvasive ocular biomarkers to prognosticate progression and measure the response to systemic treatment.

Automated Assessment of Photoreceptor Visibility in Adaptive Optics Split-Detection Images Using Edge Detection

Purpose

Adaptive optics scanning laser ophthalmoscopy (AOSLO) is a high-resolution imaging modality that allows measurements of cellular-level retinal changes in living patients. In retinal diseases, the visibility of photoreceptors in AOSLO images is affected by pathology, patient motion, and optics, which can lead to variability in analyses of the photoreceptor mosaic. Current best practice for AOSLO mosaic quantification requires manual assessment of photoreceptor visibility across overlapping images, a laborious and time-consuming task.

Methods

We propose an automated measure for quantification of photoreceptor visibility in AOSLO. Our method detects salient edge features, which can represent visible photoreceptor boundaries in each image. We evaluate our measure against two human graders and two standard automated image quality assessment algorithms.

Results

We evaluate the accuracy of pairwise ordering (PO) and the correlation of ordinal rankings (ORs) of photoreceptor visibility in 29 retinal regions, taken from five subjects with choroideremia. The proposed measure had high association with manual assessments (Grader 1: PO = 0.71, OR = 0.61; Grader 2: PO = 0.67, OR = 0.62), which is comparable with intergrader reliability (PO = 0.76, OR = 0.75) and outperforms the top standard approach (PO = 0.57; OR = 0.46).

Conclusions

Our edge-based measure can automatically assess photoreceptor visibility and order overlapping images within AOSLO montages. This can significantly reduce the manual labor required to generate high-quality AOSLO montages and enables higher throughput for quantitative studies of photoreceptors.

Translational Relevance

Automated assessment of photoreceptor visibility allows us to more rapidly quantify photoreceptor morphology in the living eye. This has applications to ophthalmic medicine by allowing detailed characterization of retinal degenerations, thus yielding potential biomarkers of treatment safety and efficacy.

Short-term Assessment of Subfoveal Injection of Adeno-Associated Virus-Mediated hCHM Gene Augmentation in Choroideremia Using Adaptive Optics Ophthalmoscopy

Importance

Subretinal injection for gene augmentation in retinal degenerations forcefully detaches the neural retina from the retinal pigment epithelium, potentially damaging photoreceptors and/or retinal pigment epithelium cells.

Objective

To use adaptive optics scanning light ophthalmoscopy (AOSLO) to assess the short-term integrity of the cone mosaic following subretinal injections of adeno-associated virus vector designed to deliver a functional version of the CHM gene (AAV2-hCHM) in patients with choroideremia.

Design, Setting, and Participants

This longitudinal case series study enrolled adult patients with choroideremia from February 2015 to January 2016 in the US. To be included in the study, study participants must have received uniocular subfoveal injections of low-dose (5 × 1010 vector genome per eye) or high-dose (1 × 1011 vector genome per eye) AAV2-hCHM. Analysis began February 2015.

Main Outcomes and Measures

The macular regions of both eyes were imaged before and 1 month after injection using a custom-built multimodal AOSLO. Postinjection cone inner segment mosaics were compared with preinjection mosaics at multiple regions of interest. Colocalized spectral-domain optical coherence tomography and dark-adapted cone sensitivity was also acquired at each time point.

Results

Nine study participants ranged in age from 26 to 50 years at the time of enrollment, and all were White men. Postinjection AOSLO images showed preservation of the cone mosaic in all 9 AAV2-hCHM-injected eyes. Mosaics appeared intact and contiguous 1 month postinjection, with the exception of foveal disruption in 1 patient. Optical coherence tomography showed foveal cone outer segment shortening postinjection. Cone-mediated sensitivities were unchanged in 8 of 9 injected and 9 of 9 uninjected eyes. One participant showed acute loss of foveal optical coherence tomography cone outer segment-related signals along with cone sensitivity loss that colocalized with disruption of the mosaic on AOSLO.

Conclusions and Relevance

Integrity of the cone mosaic is maintained following subretinal delivery of AAV2-hCHM, providing strong evidence in support of the safety of the injections. Minor foveal thinning observed following surgery corresponds with short-term cone outer segment shortening rather than cone cell loss. Foveal cone loss in 1 participant raises the possibility of individual vulnerability to the subretinal injection.

Detection of retinopathy disease using morphological gradient and segmentation approaches in fundus images

BACKGROUND AND OBJECTIVE

Diabetes-related cases can cause glaucoma, cataracts, optic neuritis, paralysis of the eye muscles, or various retinal damages over time. Diabetic retinopathy is the most common form of blindness that occurs with diabetes. Diabetic retinopathy is a disease that occurs when the blood vessels in the retina of the eye become damaged, leading to loss of vision in advanced stages. This disease can occur in any diabetic patient, and the most important factor in treating the disease is early diagnosis. Nowadays, deep learning models and machine learning methods, which are open to technological developments, are already used in early diagnosis systems. In this study, two publicly available datasets were used. The datasets consist of five types according to the severity of diabetic retinopathy. The objectives of the proposed approach in diabetic retinopathy detection are to positively contribute to the performance of CNN models by processing fundus images through preprocessing steps (morphological gradient and segmentation approaches). The other goal is to detect efficient sets from type-based activation sets obtained from CNN models using Atom Search Optimization method and increase the classification success.

METHODS

The proposed approach consists of three steps. In the first step, the Morphological Gradient method is used to prevent parasitism in each image, and the ocular vessels in fundus images are extracted using the segmentation method. In the second step, the datasets are trained with transfer learning models and the activations for each class type in the last fully connected layers of these models are extracted. In the last step, the Atom Search optimization method is used, and the most dominant activation class is selected from the extracted activations on a class basis.

RESULTS

When classified by the severity of diabetic retinopathy, an overall accuracy of 99.59% was achieved for dataset #1 and 99.81% for dataset #2.

CONCLUSIONS

In this study, it was found that the overall accuracy achieved with the proposed approach increased. To achieve this increase, the application of preprocessing steps and the selection of the dominant activation sets from the deep learning models were implemented using the Atom Search optimization method.

Absence of MMACHC in peripheral retinal cells does not lead to an ocular phenotype in mice

Combined methylmalonic aciduria with homocystinuria (cblC type) is a rare disease caused by mutations in the MMACHC gene. MMACHC encodes an enzyme crucial for intracellular vitamin B₁₂ metabolism, leading to the accumulation of toxic metabolites e.g. methylmalonic acid (MMA) and homocysteine (Hcy), and secondary disturbances in folate and one-carbon metabolism when not fully functional. Patients with cblC deficiency often present in the neonatal or early childhood period with a severe multisystem pathology, which comprises a broad spectrum of treatment-resistant ophthalmological phenotypes, including retinal degeneration, impaired vision, and vascular changes. To examine the potential function of MMACHC in the retina and how its loss may impact disease, we performed gene expression studies in human and mouse, which showed that local expression of MMACHC in the retina and retinal pigment epithelium is relatively stable over time. To study whether functional MMACHC is required for retinal function and tissue integrity, we generated a transgenic mouse lacking Mmachc expression in cells of the peripheral retina. Characterization of this mouse revealed accumulation of cblC disease related metabolites, including MMA and the folate-dependent purine synthesis intermediates AICA-riboside and SAICA-riboside in the retina. Nevertheless, fundus appearance, morphology, vasculature, and cellular composition of the retina, as well as ocular function, remained normal in mice up to 6 or 12 months of age. Our data indicates that peripheral retinal neurons do not require intrinsic expression of Mmachc for survival and function and questions whether a local MMACHC deficiency is responsible for the retinal phenotypes in patients.

RetFluidNet: Retinal Fluid Segmentation for SD-OCT Images Using Convolutional Neural Network

Age-related macular degeneration (AMD) is one of the leading causes of irreversible blindness and is characterized by fluid-related accumulations such as intra-retinal fluid (IRF), subretinal fluid (SRF), and pigment epithelial detachment (PED). Spectral-domain optical coherence tomography (SD-OCT) is the primary modality used to diagnose AMD, yet it does not have algorithms that directly detect and quantify the fluid. This work presents an improved convolutional neural network (CNN)-based architecture called RetFluidNet to segment three types of fluid abnormalities from SD-OCT images. The model assimilates different skip-connect operations and atrous spatial pyramid pooling (ASPP) to integrate multi-scale contextual information; thus, achieving the best performance. This work also investigates between consequential and comparatively inconsequential hyperparameters and skip-connect techniques for fluid segmentation from the SD-OCT image to indicate the starting choice for future related researches. RetFluidNet was trained and tested on SD-OCT images from 124 patients and achieved an accuracy of 80.05%, 92.74%, and 95.53% for IRF, PED, and SRF, respectively. RetFluidNet showed significant improvement over competitive works to be clinically applicable in reasonable accuracy and time efficiency. RetFluidNet is a fully automated method that can support early detection and follow-up of AMD.

Emulated retinal image capture (ERICA) to test, train and validate processing of retinal images

High resolution retinal imaging systems, such as adaptive optics scanning laser ophthalmoscopes (AOSLO), are increasingly being used for clinical research and fundamental studies in neuroscience. These systems offer unprecedented spatial and temporal resolution of retinal structures in vivo. However, a major challenge is the development of robust and automated methods for processing and analysing these images. We present ERICA (Emulated Retinal Image CApture), a simulation tool that generates realistic synthetic images of the human cone mosaic, mimicking images that would be captured by an AOSLO, with specified image quality and with corresponding ground-truth data. The simulation includes a self-organising mosaic of photoreceptors, the eye movements an observer might make during image capture, and data capture through a real system incorporating diffraction, residual optical aberrations and noise. The retinal photoreceptor mosaics generated by ERICA have a similar packing geometry to human retina, as determined by expert labelling of AOSLO images of real eyes. In the current implementation ERICA outputs convincingly realistic en face images of the cone photoreceptor mosaic but extensions to other imaging modalities and structures are also discussed. These images and associated ground-truth data can be used to develop, test and validate image processing and analysis algorithms or to train and validate machine learning approaches. The use of synthetic images has the advantage that neither access to an imaging system, nor to human participants is necessary for development.

DeepRetina: Layer Segmentation of Retina in OCT Images Using Deep Learning

Purpose

To automate the segmentation of retinal layers, we propose DeepRetina, a method based on deep neural networks.

Methods

DeepRetina uses the improved Xception65 to extract and learn the characteristics of retinal layers. The Xception65-extracted feature maps are inputted to an atrous spatial pyramid pooling module to obtain multiscale feature information. This information is then recovered to capture clearer retinal layer boundaries in the encoder-decoder module, thus completing retinal layer auto-segmentation of the retinal optical coherence tomography (OCT) images.

Results

We validated this method using a retinal OCT image database containing 280 volumes (40 B-scans per volume) to demonstrate its effectiveness. The results showed that the method exhibits excellent performance in terms of the mean intersection over union and sensitivity (Se), which are as high as 90.41 and 92.15%, respectively. The intersection over union and Se values of the nerve fiber layer, ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, outer nuclear layer, outer limiting membrane, photoreceptor inner segment, photoreceptor outer segment, and pigment epithelium layer were found to be above 88%.

Conclusions

DeepRetina can automate the segmentation of retinal layers and has great potential for the early diagnosis of fundus retinal diseases. In addition, our approach will provide a segmentation model framework for other types of tissues and cells in clinical practice.

Translational Relevance

Automating the segmentation of retinal layers can help effectively diagnose and monitor clinical retinal diseases. In addition, it requires only a small amount of manual segmentation, significantly improving work efficiency.

Cellular-Scale Imaging of Transparent Retinal Structures and Processes Using Adaptive Optics Optical Coherence Tomography

High-resolution retinal imaging is revolutionizing how scientists and clinicians study the retina on the cellular scale. Its exquisite sensitivity enables time-lapse optical biopsies that capture minute changes in the structure and physiological processes of cells in the living eye. This information is increasingly used to detect disease onset and monitor disease progression during early stages, raising the possibility of personalized eye care. Powerful high-resolution imaging tools have been in development for more than two decades; one that has garnered considerable interest in recent years is optical coherence tomography enhanced with adaptive optics. State-of-the-art adaptive optics optical coherence tomography (AO-OCT) makes it possible to visualize even highly transparent cells and measure some of their internal processes at all depths within the retina, permitting reconstruction of a 3D view of the living microscopic retina. In this review, we report current AO-OCT performance and its success in visualizing and quantifying these once-invisible cells in human eyes.

Cone Identification in Choroideremia: Repeatability, Reliability, and Automation Through Use of a Convolutional Neural Network

Purpose

Adaptive optics imaging has enabled the visualization of photoreceptors both in health and disease. However, there remains a need for automated accurate cone photoreceptor identification in images of disease. Here, we apply an open-source convolutional neural network (CNN) to automatically identify cones in images of choroideremia (CHM). We further compare the results to the repeatability and reliability of manual cone identifications in CHM.

Methods

We used split-detection adaptive optics scanning laser ophthalmoscopy to image the inner segment cone mosaic of 17 patients with CHM. Cones were manually identified twice by one experienced grader and once by two additional experienced graders in 204 regions of interest (ROIs). An open-source CNN either pre-trained on normal images or trained on CHM images automatically identified cones in the ROIs. True and false positive rates and Dice's coefficient were used to determine the agreement in cone locations between data sets. Interclass correlation coefficient was used to assess agreement in bound cone density.

Results

Intra- and intergrader agreement for cone density is high in CHM. CNN performance increased when it was trained on CHM images in comparison to normal, but had lower agreement than manual grading.

Conclusions

Manual cone identifications and cone density measurements are repeatable and reliable for images of CHM. CNNs show promise for automated cone selections, although additional improvements are needed to equal the accuracy of manual measurements.

Translational Relevance

These results are important for designing and interpreting longitudinal studies of cone mosaic metrics in disease progression or treatment intervention in CHM.

Light reflectivity and interference in cone photoreceptors

In several modes of retinal imaging, the primary means of visualizing cone photoreceptors is from reflected light. Understanding how such images are formed, particularly when adaptive optics techniques are used, will help to guide their interpretation. Toward this end, we used finite difference beam propagation to model reflections from cone photoreceptors. We investigated the formation of cone images in adaptive optics scanning laser ophthalmoscopy (AOSLO) and optical coherence tomography (AOOCT). Three cone models were tested, one made up of three segments of varying refractive index, the other two having additional boundaries at the inner/outer segment junction and outer segment tip. Images formed by the first model did not correspond to AOOCT observations in the literature, while the latter two did. The predicted distributions of reflected light intensity from the latter cone models were compared to the distribution from AOSLO images, both studied with light sources of varied coherence length. The cone model with the most reflections at the inner/outer segment junction best fit the data measured in vivo. These results show that variance in cone reflection can originate from light interfering from reflectors much more closely spaced than the outer segment length. We also show that subtracting images taken with different coherence length sources highlights these changes in interference. Differential coherence images of cones occasionally revealed an annular reflection profile, which modeling showed to be very sensitive to cone size and the gaps bracketing the outer segment, suggesting that such imaging may be useful for probing photoreceptor morphology.

A 2-Year Longitudinal Study of Normal Cone Photoreceptor Density

Purpose

Despite the potential for adaptive optics scanning light ophthalmoscopy (AOSLO) to quantify retinal disease progression at the cellular level, there remain few longitudinal studies investigating changes in cone density as a measure of disease progression. Here, we undertook a prospective, longitudinal study to investigate the variability of cone density measurements in normal subjects during a 2-year period.

Methods

Fourteen eyes of nine subjects with no known ocular pathology were imaged both at a baseline and a 2-year follow-up visit by using confocal AOSLO at five retinal locations. Two-year affine-registered images were created to minimize the effects of intraframe distortions. Regions of interest were cropped from baseline, 2-year manually aligned, and 2-year affine-registered images. Cones were identified (graded masked) and cone density was extracted.

Results

Mean baseline cone density (cones/mm²) was 87,300, 62,200, 45,500, 28,700, and 18,200 at 190, 350, 500, 900, and 1500 μm, respectively. The mean difference (± standard deviation [SD]) in cone density from baseline to 2-year affine-registered images was 1400 (1700), 100 (1800), 300 (800), 400 (800), and 1000 (2400) cones/mm² at the same locations. The mean difference in cone density during the 2-year period was lower for affine-registered images than manually aligned images.

Conclusions

There was no meaningful change in normal cone density during a 2-year period. Intervisit variability in cone density measurements decreased when intraframe distortions between time points were minimized. This variability must be considered when planning prospective longitudinal clinical trials using changes in cone density as an outcome measure for assessing retinal disease progression.

Earliest Evidence of Preclinical Diabetic Retinopathy Revealed Using Optical Coherence Tomography Angiography Perfused Capillary Density

PURPOSE

To compare perfused capillary density (PCD) in diabetic patients and healthy controls using optical coherence tomography angiography (OCTA).

METHODS

Forty controls, 36 diabetic subjects without clinical retinopathy (NoDR), 38 with nonproliferative retinopathy (NPDR), and 38 with proliferative retinopathy (PDR) were imaged using spectral-domain optical coherence tomography. A 3 × 3-mm full-thickness parafoveal OCTA scan was obtained from each participant. Following manual delineation of the foveal avascular zone (FAZ), FAZ area, perimeter, and acircularity index were determined. Seven consecutive equidistant 200-μm-wide annular segments were drawn at increasing eccentricities from the FAZ margin. Annular PCD (%) was defined as perfused capillary area divided by the corresponding annulus area after subtraction of noncapillary blood vessel areas. Nonparametric Kruskal-Wallis testing with Bonferroni correction was performed in pairwise comparisons of group PCD values.

RESULTS

The NoDR group demonstrated consistently higher PCD compared to the control group in all 7 annuli, reaching statistical significance (36.6% ± 3.30% vs 33.6% ± 3.98%, P = .034) at the innermost annulus (FAZ margin to 200 μm out). The NPDR and PDR groups demonstrated progressively decreasing PCD. Differences in FAZ metrics between the NoDR and control groups did not reach statistical significance.

CONCLUSIONS

Relative to healthy controls, increased PCD values in the NoDR group likely represent an autoregulatory response to increased metabolic demand, while the decrease in PCD that follows in NPDR and PDR results largely from an incremental loss of capillary segments. These findings, consistent with previous studies, demonstrate the potential of OCTA as a clinical tool for earlier objective detection of preclinical diabetic retinopathy. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Dysflective Cones

Retinal imaging has advanced to enable noninvasive in vivo visualization of macular photoreceptors with cellular resolution. Images of retinal structure are best interpreted in the context of visual function, but clinical measures of visual function lack resolution on the scale of individual cells. Combined with cross-sectional measures of retinal structure acquired with optical coherence tomography (OCT), macular photoreceptor function can be evaluated using visual acuity and fundus-guided microperimetry, but the resolution of these measures is limited to relatively large retinal areas. By incorporating adaptive optics correction of aberrations in light entering and exiting the pupil, individual photoreceptors can be visualized and stimulated to assess structure and function. Discrepancy between structural images and visual function can shed light on the origin of visible features and their relation to visual function. Dysflective cones, cones with abnormal waveguiding properties on confocal adaptive optics scanning laser ophthalmoscopy (AOSLO) images and measurable function, provide insight into the visual significance of features in retinal images and may facilitate identification of patients who could benefit from therapies.

Adaptive optics imaging of the human retina

Adaptive Optics (AO) retinal imaging has provided revolutionary tools to scientists and clinicians for studying retinal structure and function in the living eye. From animal models to clinical patients, AO imaging is changing the way scientists are approaching the study of the retina. By providing cellular and subcellular details without the need for histology, it is now possible to perform large scale studies as well as to understand how an individual retina changes over time. Because AO retinal imaging is non-invasive and when performed with near-IR wavelengths both safe and easily tolerated by patients, it holds promise for being incorporated into clinical trials providing cell specific approaches to monitoring diseases and therapeutic interventions. AO is being used to enhance the ability of OCT, fluorescence imaging, and reflectance imaging. By incorporating imaging that is sensitive to differences in the scattering properties of retinal tissue, it is especially sensitive to disease, which can drastically impact retinal tissue properties. This review examines human AO retinal imaging with a concentration on the use of the Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO). It first covers the background and the overall approaches to human AO retinal imaging, and the technology involved, and then concentrates on using AO retinal imaging to study the structure and function of the retina.

Principles of OCTA and Applications in Clinical Neurology

PURPOSE OF REVIEW

The article reviews the recent findings on the use of optical coherence tomography angiography (OCTA) in neurology.

RECENT FINDINGS

OCTA is a new addition to the powerful and complementary technology of the OCT. Due to its noninvasiveness, and reproducibility, it is possible to obtain high-resolution 3D images of the vessels of the human eye. As the vessels of the retina with the presence of endothelial cell's tight junctions resemble the brain vessels, it was hypothesized that the imaging of the retinal vessels might bring insight into brain vessels. OCTA has been effectively used to predict retinal vessel abnormalities in dementia, demyelization, optic disc neuropathies, and inherited degenerative diseases. Most common findings were decrease of vascular density and flow and an increase of avascular zones. Although OCTA is a relative new technology, recent studies show that it can be successfully applied in neurology.

Spatial summation in the human fovea: Do normal optical aberrations and fixational eye movements have an effect?

Psychophysical inferences about the neural mechanisms supporting spatial vision can be undermined by uncertainties introduced by optical aberrations and fixational eye movements, particularly in fovea where the neuronal grain of the visual system is fine. We examined the effect of these preneural factors on photopic spatial summation in the human fovea using a custom adaptive optics scanning light ophthalmoscope that provided control over optical aberrations and retinal stimulus motion. Consistent with previous results, Ricco's area of complete summation encompassed multiple photoreceptors when measured with ordinary amounts of ocular aberrations and retinal stimulus motion. When both factors were minimized experimentally, summation areas were essentially unchanged, suggesting that foveal spatial summation is limited by postreceptoral neural pooling. We compared our behavioral data to predictions generated with a physiologically-inspired front-end model of the visual system, and were able to capture the shape of the summation curves obtained with and without pre-retinal factors using a single postreceptoral summing filter of fixed spatial extent. Given our data and modeling, neurons in the magnocellular visual pathway, such as parasol ganglion cells, provide a candidate neural correlate of Ricco's area in the central fovea.

Multimodal Imaging of the Normal Eye

Multimodal imaging is the concept of "bundling" images obtained from various imaging modalities, viz., fundus photograph, fundus autofluorescence imaging, infrared (IR) imaging, simultaneous fluorescein and indocyanine angiography, optical coherence tomography (OCT), and, more recently, OCT angiography. Each modality has its pros and cons as well as its limitations. Combination of multiple imaging techniques will overcome their individual weaknesses and give a comprehensive picture. Such approach helps in accurate localization of a lesion and understanding the pathology in posterior segment. It is important to know imaging of normal eye before one starts evaluating pathology. This article describes multimodal imaging modalities in detail and discusses healthy eye features as seen on various imaging modalities mentioned above.

The Reliability of Cone Density Measurements in the Presence of Rods

Purpose

Recent advances in adaptive optics scanning light ophthalmoscopy (AOSLO) have enabled visualization of cone inner segments through nonconfocal split-detection, in addition to rod and cone outer segments revealed by confocal reflectance. Here, we examined the interobserver reliability of cone density measurements in both AOSLO imaging modalities.

Methods

Five normal subjects (nine eyes) were imaged along the horizontal and vertical meridians using a custom AOSLO with confocal and nonconfocal split-detection modalities. The resulting images were montaged using a previously described semiautomatic algorithm. Regions of interest (ROIs) were selected from the confocal montage at 190 μm, and from split-detection and confocal montages at 900 and 1800 μm from the fovea. Four observers (three experts, one naïve) manually identified cone locations in each ROI, and these locations were used to calculate bound densities. Intraclass correlation coefficients and Dice's coefficients were calculated to assess interobserver agreement.

Results

Interobserver agreement was high in cone-only images (confocal 190 μm: 0.85; split-detection 900 μm: 0.91; split-detection 1800 μm: 0.89), moderate in confocal images at 900 μm (0.68), and poor in confocal images at 1800 μm (0.24). Excluding the naïve observer data substantially increased agreement within confocal images (190 μm: 0.99; 900 μm: 0.80; 1800 μm: 0.68).

Conclusions

Interobserver measurements of cone density are more reliable in rod-free retinal images. Moreover, when using manual cell identification, it is essential that observers are trained, particularly for confocal AOSLO images.

Translational Relevance

This study underscores the need for additional reliability studies in eyes containing pathology where identifying cones can be substantially more difficult.

High speed adaptive optics ophthalmoscopy with an anamorphic point spread function

Retinal imaging working with a line scan mechanism and a line camera has the potential to image the eye with a near-confocal performance at the high frame rate, but this regime has difficulty to collect sufficient imaging light while adequately digitize the optical resolution in adaptive optics imaging. To meet this challenge, we have developed an adaptive optics line scan ophthalmoscope with an anamorphic point spread function. The instrument uses a high-speed line camera to acquire the retinal image and act as a confocal gate. Meanwhile, it employs a digital micro-mirror device to modulate the imaging light into a line of point sources illuminating the retina. The anamorphic mechanism ensures adequate digitization of the optical resolution and increases light collecting efficiency. We demonstrate imaging of the living human retina with cellular level resolution at a frame rate of 200 frames/second (FPS) with a digitization of 512 × 512 pixels over a field of view of 1.2° × 1.2°. We have assessed cone photoreceptor structure in images acquired at 100, 200, and 800 FPS in 2 normal human subjects, and confirmed that retinal images acquired at high speed rendered macular cone mosaic with improved measurement repeatability.

Optical coherence tomography angiography in the management of age-related macular degeneration

PURPOSE OF REVIEW

Optical coherence tomography angiography (OCT-A) provides rapid, flow-based imaging of the retinal and choroidal vasculature in a noninvasive manner. This review contrasts this novel technique with conventional angiography and discusses its current uses and limitations in the management of age-related macular degeneration (AMD).

RECENT FINDINGS

Initial work with OCT-A has focused on its ability to identify choriocapillaris flow alterations in dry AMD and to sensitively detect choroidal neovascular membranes (CNVs) in neovascular AMD. Reduced choriocapillaris flow beyond the borders of geographic atrophy seen on OCT-A suggests a primary vascular cause in geographic atrophy. Longitudinal OCT-A analysis of CNV morphology has demonstrated the transition from an immature to mature CNV phenotype following treatment. Current clinical applications of OCT-A include identification of asymptomatic CNV and monitoring for CNV development in the setting of an acquired vitelliform lesion.

SUMMARY

OCT-A remains a promising diagnostic tool but one still very much in evolution. Larger studies will be needed to more accurately describe its sensitivity and specificity for CNV detection and to better characterize longitudinal CNV morphologic changes. Anticipated hardware and software updates including swept-source light sources, automated montaging, and manual adjustment of interscan timing should enhance the capabilities of OCT-A in the management of AMD.

Photoreceptor-Based Biomarkers in AOSLO Retinal Imaging

Improved understanding of the mechanisms underlying inherited retinal degenerations has created the possibility of developing much needed treatments for these relentless, blinding diseases. However, standard clinical indicators of retinal health (such as visual acuity and visual field sensitivity) are insensitive measures of photoreceptor survival. In many retinal degenerations, significant photoreceptor loss must occur before measurable differences in visual function are observed. Thus, there is a recognized need for more sensitive outcome measures to assess therapeutic efficacy as numerous clinical trials are getting underway. Adaptive optics (AO) retinal imaging techniques correct for the monochromatic aberrations of the eye and can be used to provide nearly diffraction-limited images of the retina. Many groups routinely are using AO imaging tools to obtain in vivo images of the rod and cone photoreceptor mosaic, and it now is possible to monitor photoreceptor structure over time with single cell resolution. Highlighting recent work using AO scanning light ophthalmoscopy (AOSLO) across a range of patient populations, we review the development of photoreceptor-based metrics (e.g., density/geometry, reflectivity, and size) as candidate biomarkers. Going forward, there is a need for further development of automated tools and normative databases, with the latter facilitating the comparison of data sets across research groups and devices. Ongoing and future clinical trials for inherited retinal diseases will benefit from the improved resolution and sensitivity that multimodal AO retinal imaging affords to evaluate safety and efficacy of emerging therapies.

Evaluating ocular blood flow

Studies have shown that vascular impairment plays an important role in the etiology and pathogenesis of various ocular diseases including glaucoma, age-related macular degeneration, diabetic retinopathy, and retinal venous occlusive disease. Thus, qualitative and quantitative assessment of ocular blood flow (BF) is a topic of interest for early disease detection, diagnosis, and management. Owing to the rapid improvement in technology, there are several invasive and noninvasive techniques available for evaluating ocular BF, with each of these techniques having their own limitations and advantages. This article reviews these important techniques, with a particular focus on Doppler Fourier domain optical coherence tomography (OCT) and OCT-angiography.

Dysflective cones: Visual function and cone reflectivity in long-term follow-up of acute bilateral foveolitis

Purpose

Confocal adaptive optics scanning laser ophthalmoscope (AOSLO) images provide a sensitive measure of cone structure. However, the relationship between structural findings of diminished cone reflectivity and visual function is unclear. We used fundus-referenced testing to evaluate visual function in regions of apparent cone loss identified using confocal AOSLO images.

Methods

A patient diagnosed with acute bilateral foveolitis had spectral-domain optical coherence tomography (SD-OCT) (Spectralis HRA + OCT system [Heidelberg Engineering, Vista, CA, USA]) images indicating focal loss of the inner segment-outer segment junction band with an intact, but hyper-reflective, external limiting membrane. Five years after symptom onset, visual acuity had improved from 20/80 to 20/25, but the retinal appearance remained unchanged compared to 3 months after symptoms began. We performed structural assessments using SD-OCT, directional OCT (non-standard use of a prototype on loan from Carl Zeiss Meditec) and AOSLO (custom-built system). We also administered fundus-referenced functional tests in the region of apparent cone loss, including analysis of preferred retinal locus (PRL), AOSLO acuity, and microperimetry with tracking SLO (TSLO) (prototype system). To determine AOSLO-corrected visual acuity, the scanning laser was modulated with a tumbling E consistent with 20/30 visual acuity. Visual sensitivity was assessed in and around the lesion using TSLO microperimetry. Complete eye examination, including standard measures of best-corrected visual acuity, visual field tests, color fundus photos, and fundus auto-fluorescence were also performed.

Results

Despite a lack of visible cone profiles in the foveal lesion, fundus-referenced vision testing demonstrated visual function within the lesion consistent with cone function. The PRL was within the lesion of apparent cone loss at the fovea. AOSLO visual acuity tests were abnormal, but measurable: for trials in which the stimulus remained completely within the lesion, the subject got 48% correct, compared to 78% correct when the stimulus was outside the lesion. TSLO microperimetry revealed reduced, but detectible, sensitivity thresholds within the lesion.

Conclusions and importance

Fundus-referenced visual testing proved useful to identify functional cones despite apparent photoreceptor loss identified using AOSLO and SD-OCT. While AOSLO and SD-OCT appear to be sensitive for the detection of abnormal or absent photoreceptors, changes in photoreceptors that are identified with these imaging tools do not correlate completely with visual function in every patient. Fundus-referenced vision testing is a useful tool to indicate the presence of cones that may be amenable to recovery or response to experimental therapies despite not being visible on confocal AOSLO or SD-OCT images.

Efficient OCT Volume Reconstruction From Slitlamp Microscopes

Since its introduction 25 years ago, Optical Coherence Tomography (OCT) has contributed tremendously to diagnostic and monitoring capabilities of pathologies in the field of ophthalmology. Despite rapid progress in hardware and software technology however, the price of OCT devices has remained high, limiting their use in private practice, and in screening examinations. In this paper, we present a slitlamp-integrated OCT device, built with off-the-shelf components, which can generate high-quality volumetric images of the posterior eye segment. To do so, we present a novel strategy for 3D image reconstruction in this challenging domain that allows us for state-of-the-art OCT volumes to be generated at fast speeds. The result is an OCT device that can match current systems in clinical practice, at a significantly lower cost.

Multi-modal automatic montaging of adaptive optics retinal images

We present a fully automated adaptive optics (AO) retinal image montaging algorithm using classic scale invariant feature transform with random sample consensus for outlier removal. Our approach is capable of using information from multiple AO modalities (confocal, split detection, and dark field) and can accurately detect discontinuities in the montage. The algorithm output is compared to manual montaging by evaluating the similarity of the overlapping regions after montaging, and calculating the detection rate of discontinuities in the montage. Our results show that the proposed algorithm has high alignment accuracy and a discontinuity detection rate that is comparable (and often superior) to manual montaging. In addition, we analyze and show the benefits of using multiple modalities in the montaging process. We provide the algorithm presented in this paper as open-source and freely available to download.

Optical coherence angiography: A review

BACKGROUND

Retinal vascular diseases are one of the most common causes of blindness in the developed world. Optical Coherence Tomography Angiography (OCT-A) is a new noninvasive method that uses several algorithms to detect blood movement. This enables the creation of high-resolution vascular images with contrast depicting motionless tissue.

METHODS

This review presents the results of articles relevant to age-related macular degeneration (AMD), diabetic retinopathy (DR), and OCT-A. The OCT-A technique can successfully be used in the diagnosis of neovascularization, retinal vein occlusion (RVO) and retinal artery occlusion (RAO), vessel abnormalities and even anterior segment neovascularization. OCT-A can also be applied to compute data such as vessel density, and flow index in both superficial and deep plexuses.

RESULTS

Many studies have compared fluorescein angiography with OCT-A. Other studies have reported differences in vascular density in AMD patients and have compared them with people having healthy eyes. Although OCT-A offers rapid picture acquisition, high repeatability and resolution, it also has many drawbacks. The most common are: motion artifacts, projections from overlying vessels and limited field of view.An interesting new application is the possibility to assess changes during antivascular endothelial growth factor (anti-VEGF) therapy. Another function of OCT-A is the possible application in the study of choriocapillaries in many fields of ocular pathology.

CONCLUSION

OCT-A is a new promising method that allows the visualization of the retinal vascular network and the counting of blood flow parameters. This technique provides reliable images useful in clinical routines.