Imaging the Retinal Vasculature

Microglia in retinal angiogenesis and diabetic retinopathy

Diabetic retinopathy has a high probability of causing visual impairment or blindness throughout the disease progression and is characterized by the growth of new blood vessels in the retina at an advanced, proliferative stage. Microglia are a resident immune population in the central nervous system, known to play a crucial role in regulating retinal angiogenesis in both physiological and pathological conditions, including diabetic retinopathy. Physiologically, they are located close to blood vessels and are essential for forming new blood vessels (neovascularization). In diabetic retinopathy, microglia become widely activated, showing a distinct polarization phenotype that leads to their accumulation around neovascular tufts. These activated microglia induce pathogenic angiogenesis through the secretion of various angiogenic factors and by regulating the status of endothelial cells. Interestingly, some subtypes of microglia simultaneously promote the regression of neovascularization tufts and normal angiogenesis in neovascularization lesions. Modulating the state of microglial activation to ameliorate neovascularization thus appears as a promising potential therapeutic approach for managing diabetic retinopathy.

Compact Linear Flow Phantom Model for Retinal Blood-Flow Evaluation

Impaired retinal blood flow is associated with ocular diseases such as glaucoma, macular degeneration, and diabetic retinopathy. Among several ocular imaging techniques developed to measure retinal blood flow both invasively and non-invasively, adaptive optics (AO)-enabled scanning laser ophthalmoscopy (AO-SLO) resolves individual red blood cells and provides a high resolution with which to measure flow across retinal microvasculature. However, cross-validation of flow measures remains a challenge owing to instrument and patient-specific variability in each imaging technique. Hence, there is a critical need for a well-controlled clinical flow phantom for standardization and to establish blood-flow measures as clinical biomarkers for early diagnosis. Here, we present the design and validation of a simple, compact, portable, linear flow phantom based on a direct current motor and a conveyor-belt system that provides linear velocity tuning within the retinal microvasculature range (0.5-7 mm/s). The model was evaluated using a sensitive AO-SLO line-scan technique, which showed a <6% standard deviation from the true velocity. Further, a clinical SLO instrument showed a linear correlation with the phantom's true velocity (r2 > 0.997). This model has great potential to calibrate, evaluate, and improve the accuracy of existing clinical imaging systems for retinal blood flow and aid in the diagnosis of ocular diseases with abnormal blood flow.

Rapid prototyping of a retinal multivascular network phantom for optical retinal vascular imaging equipment evaluation

Retinal vascular health holds paramount importance for healthy vision. Many technologies have been developed to examine retinal vasculature non-destructively, including fundus cameras, optical coherence tomography angiography (OCTA), fluorescein angiography (FA), and so on. However, there is a lack of a proper phantom simulating the critical features of the real human retina to calibrate and evaluate the performance of these technologies. In this work, we present a rapid, high-resolution, and economical technology based on 3D printed mold-based soft lithography and spin coating for the fabrication of a multivascular network and multilayer structural retinal phantom with the appropriate optical properties. The feasibility of the retinal phantom as a test device was demonstrated with an OCTA system and a confocal retinal ophthalmoscope. Experiment results prove that the retinal phantom could provide an objective evaluation of the OCTA and confocal retinal ophthalmoscope. Furthermore, the microfluidic phantoms enabled by this fabrication technology may support the development and evaluation of other techniques.

Investigating the Utility of Near-Infrared Reflectance Imaging for Diabetic Retinopathy Screening

BACKGROUND AND OBJECTIVE

We investigated the reliability of near-infrared reflectance (NIR) imaging as a method of assessing severity of diabetic retinopathy (DR).

PATIENTS AND METHODS

One hundred ninety-five NIR images were reviewed by two graders for the number of hyporeflective foci, presence or absence of vascular abnormalities, and presumptive DR stage; these were correlated to fundus photography-defined DR stage. Interrater reliability was confirmed via one-way random effects model of intraclass correlation coefficients. Analysis of variance was used in subgroup analysis, receiver operating characteristic (ROC) curves were created to validate reliability of the model, and logistic regression was used to model foci and vascular abnormalities as predictors for moderate or worse disease.

RESULTS

A statistically significant difference in mean number of hyporeflective foci was found between no DR and moderate non-proliferative DR (NPDR; P < 0.0001), no DR and severe NPDR (P < 0.001), no DR and proliferative DR (PDR; P < 0.0001), mild and moderate NPDR (P = 0.008), mild and severe NPDR (P < 0.001), and mild NPDR and PDR (P < 0.001). The area under the ROC curve was 0.849 (CI: 0.792 to 0.905). The threshold for detection of moderate NPDR or worse was 4.75 foci, with a sensitivity of 79.0% and a false positive rate of 20.0%. Multivariate logistic regression model incorporating hyporeflective foci with vascular abnormalities (odds ratio [OR] = 1.592, 95% CI: 1.381 to 1.835; P < 0.001) was able to accurately predict moderate disease or worse, just moderate disease (OR = 1.045, 95% CI: 1.003 to 1.089; P = 0.035), severe disease (OR = 1.050, 95% CI: 1.006 to 1.096; P = 0.027), and proliferative disease (OR = 1.050, 95% CI: 1.008 to 1.095; P = 0.018).

CONCLUSIONS

NIR imaging may be an adjunct tool in screening for DR. [Ophthalmic Surg Lasers Imaging Retina 2024;55:318-325.].

The clinical evaluation of a widefield lens to expand the field of view in optical coherence tomography (OCT-A)

This study aimed to evaluate the clinical benefits of incorporating a widefield lens (WFL) in optical coherence tomography angiography (OCT-A) in patients with retinal vascular diseases in comparison to standard single-shot OCT-A scans. Sixty patients with retinal vascular diseases including diabetic retinopathy (DR) and retinal vein occlusion (RVO) were recruited. OCT-A imaging (PlexElite 9000) with and without WFL was performed in randomized order. The assessment included patient comfort, time, field of view (FoV), image quality and pathology detection. Statistical analysis included paired t-tests, Mann-Whitney U-tests and Bonferroni correction for multiple tests, with inter-grader agreement using the kappa coefficient. Using a WFL did not lead to statistically significant differences in DR and RVO group test times. Patient comfort remained high, with similar responses for WFL and non-WFL measurements. The WFL notably expanded the scan field (1.6× FoV increase), enhancing peripheral retinal visibility. However, image quality varied due to pathology and eye dominance, affecting the detection of peripheral issues in RVO and DR cases. The use of a WFL widens the scan field, aiding vascular retinal disease imaging with minor effects on comfort, time, and image quality. Further enhancements are needed for broader view angles, enabling improved quantification of non-perfused areas and more reliable peripheral proliferation detection.

One-shot Retinal Artery and Vein Segmentation via Cross-modality Pretraining

Purpose

To perform one-shot retinal artery and vein segmentation with cross-modality artery-vein (AV) soft-label pretraining.

Design

Cross-sectional study.

Subjects

The study included 6479 color fundus photography (CFP) and arterial-venous fundus fluorescein angiography (FFA) pairs from 1964 participants for pretraining and 6 AV segmentation data sets with various image sources, including RITE, HRF, LES-AV, AV-WIDE, PortableAV, and DRSplusAV for one-shot finetuning and testing.

Methods

We structurally matched the arterial and venous phase of FFA with CFP, the AV soft labels were automatically generated by utilizing the fluorescein intensity difference of the arterial and venous-phase FFA images, and the soft labels were then used to train a generative adversarial network to learn to generate AV soft segmentations using CFP images as input. We then finetuned the pretrained model to perform AV segmentation using only one image from each of the AV segmentation data sets and test on the remainder. To investigate the effect and reliability of one-shot finetuning, we conducted experiments without finetuning and by finetuning the pretrained model on an iteratively different single image for each data set under the same experimental setting and tested the models on the remaining images.

Main Outcome Measures

The AV segmentation was assessed by area under the receiver operating characteristic curve (AUC), accuracy, Dice score, sensitivity, and specificity.

Results

After the FFA-AV soft label pretraining, our method required only one exemplar image from each camera or modality and achieved similar performance with full-data training, with AUC ranging from 0.901 to 0.971, accuracy from 0.959 to 0.980, Dice score from 0.585 to 0.773, sensitivity from 0.574 to 0.763, and specificity from 0.981 to 0.991. Compared with no finetuning, the segmentation performance improved after one-shot finetuning. When finetuned on different images in each data set, the standard deviation of the segmentation results across models ranged from 0.001 to 0.10.

Conclusions

This study presents the first one-shot approach to retinal artery and vein segmentation. The proposed labeling method is time-saving and efficient, demonstrating a promising direction for retinal-vessel segmentation and enabling the potential for widespread application.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Twenty-four hour diurnal variation in retinal oxygen saturation

Retinal oxygen saturation is influenced by systemic and local vasculature, intraocular pressure (IOP), and individual cellular function. In numerous retinal pathologies, early changes take place at the level of the microvasculature, thereby affecting retinal oxygenation. The purpose of this study was to investigate diurnal variations in retinal oximetry measures and evaluate the relationship with other ocular and systemic physiological processes. Healthy adults (n = 18, mean age 27 ± 5.5 years) participated. Ocular and systemic measures were collected every four hours over 24 h and included retinal oximetry, IOP, optical coherence tomography (OCT), OCT-angiography (OCTA), biometry, blood pressure, and partial pressure of oxygen. Amplitude and acrophase for retinal oxygen saturation, axial length, retinal and choroidal thickness, OCTA parameters, and mean arterial and ocular perfusion pressure (MAP, MOPP) were determined were determined using cosine fits, and multiple regression analysis was performed to compare metrics. Retinal oxygenation saturation demonstrated a significant diurnal variation with an amplitude of 5.84 ± 3.86% and acrophase of 2.35 h. Other parameters that demonstrated significant diurnal variation included IOP, MOPP, axial length, choroidal thickness, superficial vessel density, heart rate, systolic blood pressure, and MAP. Diurnal variations in retinal oxygen saturation were in-phase with choroidal thickness, IOP, and density of the superficial vascular plexus and out-of-phase with axial length and MOPP. In conclusion, retinal oxygenation saturation undergoes diurnal variations over 24 h. These findings contribute to a better understanding of intrinsic and extrinsic factors influencing oxygenation of the area surrounding the fovea.

Quantitative study of spatial and temporal variation in retinal capillary network perfusion in rat eye by in vivo confocal imaging

Microvascular dysfunction is the underlying pathological process in many systemic diseases. However, investigation into its pathogenesis is impeded by the accessibility and complexity of the microvasculature within different organs, particularly for the central nervous system. The retina as an extension of the cerebrum provides a glimpse into the brain through which the microvasculature can be observed. Two major questions remain unanswered: How do the microvessels regulate spatial and temporal delivery to satisfy the varying cellular demands, and how can we quantify blood perfusion in the 3D capillary network? Here, quantitative measurements of red blood cell (RBC) speed in each vessel in the field were made in the in vivo rat retinal capillary network using an ultrafast confocal technique with fluorescently labelled RBCs. Retinal RBC speed and number were found to vary remarkably between microvessels ranging from 215 to 6641 microns per second with significant variations spatially and temporally. Overall, the RBC speed was significantly faster in the microvessels in the superficial retina than in the deep retina (estimated marginal means of 2405 ± 238.2 µm/s, 1641 ± 173.0 µm/s respectively). These observations point to a highly dynamic nature of microvasculature that is specific to its immediate cellular environment and is constantly changing.

Central Retinal Vein Occlusion with Three-Retinal Quadrant Involvement: Another Focus on Optic Disc Head Vascular Anatomy Variations

A 50-year-old male patient with sudden visual acuity loss in his right eye came to our clinic. Visual acuity at presentation was 1/10 in right eye and 10/10 in left. The patient was otherwise healthy Caucasian man without any history of previous systemic or ophthalmic disease. There was not any history of amblyopia and refractive error. Anterior segment findings were unremarkable. Three quadrants of retina were fully involved with central retinal vein occlusion (CRVO) features including retinal hemorrhages, retinal edema obscuring retinal details, and cotton wool spots while sparing inferior temporal quadrant. Inferior temporal quadrant sparing in this patient is due to a specific retinal vascular anatomical variation. In conclusion, in unusual presentations of retinal vascular branch obstructions, considering retinal vascular anatomy variations would help us to explain the clinical presentation more precisely in some cases.

Quantitative Parameters Relevant for Diabetic Macular Edema Evaluation by Optical Coherence Tomography Angiography

Diabetic macular edema (DME) is one of the main ocular complications of diabetes mellitus (DM) that can lead to important vision loss in diabetic patients. In clinical practice, there are cases of DME with unsatisfying treatment responses, despite adequate therapeutic management. Diabetic macular ischemia (DMI) is one of the causes suggested to be associated with the persistence of fluid accumulation. Optical coherence tomography angiography (OCTA) is a non-invasive imaging modality, able to give in-depth information about retinal vascularization in a 3-dimensional manner. The OCTA devices currently available can provide various OCTA metrics that quantitatively assess the retinal microvasculature. In this paper, we reviewed the results of multiple studies that investigated the changes in OCTA metrics in the setting of DME and their possible contribution to the diagnosis, therapeutic management, follow-up and prognosis of patients with DME. We analyzed and compared relevant studies that investigated OCTA parameters related to changes in macular perfusion in the setting of DME and we evaluated the correlations between DME and several quantitative parameters, such as vessel density (VD), perfusion density (PD), foveal avascular zone (FAZ)-related parameters, as well as complexity indices of retinal vasculature. The results of our research showed that OCTA metrics, evaluated especially at the level of the deep vascular plexus (DVP), are useful instruments that can contribute to the assessment of patients with DME.

2022 Prentice Award Lecture: Advancing Retinal Imaging and Visual Function in Patient Management and Disease Mechanisms

SIGNIFICANCE

Patient-based research plays a key role in probing basic visual mechanisms. Less-well recognized is the role of patient-based retinal imaging and visual function studies in elucidating disease mechanisms, which are accelerated by advances in imaging and function techniques and are most powerful when combined with the results from histology and animal models.A patient's visual complaints can be one key to patient management, but human data are also key to understanding disease mechanisms. Unfortunately, pathological changes can be difficult to detect. Before advanced retinal imaging, the measurement of visual function indicated the presence of pathological changes that were undetectable with existing clinical examination. Over the past few decades, advances in retinal imaging have increasingly revealed the unseen. This has led to great strides in the management of many diseases, particularly diabetic retinopathy and macular edema, and age-related macular degeneration. It is likely widely accepted that patient-based research, as in clinical trials, led to such positive outcomes. Both visual function measures and advanced retinal imaging have clearly demonstrated differences among retinal diseases. Contrary to initial thinking, sight-threatening damage in diabetes occurs to the outer retina and not only to the inner retina. This has been clearly indicated in patient results but has only gradually entered the clinical classifications and understanding of disease etiology. There is strikingly different pathophysiology for age-related macular degeneration compared with photoreceptor and retinal pigment epithelial genetic defects, yet research models and even some treatments confuse these. It is important to recognize the role that patient-based research plays in probing basic visual mechanisms and elucidating disease mechanisms, combining these findings with the concepts from histology and animal models. Thus, this article combines sample instrumentation from my laboratory and progress in the fields of retinal imaging and visual function.

Retinal blood flow speed quantification at the capillary level using temporal autocorrelation fitting OCTA [Invited]

Optical coherence tomography angiography (OCTA) can visualize vasculature structures, but provides limited information about blood flow speed. Here, we present a second generation variable interscan time analysis (VISTA) OCTA, which evaluates a quantitative surrogate marker for blood flow speed in vasculature. At the capillary level, spatially compiled OCTA and a simple temporal autocorrelation model, ρ(τ) = exp(-ατ), were used to evaluate a temporal autocorrelation decay constant, α, as the blood flow speed marker. A 600 kHz A-scan rate swept-source OCT prototype instrument provides short interscan time OCTA and fine A-scan spacing acquisition, while maintaining multi mm² field of views for human retinal imaging. We demonstrate the cardiac pulsatility and assess repeatability of α measured with VISTA. We show different α for different retinal capillary plexuses in healthy eyes and present representative VISTA OCTA in eyes with diabetic retinopathy.

High-speed measurement of retinal arterial blood flow in the living human eye with adaptive optics ophthalmoscopy

We present a technique to measure the rapid blood velocity in large retinal vessels with high spatiotemporal resolution. Red blood cell motion traces in the vessels were non-invasively imaged using an adaptive optics near-confocal scanning ophthalmoscope at a frame rate of 200 fps. We developed software to measure blood velocity automatically. We demonstrated the ability to measure the spatiotemporal profiles of the pulsatile blood flow with a maximum velocity of 95-156 mm/s in retinal arterioles with a diameter >100 µm. High-speed and high-resolution imaging increased the dynamic range, enhanced sensitivity, and improved the accuracy when studying retinal hemodynamics.

Long non-coding RNAs in retinal neovascularization: current research and future directions

PURPOSE

Retinal neovascularization (RNV) is an intractable pathological hallmark of numerous ocular blinding diseases, including diabetic retinopathy, retinal vein occlusion, and retinopathy of prematurity. However, current therapeutic methods have potential side effects and limited efficacy. Thus, further studies on the pathogenesis of RNV-related disorders and novel therapeutic targets are critically required. Long non-coding RNAs (lncRNAs) have various functions and participate in almost all biological processes in living cells, such as translation, transcription, signal transduction, and cell cycle control. In addition, recent research has demonstrated critical modulatory roles of various lncRNAs in RNV. In this review, we summarize current knowledge about the expression and regulatory functions of lncRNAs related to the progression of pathological RNV.

METHODS

We searched databases such as PubMed and Web of Science to gather and review information from the published literature.

CONCLUSIONS

In general, lncRNA MEG3 attenuates RNV, thus protecting the retina from excessive and dysregulated angiogenesis under high glucose stress. In contrast, lncRNAs MALAT1, MIAT, ANRIL, HOTAIR, HOTTIP, and SNHG16, have been identified as causative molecules in the pathological progression of RNV. Comprehensive and in-depth studies of the roles of lncRNAs in RNV indicate that targeting lncRNAs may be an alternative therapeutic approach in the near future, enabling new options for attenuating RNV progression and treating RNV-related retinal diseases.

Essential Role of Multi-Omics Approaches in the Study of Retinal Vascular Diseases

Retinal vascular disease is a highly prevalent vision-threatening ocular disease in the global population; however, its exact mechanism remains unclear. The expansion of omics technologies has revolutionized a new medical research methodology that combines multiple omics data derived from the same patients to generate multi-dimensional and multi-evidence-supported holistic inferences, providing unprecedented opportunities to elucidate the information flow of complex multi-factorial diseases. In this review, we summarize the applications of multi-omics technology to further elucidate the pathogenesis and complex molecular mechanisms underlying retinal vascular diseases. Moreover, we proposed multi-omics-based biomarker and therapeutic strategy discovery methodologies to optimize clinical and basic medicinal research approaches to retinal vascular diseases. Finally, the opportunities, current challenges, and future prospects of multi-omics analyses in retinal vascular disease studies are discussed in detail.

Visualizing retinal cells with adaptive optics imaging modalities using a translational imaging framework

Adaptive optics reflectance-based retinal imaging has proved a valuable tool for the noninvasive visualization of cells in the living human retina. Many subcellular features that remain at or below the resolution limit of current in vivo techniques may be more easily visualized with the same modalities in an ex vivo setting. While most microscopy techniques provide significantly higher resolution, enabling the visualization of fine cellular detail in ex vivo retinal samples, they do not replicate the reflectance-based imaging modalities of in vivo retinal imaging. Here, we introduce a strategy for imaging ex vivo samples using the same imaging modalities as those used for in vivo retinal imaging, but with increased resolution. We also demonstrate the ability of this approach to perform protein-specific fluorescence imaging and reflectance imaging simultaneously, enabling the visualization of nearly transparent layers of the retina and the classification of cone photoreceptor types.

Cone Photoreceptors in Diabetic Patients

Purpose

Cones in diabetic patients are at risk due to metabolic and vascular changes. By imaging retinal vessel modeling at high magnification, we reduced its impact on cone distribution measurements. The retinal vessel images and retinal thickness measurements provided information about cone microenvironment.

Methods

We compared cone data in 10 diabetic subjects (28–78 yr) to our published norms from 36 younger and 10 older controls. All subjects were consented and tested in a manner approved by the Indiana University Institutional Review Board, which adhered to the Declaration of Helsinki. Custom adaptive optics scanning laser ophthalmoscopy (AOSLO) was used to image cones and retinal microcirculation. We counted cones in a montage of foveal and temporal retina, using four non-contiguous samples within 0.9–7 deg that were selected for best visibility of cones and least pathology. The data were fit with a two parameter exponential model: ln(cone density) = a ^* microns eccentricity + b. These results were compared to retinal thickness measurements from SDOCT.

Results

Diabetic cone maps were more variable than in controls and included patches, or unusually bright and dark cones, centrally and more peripherally. Model parameters and total cones within the central 14 deg of the macula differed across diabetic patients. Total cones fell into two groups: similar to normal for 5 vs. less than normal for 2 of 2 younger diabetic subjects and 3 older subjects, low but not outside the confidence limits. Diabetic subjects had all retinal vascular remodeling to varying degrees: microaneurysms; capillary thickening, thinning, or bends; and vessel elongation including capillary loops, tangles, and collaterals. Yet SD-OCT showed that no diabetic subject had a Total Retinal Thickness in any quadrant that fell outside the confidence limits for controls.

Conclusions

AOSLO images pinpointed widespread retinal vascular remodeling in all diabetic eyes, but the SDOCT showed no increased retinal thickness. Cone reflectivity changes were found in all diabetic patients, but significantly low cone density in only some. These results are consistent with early changes to neural, glial, or vascular components of the retinal without significant retinal thickening due to exudation.