In Vivo Imaging of Schlemm's Canal and Limbal Vascular Network in Mouse Using Visible-Light OCT

High refresh rate display for natural monocular viewing in AOSLO psychophysics experiments

By combining an external display operating at 360 frames per second with an Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) for human foveal imaging, we demonstrate color stimulus delivery at high spatial and temporal resolution in AOSLO psychophysics experiments. A custom pupil relay enables viewing of the stimulus through a 3-mm effective pupil diameter and provides refractive error correction from -8 to +4 diopters. Performance of the assembled and aligned pupil relay was validated by measuring the wavefront error across the field of view and correction range, and the as-built Strehl ratio was 0.64 or better. High-acuity stimuli were rendered on the external display and imaged through the pupil relay to demonstrate that spatial frequencies up to 54 cycles per degree, corresponding to 20/11 visual acuity, are resolved. The completed external display was then used to render fixation markers across the field of view of the monitor, and a continuous retinal montage spanning 9.4 by 5.4 degrees of visual angle was acquired with the AOSLO. We conducted eye-tracking experiments during free-viewing and high-acuity tasks with polychromatic images presented on the external display. Sub-arcminute eye position uncertainty was achieved, enabling precise localization of the line of sight on the monitor while simultaneously imaging the fine structure of the human central fovea. This high refresh rate display overcomes the temporal, spectral, and field of view limitations of AOSLO-based stimulus presentation, enabling natural monocular viewing of stimuli in psychophysics experiments conducted with AOSLO.

Freeform robotic optical coherence tomography beyond the optical field-of-view limit

Imaging complex, non-planar anatomies with optical coherence tomography (OCT) is limited by the optical field of view (FOV) in a single volumetric acquisition. Combining linear mechanical translation with OCT extends the FOV but suffers from inflexibility in imaging non-planar anatomies. We report the freeform robotic OCT to fill this gap. To address challenges in volumetric reconstruction associated with the robotic movement accuracy being two orders of magnitudes worse than OCT imaging resolution, we developed a volumetric registration algorithm based on simultaneous localization and mapping (SLAM) to overcome this limitation. We imaged the entire aqueous humor outflow pathway, whose imaging has the potential to customize glaucoma surgeries but is typically constrained by the FOV, circumferentially in mice as a test. We acquired volumetric OCT data at different robotic poses and reconstructed the entire anterior segment of the eye. The reconstructed volumes showed heterogeneous Schlemm's canal (SC) morphology in the reconstructed anterior segment and revealed a segmental nature in the circumferential distribution of collector channels (CC) with spatial features as small as a few micrometers.

Multiscale imaging of corneal endothelium damage and Rho-kinase inhibitor application in mouse models of acute ocular hypertension

We developed a multiscale optical imaging workflow, integrating and correlating visible-light optical coherence tomography, confocal laser scanning microscopy, and single-molecule localization microscopy to investigate mouse cornea damage from the in-vivo tissue level to the nanoscopic single-molecule level. We used electron microscopy to validate the imaged nanoscopic structures. We imaged wild-type mice and mice with acute ocular hypertension and examined the effects of Rho-kinase inhibitor application. We defined four types of intercellular tight junction structures as healthy, compact, partially-distorted, and fully-distorted types by labeling the zonula occludens-1 protein in the corneal endothelial cell layer. We correlated the statistics of the four types of tight junction structures with cornea thickness and intraocular pressure. We found that the population of fully-distorted tight junctions correlated well with the level of corneal edema, and applying Rho-kinase inhibitor reduced the population of fully-distorted tight junctions under acute ocular hypertension. Together, these data point to the utility of multiscale optical imaging in revealing fundamental biology relevant to disease and therapeutics.

From Soma to Synapse: Imaging Age-Related Rod Photoreceptor Changes in the Mouse with Visible Light OCT

Purpose

Although the outer nuclear layer (ONL) and outer plexiform layer (OPL) each exhibit a complex internal organization, near-infrared OCT depicts both as monolithic bands. Here, using visible light OCT in the C57BL/6J mouse retina, sublaminar age-related changes in photoreceptor features were imaged and interpreted. These features were (1) oscillations in reflectivity, or striations, in the ONL and (2) a moderately reflective subband in the OPL.

Design

Cross-sectional study.

Participants

Pigmented mice (C57BL/6J, n = 14).

Methods

A 1.0-μm axial resolution visible light spectral/Fourier domain OCT system was used for in vivo retinal imaging. Light and electron microscopy were performed ex vivo. Linear mixed effects models or regression were employed for statistical analysis.

Main Outcome Measures

Comparison of OCT subbands with corresponding histological features, as well as quantification of subband thickness and reflectivity.

Results

Corresponding histological comparisons confirm that striations in the ONL arise from the rowlike arrangement of photoreceptor nuclei and reveal that the moderately reflective OPL subband arises from rod spherules. Compression of outer ONL striations with age suggests changes in soma organization. Thinning of the moderately reflective OPL subband with age supports a reduction of synapses in the OPL. Critically, the ONL somas are tightly correlated with the purported spherule layer but not with the rest of the OPL.

Conclusions

Visible light OCT imaging of the mouse OPL resolves postsynaptic and synaptic differences. Visible light OCT can study rod photoreceptor changes from the soma to the synapse in the living mouse retina.

Financial Disclosures

Proprietary or commercial disclosure may be found after the references.

Current clinical applications of anterior segment optical coherence tomography angiography: a review

Optical coherence tomography (OCT) is a revolutionary in vivo imaging technology that presents real-time information on ocular structures. Angiography based on OCT, known as optical coherence tomography angiography (OCTA), is a noninvasive and time-saving technique originally utilized for visualizing retinal vasculature. As devices and built-in systems have evolved, high-resolution images with depth-resolved analysis have assisted ophthalmologists in accurately localizing pathology and monitoring disease progression. With the aforementioned advantages, application of OCTA has extended from the posterior to anterior segment. This nascent adaptation showed good delineation of the vasculature in the cornea, conjunctiva, sclera, and iris. Thus, neovascularization of the avascular cornea and hyperemia or ischemic changes involving the conjunctiva, sclera, and iris has become prospective applications for AS-OCTA. Although traditional dye-based angiography is regarded as the gold standard in demonstrating vasculature in the anterior segment, AS-OCTA is expected to be a comparable but more patient-friendly alternative. In its initial stage, AS-OCTA has exhibited great potential in pathology diagnosis, therapeutic evaluation, presurgical planning, and prognosis assessments in anterior segment disorders. In this review of AS-OCTA, we aim to summarize scanning protocols, relevant parameters, and clinical applications as well as limitations and future directions. We are sanguine about its wide application in the future with the development of technology and refinement in built-in systems.

Scanning darkfield high-resolution microendoscope for label-free microvascular imaging

Characterization of microvascular changes during neoplastic progression has the potential to assist in discriminating precancer and early cancer from benign lesions. Here, we introduce a novel high-resolution microendoscope that leverages scanning darkfield reflectance imaging to characterize angiogenesis without exogenous contrast agents. Scanning darkfield imaging is achieved by coupling programmable illumination with a complementary metal-oxide semiconductor (CMOS) camera rolling shutter, eliminating the need for complex optomechanical components and making the system portable, low-cost (<$5,500) and simple to use. Imaging depth is extended by placing a gradient-index (GRIN) lens at the distal end of the imaging fiber to resolve subepithelial microvasculature. We validated the capability of the scanning darkfield microendoscope to visualize microvasculature at different anatomic sites in vivo by imaging the oral cavity of healthy volunteers. Images of cervical specimens resected for suspected neoplasia reveal distinct microvascular patterns in columnar and squamous epithelium with different grades of precancer, indicating the potential of scanning darkfield microendoscopy to aid in efforts to prevent cervical cancer through early diagnosis.

Differential roles of FOXC2 in the trabecular meshwork and Schlemm's canal in glaucomatous pathology

Impaired development and maintenance of Schlemm's canal (SC) are associated with perturbed aqueous humor outflow and intraocular pressure. The angiopoietin (ANGPT)/TIE2 signaling pathway regulates SC development and maintenance, whereas the molecular mechanisms of crosstalk between SC and the neural crest (NC)-derived neighboring tissue, the trabecular meshwork (TM), are poorly understood. Here, we show NC-specific forkhead box (Fox)c2 deletion in mice results in impaired SC morphogenesis, loss of SC identity, and elevated intraocular pressure. Visible-light optical coherence tomography analysis further demonstrated functional impairment of the SC in response to changes in intraocular pressure in NC-Foxc2 -/- mice, suggesting altered TM biomechanics. Single-cell RNA-sequencing analysis identified that this phenotype is predominately characterized by transcriptional changes associated with extracellular matrix organization and stiffness in TM cell clusters, including increased matrix metalloproteinase expression, which can cleave the TIE2 ectodomain to produce soluble TIE2. Moreover, endothelial-specific Foxc2 deletion impaired SC morphogenesis because of reduced TIE2 expression, which was rescued by deleting the TIE2 phosphatase VE-PTP. Thus, Foxc2 is critical in maintaining SC identity and morphogenesis via TM-SC crosstalk.

Iridocorneal angle imaging of a human donor eye by spectral-domain optical coherence tomography

Iridocorneal angle (ICA) details particularly the trabecular meshwork (TM), Schlemm's canal (SC), and collector channels (CCs) play crucial roles in the regulation of the aqueous outflow in the eyes and are closely associated with glaucoma. Current clinical gonioscopy imaging provides no depth information, and studies of 3D high-resolution optical coherence tomography (OCT) imaging of these structures are limited. We developed a custom-built spectral-domain (SD-) OCT imaging system to fully characterize the angle details. Imaging of a human cadaver eye reveals the visibility of details in the TM/SC/CC region via a 'crossline' scanning and a series of image processing. This shows that ICA imaging can be used for preoperative glaucoma inspections in the clinical setting with the proposed prototype.

Multiscale imaging of corneal endothelium damage and effects of Rho Kinase inhibitor application in mouse models of acute ocular hypertension

We developed a multiscale optical imaging workflow, integrating and correlating visible-light optical coherence tomography, confocal laser scanning microscopy, and single-molecule localization microscopy to investigate the mouse cornea damages from the in-vivo tissue level to the nanoscopic single-molecule level. We used electron microscopy to validate the imaged nanoscopic structures. We imaged wild-type mice and mice with acute ocular hypertension and examined the effects of Rho Kinase inhibitor application. We defined four types of intercellular tight junction structures as healthy, compact, partially-distorted, and fully-distorted types by labeling the Zonula occludens-1 protein in the corneal endothelial cell layer. We correlated the statistics of the four types of tight junction structures with cornea thickness and intraocular pressure. We found that the population of fully-distorted tight junctions correlated well with the level of cornea edema, and applying Rho Kinase inhibitor reduced the population of fully-distorted tight junctions under acute ocular hypertension.

Recent advances in optical coherence tomography for anterior segment imaging in small animals and their clinical implications

Anterior segment optical coherence tomography (AS-OCT) is a rapidly evolving area of OCT imaging, providing high-resolution and non-invasive volumetric imaging of the anterior segment. This review focuses on recent advances in AS-OCT imaging in small animals, which we categorize into ultrahigh-resolution, spectroscopic, magnetomotive, polarization-sensitive, and angiographic AS-OCTs. We summarize their technical foundations, review their applications to small animal imaging, and briefly discuss their current and future clinical applications.

Multiple forward scattering reduces the measured scattering coefficient of whole blood in visible-light optical coherence tomography

The optical properties of blood encode oxygen-dependent information. Noninvasive optical detection of these properties is increasingly desirable to extract biomarkers for tissue health. Recently, visible-light optical coherence tomography (vis-OCT) demonstrated retinal oxygen saturation (sO2) measurements by inversely measuring the oxygen-dependent absorption and scattering coefficients of whole blood. However, vis-OCT may be sensitive to optical scattering properties of whole blood, different from those reported in the literature. Incorrect assumptions of such properties can add additional uncertainties or biases to vis-OCT's sO2 model. This work investigates whole blood's scattering coefficient measured by vis-OCT. Using Monte Carlo simulation of a retinal vessel, we determined that vis-OCT almost exclusively detects multiple-scattered photons in whole blood. Meanwhile, photons mostly forward scatter in whole blood within the visible spectral range, allowing photons to maintain ballistic paths and penetrate deeply, leading to a reduction in the measured scattering coefficient. We defined a scattering scaling factor (SSF) to account for such a reduction and found that SSF varied with measurement conditions, such as numerical aperture, depth resolution, and depth selection. We further experimentally validated SSF in ex vivo blood phantoms with pre-set sO2 levels and in the human retina, both of which agreed well with our simulation.

The Development and Clinical Application of Innovative Optical Ophthalmic Imaging Techniques

The field of ophthalmic imaging has grown substantially over the last years. Massive improvements in image processing and computer hardware have allowed the emergence of multiple imaging techniques of the eye that can transform patient care. The purpose of this review is to describe the most recent advances in eye imaging and explain how new technologies and imaging methods can be utilized in a clinical setting. The introduction of optical coherence tomography (OCT) was a revolution in eye imaging and has since become the standard of care for a plethora of conditions. Its most recent iterations, OCT angiography, and visible light OCT, as well as imaging modalities, such as fluorescent lifetime imaging ophthalmoscopy, would allow a more thorough evaluation of patients and provide additional information on disease processes. Toward that goal, the application of adaptive optics (AO) and full-field scanning to a variety of eye imaging techniques has further allowed the histologic study of single cells in the retina and anterior segment. Toward the goal of remote eye care and more accessible eye imaging, methods such as handheld OCT devices and imaging through smartphones, have emerged. Finally, incorporating artificial intelligence (AI) in eye images has the potential to become a new milestone for eye imaging while also contributing in social aspects of eye care.

Open-source deep learning-based automatic segmentation of mouse Schlemm's canal in optical coherence tomography images

The purpose of this study was to develop an automatic deep learning-based approach and corresponding free, open-source software to perform segmentation of the Schlemm's canal (SC) lumen in optical coherence tomography (OCT) scans of living mouse eyes. A novel convolutional neural network (CNN) for semantic segmentation grounded in a U-Net architecture was developed by incorporating a late fusion scheme, multi-scale input image pyramid, dilated residual convolution blocks, and attention-gating. 163 pairs of intensity and speckle variance (SV) OCT B-scans acquired from 32 living mouse eyes were used for training, validation, and testing of this CNN model for segmentation of the SC lumen. The proposed model achieved a mean Dice Similarity Coefficient (DSC) of 0.694 ± 0.256 and median DSC of 0.791, while manual segmentation performed by a second expert grader achieved a mean and median DSC of 0.713 ± 0.209 and 0.763, respectively. This work presents the first automatic method for segmentation of the SC lumen in OCT images of living mouse eyes. The performance of the proposed model is comparable to the performance of a second human grader. Open-source automatic software for segmentation of the SC lumen is expected to accelerate experiments for studying treatment efficacy of new drugs affecting intraocular pressure and related diseases such as glaucoma, which present as changes in the SC area.

Surface Engineering of FLT4-Targeted Nanocarriers Enhances Cell-Softening Glaucoma Therapy

Primary open-angle glaucoma is associated with elevated intraocular pressure (IOP) that damages the optic nerve and leads to gradual vision loss. Several agents that reduce the stiffness of pressure-regulating Schlemm's canal (SC) endothelial cells, in the conventional outflow pathway of the eye, lower IOP in glaucoma patients and are approved for clinical use. However, poor drug penetration and uncontrolled biodistribution limit their efficacy and produce local adverse effects. Compared to other ocular endothelia, FLT4/VEGFR3 is expressed at elevated levels by SC endothelial cells and can be exploited for targeted drug delivery. Here, we validate FLT4 receptors as clinically relevant targets on SC cells from glaucomatous human donors and engineer polymeric self-assembled nanocarriers displaying lipid-anchored targeting ligands that optimally engage this receptor. Targeting constructs were synthesized as lipid-PEGx-peptide, differing in the number of PEG spacer units (x), and were embedded in micelles. We present a novel proteolysis assay for quantifying ligand accessibility that we employ to design and optimize our FLT4-targeting strategy for glaucoma nanotherapy. Peptide accessibility to proteases correlated with receptor-mediated targeting enhancements. Increasing the accessibility of FLT4-binding peptides enhanced nanocarrier uptake by SC cells while simultaneously decreasing the uptake by off-target vascular endothelial cells. Using a paired longitudinal IOP study in vivo, we show that this enhanced targeting of SC cells translates to IOP reductions that are sustained for a significantly longer time as compared to controls. Confocal microscopy of murine anterior segment tissue confirmed nanocarrier localization to SC within 1 h after intracameral administration. This work demonstrates that steric effects between surface-displayed ligands and PEG coronas significantly impact the targeting performance of synthetic nanocarriers across multiple biological scales. Minimizing the obstruction of modular targeting ligands by PEG measurably improved the efficacy of glaucoma nanotherapy and is an important consideration for engineering PEGylated nanocarriers for targeted drug delivery.

Full circumferential morphological analysis of Schlemm's canal in human eyes using megahertz swept source OCT

We performed full circumferential imaging of the Schlemm's canal (SC) of two human eyes using a Fourier domain mode-lock laser (FDML) based 1.66-MHz SS-OCT prototype at 1060 nm. Eight volumes with overlapping margins were acquired around the limbal area with customized raster scanning patterns designed to fully cover the SC while minimizing motion artifacts. The SC was segmented from the volumes using a semi-automated active contour segmentation algorithm, whose mean dice similarity coefficient was 0.76 compared to the manual segmentation results. We also reconstructed three-dimensional (3D) renderings of the 360° SC by stitching the segmented SCs from the volumetric datasets. Quantitative metrics of the full circumferential SC were provided, including the mean and standard deviation (SD) of the cross-sectional area (CSA), the maximum CSA, the minimum and maximum SC opening width, and the number of collector channels (CC) stemming from the SC.

Comparison of 1-year surgical outcomes of combined cataract surgery and gonioscopy-assisted transluminal trabeculotomy (GATT) versus cataract surgery and iStent Inject

PURPOSE

Evaluate the efficacy, safety, and complication rates of phacoemulsification cataract surgery when combined with either gonioscopy-assisted transluminal trabeculotomy (GATT) or iStent Inject.

METHODS

This is a retrospective case-control study to compare the surgical outcomes of combined phacoemulsification cataract surgery with either GATT (phaco-GATT) or iStent Inject (phaco-iStent). Both groups had at least 1-year follow-up. The primary outcome measures were IOP and number of glaucoma medications (NGMs) at 1 year. Secondary outcomes measures were best corrected visual acuity (BCVA) and intra- or postoperative complications within the first year of follow-up. Success was defined as intraocular pressure (IOP) < 21 mmHg and ≥ 20% reduction in IOP at 1 year regardless of the NGM.

RESULTS

Each group included 37 patients. The median baseline IOP (24 vs 17) and NGM (3 vs 2) were higher in the phaco-GATT group (p < 0.001). Phaco-GATT achieved a 38% (p < 0.0001) reduction in IOP compared to 13.2% (p < 0.001) in the phaco-iStent group at 1-year follow-up. The reduction in IOP and NGM was significantly higher in the phaco-GATT group (p < 0.01). After adjusting for baseline IOP, the reduction in IOP at 12 months was still significantly higher in the phaco-GATT group (p = 0.042). At 1 year, 86.4% of patients in the phaco-GATT group met the success criteria compared to 35.1% in the phaco-iStent group. Safety outcomes were slightly favourable in the phaco-iStent group.

CONCLUSION

Phaco-GATT and phaco-iStent showed a significant reduction in IOP and NGM, with phaco-GATT having a significantly higher reduction. Phaco-iStent appears to have a higher safety profile and is probably preferable in monocular patients and those with a high risk of bleeding.

Effect of postoperative corticosteroids on surgical outcome and aqueous autotaxin following combined cataract and microhook ab interno trabeculotomy

To evaluate the effect of postoperative corticosteroids on surgical outcome and autotaxin (ATX) levels after microhook ab interno trabeculotomy combined with cataract surgery (μLOT-CS), prospective, consecutive non-randomized case series comparing outcomes of 30 eyes with primary open angle glaucoma was performed. The aqueous ATX, intraocular pressure (IOP) and glaucoma medications were monitored for 3 months postoperatively. An in-vivo mouse μLOT model was generated. In vitro, ATX and fibrotic changes induced by dexamethasone (Dex) treatment following scratch (S) in cultured human trabecular meshwork (hTM) cells were assessed by immunofluorescence, immunoenzymatic assay, and RT-qPCR. Postoperative ATX at 1 week and the number of antiglaucoma medications at 3 months were significantly lower in non-steroid group, and steroid use was the only variable significantly associated with postoperative medications at 3 months in multiregression analyses. In vitro, ATX activity was significantly upregulated in the Dex + S group, and αSMA was significantly upregulated in the Dex and Dex + S groups. Fibronectin and COL1A1 were significantly upregulated in the S group. μLOT-CS decreased IOP and medications in the overall cohort, and non-use of postoperative steroids resulted in a smaller number of postoperative medications. Limiting postoperative steroids in μLOT may minimize IOP elevation and postoperative fibrosis.

Diagnosing colorectal abnormalities using scattering coefficient maps acquired from optical coherence tomography

Optical coherence tomography (OCT) has shown potential in differentiating normal colonic mucosa from neoplasia. In this study of 33 fresh human colon specimens, we report the first use of texture features and computer vision-based imaging features acquired from en face scattering coefficient maps to characterize colorectal tissue. En face scattering coefficient maps were generated automatically using a new fast integral imaging algorithm. From these maps, a gray-level cooccurrence matrix algorithm was used to extract texture features, and a scale-invariant feature transform algorithm was used to derive novel computer vision-based features. In total, 25 features were obtained, and the importance of each feature in diagnosis was evaluated using a random forest model. Two classifiers were assessed on two different classification tasks. A support vector machine model was found to be optimal for distinguishing normal from abnormal tissue, with 94.7% sensitivity and 94.0% specificity, while a random forest model performed optimally in further differentiating abnormal tissues (i.e., cancerous tissue and adenomatous polyp) with 86.9% sensitivity and 85.0% specificity. These results demonstrated the potential of using OCT to aid the diagnosis of human colorectal disease.

Integral role for lysyl oxidase-like-1 in conventional outflow tissue function and behavior

Lysyl oxidase-like-1 (LOXL1), a vital crosslinking enzyme in elastin fiber maintenance, is essential for the stability and strength of elastic vessels and tissues. Variants in the LOXL1 locus associate with a dramatic increase in risk of exfoliation syndrome (XFS), a systemic fibrillopathy, which often presents with ocular hypertension and exfoliation glaucoma (XFG). We examined the role of LOXL1 in conventional outflow function, the prime regulator of intraocular pressure (IOP). Using Loxl1^-/- , Loxl1^+/- , and Loxl1^+/+ mice, we observed an inverse relationship between LOXL1 expression and IOP, which worsened with age. Elevated IOP in Loxl1^-/- mice was associated with a larger globe, decreased ocular compliance, increased outflow facility, extracellular matrix (ECM) abnormalities, and dilated intrascleral veins, yet, no dilation of arteries or capillaries. Interestingly, in living Loxl1^-/- mouse eyes, Schlemm's canal (SC) was less susceptible to collapse when challenged with acute elevations in IOP, suggesting elevated episcleral venous pressure (EVP). Thus, LOXL1 expression is required for normal IOP control, while ablation results in altered ECM repair/homeostasis and conventional outflow physiology. Dilation of SC and distal veins, but not arteries, is consistent with key structural and functional roles for elastin in low-pressure vessels subjected to cyclical mechanical stress.