Optophysiology: depth-resolved probing of retinal physiology with functional ultrahigh-resolution optical coherence tomography

Bichromatic tetraphasic full-field optical coherence microscopy

Significance

Full-field optical coherence microscopy (FF-OCM) is a prevalent technique for backscattering and phase imaging with epi-detection. Traditional methods have two limitations: suboptimal utilization of functional information about the sample and complicated optical design with several moving parts for phase contrast.

Aim

We report an OCM setup capable of generating dynamic intensity, phase, and pseudo-spectroscopic contrast with single-shot full-field video-rate imaging called bichromatic tetraphasic (BiTe) full-field OCM with no moving parts.

Approach

BiTe OCM resourcefully uses the phase-shifting properties of anti-reflection (AR) coatings outside the rated bandwidths to create four unique phase shifts, which are detected with two emission filters for spectroscopic contrast.

Results

BiTe OCM overcomes the disadvantages of previous FF-OCM setup techniques by capturing both the intensity and phase profiles without any artifacts or speckle noise for imaging scattering samples in three-dimensional (3D). BiTe OCM also utilizes the raw data effectively to generate three complementary contrasts: intensity, phase, and color. We demonstrate BiTe OCM to observe cellular dynamics, image live, and moving micro-animals in 3D, capture the spectroscopic hemodynamics of scattering tissues along with dynamic intensity and phase profiles, and image the microstructure of fall foliage with two different colors.

Conclusions

BiTe OCM can maximize the information efficiency of FF-OCM while maintaining overall simplicity in design for quantitative, dynamic, and spectroscopic characterization of biological samples.

Parafoveal cone function in choroideremia assessed with adaptive optics optoretinography

Choroideremia (CHM) is an X-linked retinal degeneration leading to loss of the photoreceptors, retinal pigment epithelium (RPE), and choroid. Adaptive optics optoretinography is an emerging technique for noninvasive, objective assessment of photoreceptor function. Here, we investigate parafoveal cone function in CHM using adaptive optics optoretinography and compare with cone structure and clinical assessments of vision. Parafoveal cone mosaics of 10 CHM and four normal-sighted participants were imaged with an adaptive optics scanning light ophthalmoscope. While acquiring video sequences, a 2 s 550Δ10 nm, 450 nW/deg2 stimulus was presented. Videos were registered and the intensity of each cone in each frame was extracted, normalized, standardized, and aggregated to generate the population optoretinogram (ORG) over time. A gamma-pdf was fit to the ORG and the peak was extracted as ORG amplitude. CHM ORG amplitudes were compared to normal and were correlated with bound cone density, ellipsoid zone to RPE/Bruch's membrane (EZ-to-RPE/BrM) distance, and foveal sensitivity using Pearson correlation analysis. ORG amplitude was significantly reduced in CHM compared to normal (0.22 ± 0.15 vs. 1.34 ± 0.31). In addition, CHM ORG amplitude was positively correlated with cone density, EZ-to-RPE/BrM distance, and foveal sensitivity. Our results demonstrate promise for using ORG as a biomarker of photoreceptor function.

The Impact of Photopigment Bleaching on the Human Rod Photoreceptor Subretinal Space Measured Via Optical Coherence Tomography

Purpose

The purpose of this study was to investigate rod photopigment bleaching-driven intrinsic optical signals (IOS) in the human outer retina and its measurement repeatability based on a commercial optical coherence tomography (OCT) platform.

Methods

The optical path length of the rod photoreceptor subretinal space (SRS), that is, the distance between signal bands of rod outer segment tips and retinal pigment epithelium, was measured in 15 healthy subjects in ambient light and during a long-duration bleaching white-light exposure.

Results

On 2 identical study days (day 1 and day 2 [D1 and D2]), light stimulation resulted in a significant decrease in rod SRS by 21.3 ± 7.6% and 19.8 ± 8.5% (both P < 0.001), respectively. The test-retest reliability of the SRS maximum change of an individual subject was moderate for single measures (intraclass correlation coefficient [ICC] = 0.730, 95% confidence interval [CI] = 0.376, 0.900, P < 0.001) and good for average measures (ICC = 0.844, 95% CI = 0.546, 0.947, P < 0.001). The mean area under the stimulus response curve with values of 14.8 ± 9.4 and 15.5 ± 7.5 µm × minutes (P = 0.782) showed excellent agreement between the stimulus response on D1 and D2. Intermittent dark adaptation of the retina led to an initial increase of the SRS by 6.1% (P = 0.018) and thereafter showed a decrease toward baseline, despite continued dark adaptation.

Conclusions

The data indicate the potential of commercial OCT in measuring slow IOS in the outer retina suggesting that the rod SRS could serve as a biomarker for photoreceptor function. The presented approach could provide an easily implementable clinical tool for the early detection of diseases affecting photoreceptor health.

Intrinsic signal optoretinography of dark adaptation abnormality due to rod photoreceptor degeneration

This research aims to investigate the potential of using intrinsic optical signal (IOS) optoretinography (ORG) to objectively detect dark adaptation (DA) abnormalities related to rod photoreceptor degeneration. Functional optical coherence tomography (OCT) was employed in both wild-type (WT) and retinal degeneration 10 (rd10) mice to conduct this assessment. Dynamic OCT measurements captured the changes in retinal thickness and reflectance from light-to-dark transition. Comparative analysis revealed significant IOS alterations within the outer retina. Specifically, a reduction in thickness from external limiting membrane (ELM) peak to retinal pigment epithelium (RPE) peak was observed (WT: 1.13 ± 0.69 µm, 30 min DA; rd10: 2.64 ± 0.86 µm, 30 min DA), as well as a decrease in the intensity of the inner segment ellipsoid zone (EZ) in 30 min DA compared to light adaptation (LA). The reduction of relative EZ intensity was notable in rd10 after 5 min DA and in WT after 15 min DA, with a distinguishable difference between rd10 and WT after 10 min DA. Furthermore, our findings indicated a significant decrease in the relative intensity of the hypo-reflective band between EZ and RPE in rd10 retinas during DA, which primarily corresponds to the outer segment (OS) region. In conclusion, the observed DA-IOS abnormalities, including changes in ELM-RPE thickness, EZ, and OS intensity, hold promise as differentiators between WT and rd10 mice before noticeable morphological abnormalities occur. These findings suggest the potential of this non-invasive imaging technique for the early detection of dysfunction in retinal photoreceptors.

Transducin-Deficient Rod Photoreceptors Evaluated With Optical Coherence Tomography and Oxygen Consumption Rate Energy Biomarkers

Purpose

To test the hypothesis that rod energy biomarkers in light and dark are similar in mice without functional rod transducin (Gnat1rd17).

Methods

Gnat1rd17 and wildtype (WT) mice were studied in canonically low energy demand (light) and high energy demand (dark) conditions. We measured rod inner segment ellipsoid zone (ISez) profile shape, external limiting membrane-retinal pigment epithelium (ELM-RPE) thickness, and magnitude of a hyporeflective band (HB) intensity dip located between photoreceptor tips and apical RPE; antioxidants were given in a subset of mice. Oxygen consumption rate (OCR) and visual performance indexes were also measured.

Results

The lower energy demand expected in light-adapted wildtype retinas was associated with an elongated ISez, thicker ELM-RPE, and higher HB magnitude, and lower OCR compared to high energy demand conditions in the dark. Gnat1rd17 mice showed a wildtype-like ISez profile shape at 20 minutes of light that became rounder at 60 minutes; at both times, ELM-RPE was smaller than wildtype values, and the HB magnitude was unmeasurable. OCR was higher than in the dark. Light-adapted Gnat1rd17 mice biomarkers were unaffected by anti-oxidants. Gnat1rd17 mice showed modest outer nuclear layer thinning and no reduction in visual performance indexes.

Conclusions

Light-stimulated changes in all biomarkers in WT mice are consistent with the established light-induced decrease in net energy demand. In contrast, biomarker changes in Gnat1rd17 mice raise the possibility that light increases net energy demand in the absence of rod phototransduction.

Retinal imaging using adaptive optics optical coherence tomography with fast and accurate real-time tracking

One of the main obstacles in high-resolution 3-D retinal imaging is eye motion, which causes blur and distortion artifacts that require extensive post-processing to be corrected. Here, an adaptive optics optical coherence tomography (AOOCT) system with real-time active eye motion correction is presented. Correction of ocular aberrations and of retinal motion is provided by an adaptive optics scanning laser ophthalmoscope (AOSLO) that is optically and electronically combined with the AOOCT system. We describe the system design and quantify its performance. The AOOCT system features an independent focus adjustment that allows focusing on different retinal layers while maintaining the AOSLO focus on the photoreceptor mosaic for high fidelity active motion correction. The use of a high-quality reference frame for eye tracking increases revisitation accuracy between successive imaging sessions, allowing to collect several volumes from the same area. This system enables spatially targeted retinal imaging as well as volume averaging over multiple imaging sessions with minimal correction of motion in post processing.

Coarse-scale optoretinography (CoORG) with extended field-of-view for normative characterization

Optoretinography (ORG) has the potential to be an effective biomarker for light-evoked retinal activity owing to its sensitive, objective, and precise localization of retinal function and dysfunction. Many ORG implementations have used adaptive optics (AO) to localize activity on a cellular scale. However, the use of AO restricts field-of-view (FOV) to the isoplanatic angle, necessitating the montaging of multiple regions-of-interest to cover an extended field. In addition, subjects with lens opacities, increased eye movements and decreased mobility pose challenges for effective AO operation. Here, we developed a coarse-scale ORG (CoORG) system without AO, which accommodates FOVs up to 5.5 deg. in a single acquisition. The system is based on a line-scan spectral domain OCT with volume rates of up to 32 Hz (16,000 B-frames per second). For acquiring ORGs, 5.5 deg. wide OCT volumes were recorded after dark adaptation and two different stimulus bleaches. The stimulus-evoked optical phase change was calculated from the reflections encasing the cone outer segments and its variation was assessed vs. eccentricity in 12 healthy subjects. The general behavior of ΔOPL vs. time mimicked published reports. High trial-to-trial repeatability was observed across subjects and with eccentricity. Comparison of ORG between CoORG and AO-OCT based ORG at 1.5°, 2.5°, and 3.5° eccentricity showed an excellent agreement in the same 2 subjects. The amplitude of the ORG response decreased with increasing eccentricity. The variation of ORG characteristics between subjects and versus eccentricity was well explained by the photon density of the stimulus on the retina and the outer segment length. Overall, the high repeatability and rapid acquisition over an extended field enabled the normative characterization of the cone ORG response in healthy eyes, and provides a promising avenue for translating ORG for widespread clinical application.

Axonal architecture of the mouse inner retina revealed by second harmonic generation

We describe a novel method for visualizing the network of axons in the unlabeled fresh wholemount retina. The intrinsic radiation of second harmonic generation (SHG) was utilized to visualize single axons of all major retinal neurons, i.e., photoreceptors, horizontal cells, bipolar cells, amacrine cells, and the retinal ganglion cells. The cell types of SHG+ axons were determined using transgenic GFP/YFP mice. New findings were obtained with retinal SHG imaging: Müller cells do not maintain uniformly polarized microtubules in the processes; SHG+ axons of bipolar cells terminate in the inner plexiform layer (IPL) in a subtype-specific manner; a subset of amacrine cells, presumably the axon-bearing types, emits SHG; and the axon-like neurites of amacrine cells provide a cytoskeletal scaffolding for the IPL stratification. To demonstrate the utility, retinal SHG imaging was applied to testing whether the inner retina is preserved in glaucoma, using DBA/2 mice as a model of glaucoma and DBA/2-Gpnmb+ as the nonglaucomatous control. It was found that the morphology of the inner retina was largely intact in glaucoma and the presynaptic compartments to the retinal ganglion cells were uncompromised. It proves retinal SHG imaging as a promising technology for studying the physiological and diseased retinas in 3D.

Functional Optical Coherence Tomography for Intrinsic Signal Optoretinography: Recent Developments and Deployment Challenges

Intrinsic optical signal (IOS) imaging of the retina, also termed as optoretinogram or optoretinography (ORG), promises a non-invasive method for the objective assessment of retinal function. By providing the unparalleled capability to differentiate individual retinal layers, functional optical coherence tomography (OCT) has been actively investigated for intrinsic signal ORG measurements. However, clinical deployment of functional OCT for quantitative ORG is still challenging due to the lack of a standardized imaging protocol and the complication of IOS sources and mechanisms. This article aims to summarize recent developments of functional OCT for ORG measurement, OCT intensity- and phase-based IOS processing. Technical challenges and perspectives of quantitative IOS analysis and ORG interpretations are discussed.

Light-adapted flicker optoretinograms captured with a spatio-temporal optical coherence-tomography (STOC-T) system

For many years electroretinography (ERG) has been used for obtaining information about the retinal physiological function. More recently, a new technique called optoretinography (ORG) has been developed. In one form of this technique, the physiological response of retinal photoreceptors to visible light, resulting in a nanometric photoreceptor optical path length change, is measured by phase-sensitive optical coherence tomography (OCT). To date, a limited number of studies with phase-based ORG measured the retinal response to a flickering light stimulation. In this work, we use a spatio-temporal optical coherence tomography (STOC-T) system to capture optoretinograms with a flickering stimulus over a 1.7 × 0.85 mm² area of a light-adapted retina located between the fovea and the optic nerve. We show that we can detect statistically-significant differences in the photoreceptor optical path length (OPL) modulation amplitudes in response to different flicker frequencies and with better signal to noise ratios (SNRs) than for a dark-adapted eye. We also demonstrate the ability to spatially map such response to a patterned stimulus with light stripes flickering at different frequencies, highlighting the prospect of characterizing the spatially-resolved temporal-frequency response of the retina with ORG.

Volumetric data analysis enabled spatially resolved optoretinogram to measure the functional signals in the living retina

Optoretinogram, a technique in which optical coherence tomography (OCT) is used to measure retinal functions in response to a visible light stimulus, can be a potentially useful tool to quantify retinal health alterations. Existing experimental studies on animals have focused on measuring the global retinal response by transversally averaging 3D data across the retina, which minimizes the spatial resolution of the signals, and limits the signal-to-noise ratio because only central B-scans are collected and analyzed. These problems were addressed in this study by collecting volumetric data to probe functional signals and developing an improved 3D registration approach to align such series-acquired OCT volumes. These data were then divided into small blocks and subject to a spatiotemporal analysis, whose results confirmed the spatial-dependence of functional signals. By further averaging, the overall measurement accuracies for the position and the scattering signals were estimated to be approximately 30 nm and 1.1 %, respectively. With improved accuracy, this method revealed certain novel functional signals that have not been previously reported. In conclusion, this work provides a powerful tool to monitor retinal local and global functional changes in aging, diseased, or treated rodent eyes.

Correlation Between Retinal Microstructure Detected by Optical Coherence Tomography and Best Corrected Visual Acuity in Diabetic Retinopathy Macular Edema

OBJECTIVE

This study aimed to investigate the correlation between best corrected visual acuity (BCVA) and retinal microstructural parameters detected by optical coherence tomography (OCT) in diabetic retinopathy macular edema (DRME).

METHODS

Thirty-nine patients (64 eyes) with DRME were enrolled in this study. These patients underwent OCT to measure the fracture distance of the external limiting membrane (ELM), junction between the inner and outer segments (IS/OS), central foveal thickness (CFT), and edema layer. The correlation between the above parameters and BCVA was discussed.

RESULTS

CFT and the fracture distances of the ELM and IS/OS layers were negatively correlated with BCVA (p<0.05 for all). There was significant difference in Logarithm of the minimum angle of resolution (LogMAR) BCVA among patients with inner retinal edema, outer retinal edema, and mixed retinal edema (F = 5.57, p = 0.01). The LogMAR BCVA of inner retinal edema was the lowest (p < 0.05), and the LogMAR BCVA of outer retinal edema and mixed retinal edema were comparable (p > 0.05).

CONCLUSION

In eyes with DRME, thin CFT, intact ELM and IS/OS layers, and edema in inner retina is closely correlated with good BCVA.

Quantification of intrinsic optical signals in the outer human retina using optical coherence tomography

Intrinsic optical signals constitute a noninvasive biomarker promising the objective assessment of retinal photoreceptor function. We employed a commercial optical coherence tomography (OCT) system and an OCT signal model for evaluation of optical path length (OPL) changes in the temporal outer retina of five healthy subjects during light adaptation. Data were acquired at 30 time points, in ambient light and during long duration stimulation with white light, and analyzed, employing a signal model based on the sum of seven Gaussian curves corresponding to all relevant anatomical structures of the outer retina. During light stimulation, mean OPL between rod outer segment tips (ROST) and the retinal pigment epithelium (RPE) decreased by 21.4 ± 3.5%. Further, OPL between the external‐limiting membrane (ELM) and the RPE decreased by 5.2 ± 0.9% versus baseline, while OPL between ELM and ROST showed an initial decrease by 2.1 ± 1.6% versus baseline and, thereafter, increased by 2.8 ± 2.1% versus baseline. Thus, the presented approach allowed for assess to dynamic changes in the outer retina in response to light. The change in the subretinal space occurring in the context of light adaptation could be measured using a standard OCT platform and a dedicated signal model.

Challenges Associated With Ellipsoid Zone Intensity Measurements Using Optical Coherence Tomography

Translational Relevance

Qualitative evaluation of the ellipsoid zone band on optical coherence tomography is a valuable clinical tool for assessing photoreceptor structure, though more quantitative metrics are emerging. Awareness of the challenges involved in interpreting quantitative metrics is important for their clinical translation.

Enhanced medical diagnosis for dOCTors: a perspective of optical coherence tomography

SIGNIFICANCE

After three decades, more than 75,000 publications, tens of companies being involved in its commercialization, and a global market perspective of about USD 1.5 billion in 2023, optical coherence tomography (OCT) has become one of the fastest successfully translated imaging techniques with substantial clinical and economic impacts and acceptance.

AIM

Our perspective focuses on disruptive forward-looking innovations and key technologies to further boost OCT performance and therefore enable significantly enhanced medical diagnosis.

APPROACH

A comprehensive review of state-of-the-art accomplishments in OCT has been performed.

RESULTS

The most disruptive future OCT innovations include imaging resolution and speed (single-beam raster scanning versus parallelization) improvement, new implementations for dual modality or even multimodality systems, and using endogenous or exogenous contrast in these hybrid OCT systems targeting molecular and metabolic imaging. Aside from OCT angiography, no other functional or contrast enhancing OCT extension has accomplished comparable clinical and commercial impacts. Some more recently developed extensions, e.g., optical coherence elastography, dynamic contrast OCT, optoretinography, and artificial intelligence enhanced OCT are also considered with high potential for the future. In addition, OCT miniaturization for portable, compact, handheld, and/or cost-effective capsule-based OCT applications, home-OCT, and self-OCT systems based on micro-optic assemblies or photonic integrated circuits will revolutionize new applications and availability in the near future. Finally, clinical translation of OCT including medical device regulatory challenges will continue to be absolutely essential.

CONCLUSIONS

With its exquisite non-invasive, micrometer resolution depth sectioning capability, OCT has especially revolutionized ophthalmic diagnosis and hence is the fastest adopted imaging technology in the history of ophthalmology. Nonetheless, OCT has not been completely exploited and has substantial growth potential-in academics as well as in industry. This applies not only to the ophthalmic application field, but also especially to the original motivation of OCT to enable optical biopsy, i.e., the in situ imaging of tissue microstructure with a resolution approaching that of histology but without the need for tissue excision.

Toward a clinical optoretinogram: a review of noninvasive, optical tests of retinal neural function

The past few years have witnessed rapid development of the optoretinogram—a noninvasive, optical measurement of neural function in the retina, and especially the photoreceptors (Ph). While its recent development has been rapid, it represents the culmination of hundreds of experiments spanning decades. Early work showed measurable and reproducible changes in the optical properties of retinal explants and suspensions of Ph, and uncovered some of the biophysical and biochemical mechanisms underlying them. That work thus provided critical motivation for more recent work based on clinical imaging platforms, whose eventual goal is the improvement of ophthalmic care and streamlining the discovery of novel therapeutics. The first part of this review consists of a selective summary of the early work, and identifies four kinds of stimulus-evoked optical signals that have emerged from it: changes in light scattered from the membranous discs of the Ph’s outer segment (OS), changes in light scattered by the front and back boundaries of the OS, rearrangement of scattering material in and near the OS, and changes in the OS length. In the past decade, all four of these signals have continued to be investigated using imaging systems already used in the clinic or intended for clinical and translational use. The second part of this review discusses these imaging modalities, their potential to detect and quantify the signals of interest, and other factors influencing their translational promise. Particular attention is paid to phase-sensitive optical coherence tomography (OCT) with adaptive optics (AO), a method in which both the amplitude and the phase of light reflected from individual Ph is monitored as visible stimuli are delivered to them. The record of the light’s phase is decoded to reveal a reproducible pattern of deformation in the OS, while the amplitude reveals changes in scattering and structural rearrangements. The method has been demonstrated in a few labs and has been used to measure responses from both rods and cones. With the ability to detect responses to stimuli isomerizing less than 0.01% of photopigment, this technique may prove to be a quick, noninvasive, and objective way to measure subtle disease-related dysfunction at the cellular level, and thus to provide an entirely new and complementary biomarker for retinal disease and recovery.

Millimeter-scale chip-based supercontinuum generation for optical coherence tomography

Supercontinuum sources for optical coherence tomography (OCT) have raised great interest as they provide broad bandwidth to enable high resolution and high power to improve imaging sensitivity. Commercial fiber-based supercontinuum systems require high pump powers to generate broad bandwidth and customized optical filters to shape/attenuate the spectra. They also have limited sensitivity and depth performance. We introduce a supercontinuum platform based on a 1-mm² Si₃N₄ photonic chip for OCT. We directly pump and efficiently generate supercontinuum near 1300 nm without any postfiltering. With a 25-pJ pump pulse, we generate a broadband spectrum with a flat 3-dB bandwidth of 105 nm. Integrating the chip into a spectral domain OCT system, we achieve 105-dB sensitivity and 1.81-mm 6-dB sensitivity roll-off with 300-μW optical power on sample. We image breast tissue to demonstrate strong imaging performance. Our chip will pave the way toward portable OCT and incorporating integrated photonics into optical imaging technologies.

Correlation of Visual Recovery and Increased Ellipsoid Zone Reflectivity After Successful Macular Hole Surgery

Objectives:

To assess changes in reflectivity of the retinal pigment epithelium (RPE), ellipsoid zone (EZ), and external limiting membrane (ELM) on spectral domain-optical coherence tomography (SD-OCT) images and the effects of reflectivity changes on visual acuity improvement after vitrectomy in macular hole patients.

Materials and Methods:

Twenty-four eyes of 24 patients with idiopathic full-thickness macular hole closed after vitrectomy were retrospectively reviewed. The “plot profile” function of the medical imaging software was used by a single masked physician to analyze RPE, EZ, and ELM reflectivity on OCT images at postoperative 1 month and 12 months.

Results:

Absolute and relative EZ reflectivity showed highly significant increases at postoperative 12 months compared to 1 month (p<0.001 and p<0.001, respectively). Absolute and relative EZ reflectivity changes from postoperative month 1 to month 12 after macular hole surgery were significantly correlated with best corrected visual acuity improvement (p=0.012 and p=0.020, respectively).

Conclusion:

EZ reflectivity can be a predictor of functional and anatomical improvement after macular hole surgery.

Measures of Function and Structure to Determine Phenotypic Features, Natural History, and Treatment Outcomes in Inherited Retinal Diseases

Inherited retinal diseases (IRDs) are at the forefront of innovative gene-specific treatments because of the causation by single genes, the availability of microsurgical access for treatment delivery, and the relative ease of quantitative imaging and vision measurement. However, it is not always easy to choose a priori, from scores of potential measures, an appropriate subset to evaluate efficacy outcomes considering the wide range of disease stages with different phenotypic features. This article reviews measurements of visual function and retinal structure that our group has used over the past three decades to understand the natural history of IRDs. We include measures of light sensitivity, retinal structure, mapping of natural fluorophores, evaluation of pupillary light reflex, and oculomotor control. We provide historical context and examples of applicability. We also review treatment trial outcomes using these measures of function and structure. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Functional regulation of an outer retina hyporeflective band on optical coherence tomography images

Human and animal retinal optical coherence tomography (OCT) images show a hyporeflective band (HB) between the photoreceptor tip and retinal pigment epithelium layers whose mechanisms are unclear. In mice, HB magnitude and the external limiting membrane-retinal pigment epithelium (ELM-RPE) thickness appear to be dependent on light exposure, which is known to alter photoreceptor mitochondria respiration. Here, we test the hypothesis that these two OCT biomarkers are linked to metabolic activity of the retina. Acetazolamide, which acidifies the subretinal space, had no significant impact on HB magnitude but produced ELM-RPE thinning. Mitochondrial stimulation with 2,4-dinitrophenol reduced both HB magnitude and ELM-RPE thickness in parallel, and also reduced F-actin expression in the same retinal region, but without altering ERG responses. For mice strains with relatively lower (C57BL/6J) or higher (129S6/ev) rod mitochondrial efficacy, light-induced changes in HB magnitude and ELM-RPE thickness were correlated. Humans, analyzed from published data captured with a different protocol, showed a similar light–dark change pattern in HB magnitude as in the mice. Our results indicate that mitochondrial respiration underlies changes in HB magnitude upstream of the pH-sensitive ELM-RPE thickness response. These two distinct OCT biomarkers could be useful indices for non-invasively evaluating photoreceptor mitochondrial metabolic activity.

In vivo optoretinography of phototransduction activation and energy metabolism in retinal photoreceptors

The objective of this study is to verify the anatomic correlate of the second (2nd) outer retina band in optical coherence tomography (OCT), and to demonstrate the potential of using intrinsic optical signal (IOS) imaging for concurrent optoretinography (ORG) of phototransduction activation and energy metabolism in stimulus activated retinal photoreceptors. A custom-designed OCT was employed for depth-resolved IOS imaging in mouse retina activated by a visible light flicker stimulation. The spatiotemporal properties of the IOS changes at the photoreceptor outer segment (OS) and inner segment (IS) were quantitatively evaluated. Rapid IOS change was observed at the OS almost right away, and the IOS at the IS was relatively slow. Comparative analysis indicates that the OS-IOS reflects transient OS deformation caused by the phototransduction activation, and IS-IOS might reflect the energy metabolism caused by mitochondria activation in retinal photoreceptors. The consistency of the distribution of the IS-IOS and the 2nd OCT band supports the IS ellipsoid (ISe), which has abundant mitochondria, as the signal source of the 2nd OCT band of the outer retina.

Functional optoretinography: concurrent OCT monitoring of intrinsic signal amplitude and phase dynamics in human photoreceptors

Intrinsic optical signal (IOS) imaging promises a noninvasive method for objective assessment of retinal function. This study demonstrates concurrent optical coherence tomography (OCT) of amplitude-IOS and phase-IOS changes in human photoreceptors. A new procedure for differential-phase-mapping (DPM) is validated to enable depth-resolved phase-IOS imaging. Dynamic OCT revealed rapid amplitude-IOS and phase-IOS changes, which occur almost right away after the stimulus onset. These IOS changes were predominantly observed within the photoreceptor outer segment (OS), particularly two boundaries connecting to the inner segment and retinal pigment epithelium. The comparative analysis supports that both amplitude-IOS and phase-IOS attribute to transient OS morphological change associated with phototransduction activation in retinal photoreceptors. A simulation modeling is proposed to discuss the relationship between the photoreceptor OS length and phase-IOS changes.

Malfunction of outer retinal barrier and choroid in the occurrence and progression of diabetic macular edema

Diabetic macular edema (DME) is the most common cause of vision loss in diabetic retinopathy, affecting 1 in 15 patients with diabetes mellitus (DM). The disruption of the inner blood-retina barrier (BRB) has been largely investigated and attributed the primary role in the pathogenesis and progression in DME, but there is increasing evidence regarding the role of outer BRB, separating the RPE from the underlying choriocapillaris, in the occurrence and evolution of DME. The development of novel imaging technologies has led to major improvement in the field of in vivo structural analysis of the macula allowing us to delve deeper into the pathogenesis of DME and expanding our vision regarding this condition. In this review we gathered the results of studies that investigated specific outer BRB optical coherence tomography parameters in patients with DM with the aim to outline the current status of its role in the pathogenesis and progression of DME and identify new research pathways contributing to the advancement of knowledge in the understanding of this condition.

Functional intrinsic optical signal imaging for objective optoretinography of human photoreceptors

Functional mapping of photoreceptor physiology is important for better disease diagnosis and treatment assessment. Fast intrinsic optical signal (IOS), which arises before light-evoked pupillary response, promises a unique biomarker of photoreceptor physiology for objective optoretinography with high resolution. This study is to test the feasibility of non-mydriatic IOS mapping of retinal photoreceptors in awake human. Depth-resolved optical coherence tomography verified outer segment (OS) as the anatomic origin of fast photoreceptor-IOS. Dynamic IOS changes are primarily confined at OS boundaries connected with inner segment and retinal pigment epithelium, supporting transient OS shrinkage due to phototransduction process as the mechanism of the fast photoreceptor-IOS response.

Super-resolution ophthalmoscopy: Virtually structured detection for resolution improvement in retinal imaging

Quantitative retinal imaging is essential for advanced study and clinical management of eye diseases. However, spatial resolution of retinal imaging has been limited due to available numerical aperture and optical aberration of the ocular optics. Structured illumination microscopy has been established to break the diffraction-limit resolution in conventional light microscopy. However, practical implementation of structured illumination microscopy for in vivo ophthalmoscopy of the retina is challenging due to inevitable eye movements that can produce phase artifacts. Recently, we have demonstrated the feasibility of using virtually structured detection as one alternative to structured illumination microscopy for super-resolution imaging. By providing the flexibility of digital compensation of eye movements, the virtually structured detection provides a feasible, phase-artifact-free strategy to achieve super-resolution ophthalmoscopy. In this article, we summarize the technical rationale of virtually structured detection, and its implementations for super-resolution imaging of freshly isolated retinas, intact animals, and awake human subjects.

Optoretinography of individual human cone photoreceptors

Photoreceptors mediate the first step of vision, transducing light and passing signals to retinal neurons that ultimately send signals along the optic nerve to the brain. A functional deficiency in the photoreceptors, due to either congenital or acquired disease, can significantly affect an individual's sight and quality of life. Methods for quantifying the health and function of photoreceptors are essential for understanding both the progression of disease and the efficacy of treatment. Given that emerging treatments such as gene, stem cell, and small molecule therapy are designed to operate at the cellular scale, it is desirable to monitor function at the commensurate resolution of individual photoreceptors. Previously, non-invasive imaging methods for visualizing photoreceptor mosaic structure have been used to infer photoreceptor health, but these methods do not assess function directly. Conversely, most functional techniques, such as ERG and conventional microperimetry, measure function by aggregating the effects of signals from many photoreceptors. We have previously shown that stimulus-evoked intrinsic changes in intensity can be measured reliably in populations of cone photoreceptors in the intact human eye, a measurement we refer to more generally as the cone optoretinogram. Here we report that we can resolve the intensity optoretinogram at the level of individual cones. Moreover, we show that the individual cone optoretinogram exhibits two key signatures expected of a functional measure. First, responses in individual cones increase systematically as a function of stimulus irradiance. Second, we can use the amplitude of the functional response to middle wavelength (545 nm) light to separate the population of short-wavelength-sensitive (S) cones from the population of middle- and long-wavelength-sensitive (L and M) cones. Our results demonstrate the promise of optoretinography as a direct diagnostic measure of individual cone function in the living human eye.

Functional optical coherence tomography enables in vivo optoretinography of photoreceptor dysfunction due to retinal degeneration

Stimulus-evoked intrinsic optical signal (IOS), which occurs almost immediately after the onset of retinal stimulus has been observed in retinal photoreceptors, promises to be a unique biomarker for objective optoretinography (ORG) of photoreceptor function. We report here the first-time in vivo ORG detection of photoreceptor dysfunction due to retinal degeneration. A custom-designed optical coherence tomography (OCT) was employed for longitudinal ORG monitoring of photoreceptor-IOS distortions in retinal degeneration mice. Depth-resolved OCT analysis confirmed the outer segment (OS) as the physical source of the photoreceptor-IOS. Comparative ERG measurement verified the phototransduction activation as the physiological correlator of the photoreceptor-IOS. Histological examination revealed disorganized OS discs, i.e. the pathological origin of the photoreceptor-IOS distortion.

High-speed adaptive optics line-scan OCT for cellular-resolution optoretinography

Optoretinography-the non-invasive, optical imaging of light-induced functional activity in the retina-stands to provide a critical biomarker for testing the safety and efficacy of new therapies as well as their rapid translation to the clinic. Optical phase change in response to light, as readily accessible in phase-resolved OCT, offers a path towards all-optical imaging of retinal function. However, typical human eye motion adversely affects phase stability. In addition, recording fast light-induced retinal events necessitates high-speed acquisition. Here, we introduce a high-speed line-scan spectral domain OCT with adaptive optics (AO), aimed at volumetric imaging and phase-resolved acquisition of retinal responses to light. By virtue of parallel acquisition of an entire retinal cross-section (B-scan) in a single high-speed camera frame, depth-resolved tomograms at speeds up to 16 kHz were achieved with high sensitivity and phase stability. To optimize spectral and spatial resolution, an anamorphic detection paradigm was introduced, enabling improved light collection efficiency and signal roll-off compared to traditional methods. The benefits in speed, resolution and sensitivity were exemplified in imaging nanometer-millisecond scale light-induced optical path length changes in cone photoreceptor outer segments. With 660 nm stimuli, individual cone responses readily segregated into three clusters, corresponding to long, middle, and short-wavelength cones. Recording such optoretinograms on spatial scales ranging from individual cones, to 100 µm-wide retinal patches offers a robust and sensitive biomarker for cone function in health and disease.

Optoretinogram: optical measurement of human cone and rod photoreceptor responses to light

Noninvasive, objective measurement of rod function is as significant as that of cone function, and for retinal diseases such as retinitis pigmentosa and age-related macular degeneration, rod function may be a more sensitive biomarker of disease progression and efficacy of treatment than cone function. Functional imaging of single human rod photoreceptors, however, has proven difficult because their small size and rapid functional response pose challenges for the resolution and speed of the imaging system. Here, we describe light-evoked, functional responses of human rods and cones, measured noninvasively using a synchronized adaptive optics optical coherence tomography (OCT) and scanning light ophthalmoscopy (SLO) system. The higher lateral resolution of the SLO images made it possible to confirm the identity of rods in the corresponding OCT volumes.

Fast intrinsic optical signal correlates with activation phase of phototransduction in retinal photoreceptors

IMPACT STATEMENT

As the center of phototransduction, retinal photoreceptors are responsible for capturing and converting photon energy to bioelectric signals for following visual information processing in the retina. This article summarizes experimental observation and discusses biophysical mechanism of fast photoreceptor-intrinsic optical signal (IOS) correlated with early phase of phototransduction. Quantitative imaging of fast photoreceptor-IOS may provide objective optoretinography to advance the study and diagnosis of age-related macular degeneration, retinitis pigmentosa, diabetic retinopathy, and other eye diseases that can cause photoreceptor dysfunctions.

Optical coherence tomography reveals light-dependent retinal responses in Alzheimer's disease

Spectral-domain optical coherence tomography (SD-OCT) is an accessible clinical tool for measuring structural changes to the retina, and increasingly as a biomarker for brain-predominant neurodegenerative diseases like Alzheimer's. Information about retinal function can also be extracted from OCT images, but is under-studied, with literature examples often employing challenging protocols or requiring specialized hardware. The first goal of this study was to verify that functional retinal imaging was feasible with a commercially-available SD-OCT device and a clinically practical protocol. Inspired by methods from other functional imaging modalities, we acquired images while repeatedly cycling lights on and off, and spatially normalized retinas to facilitate intra- and inter-individual analyses. In eight healthy young adults, light-dependent increases in reflectivity were easily demonstrated at photoreceptor inner and outer segments, changing by ~7% in bright light and ~3% in dim light. Bright light elicited a subtle (~2%) but consistent light-dependent decrease in reflectivity through much of the rest of the retina, including the avascular outer nuclear layer (ONL). We speculated that some of these changes are influenced by glial function - as through water management - a topic of high interest in neurodegenerative diseases that may involve the glymphatic system. Functional abnormalities in patients with antibodies against aquaporin-4 (n ​= ​3) supported this interpretation. We next compared patients with early-onset Alzheimer's disease (n ​= ​14) to age-matched controls (n ​= ​14), revealing that patients had a relatively exaggerated light-induced change in ONL reflectivity (p ​< ​0.05). Because these measurements can be obtained within 30 ​min, regular use in research and limited clinical settings is feasible.

Measurement of Diurnal Variation in Rod Outer Segment Length In Vivo in Mice With the OCT Optoretinogram

Purpose

To investigate diurnal variation in the length of mouse rod outer segments in vivo.

Methods

The lengths of rod inner and outer segments (RIS, ROS) of dark-adapted albino mice maintained on a 12-hour dark:12-hour light cycle with light onset 7 AM were measured at prescribed times (6:30 AM, 11 AM, 3:30 PM) during the diurnal cycle with optical coherence tomography (OCT), taking advantage of increased visibility, after a brief bleaching exposure, of the bands corresponding to RIS/ROS boundaries and ROS tips (ROST).

Results

Deconvolution of OCT depth profiles resolved two backscatter bands located 7.4 ± 0.1 and 10.8 ± 0.2 µm (mean ± SEM) proximal to Bruch's membrane (BrM). These bands were identified with histology as arising from the apical surface of RPE and ROST, respectively. The average length of dark-adapted ROS at 6:30 AM was 17.7 ± 0.8 µm. By 11 AM, the average ROS length had decreased by 10% to 15.9 ± 0.7 µm. After 11 AM, the ROS length increased steadily at an average rate of 0.12 µm/h, returning to baseline length by 23.5 hours in the cycle.

Conclusions

The diurnal variation in ROS length measured in these experiments is consistent with prior histological investigations showing that rodent rod discs are phagocytosed by the RPE maximally over several hours around the time of normal light onset. The rate of recovery of ROS to baseline length before normal light onset is consistent with the hypothesis that disc membrane synthesis is fairly constant over the diurnal cycle.

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

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

Retinal neurovascular responses to transcorneal electrical stimulation measured with optical coherence tomography

Noninvasive transcorneal electrical stimulation (TES) has emerged as a potential strategy to facilitate visual restoration and promote retinal cell survival for certain retinal and optic nerve diseases owing to its neuroprotective effects. However, the neurovascular responses of retinal neurons evoked by TES have not been completely determined. To investigate this issue, we utilized a custom-designed spectral-domain optical coherence tomography (SD-OCT) to record the retinal neural and vascular responses under TES in vivo simultaneously. Significant increases of both positive and negative intrinsic optical signal (IOS) changes were recorded in all three segmented retinal layers, which mainly related to neural activities. However, the changes of TES-induced retinal vascular responses, including blood velocity, cross-sectional area of vessel, and blood flow, were not significant. It suggests that TES mainly elicited neural responses in retina, while no significant vascular responses were evoked. Our results provide experimental evidence to the mechanism of retinal neurovascular coupling under TES. Additionally, the present study also suggests that SD-OCT could be utilized as a promoting method to explore neurovascular responses under retinal stimulation in clinical treatment and technology.

Impact statement

Noninvasive transcorneal electrical stimulation (TES) has emerged as an effective treatment for certain retinal and optic nerve diseases owing to its neuroprotective effects. However, the retinal neurovascular responses evoked by TES have not been completely determined. To investigate this issue, we utilized a custom-designed spectral-domain optical coherence tomography (SD-OCT) to record the retinal neural and vascular responses evoked by TES in vivo simultaneously. The present study suggested that TES mainly elicited neural responses in retina, while no significant vascular responses were evoked. Our results provide experimental evidence to the mechanism of retinal neurovascular coupling evoked by TES. Additionally, the present study also suggests that SD-OCT could be utilized as a promoting method to explore neurovascular responses under retinal electrical stimulation.

Comparative study of wild-type and rd10 mice reveals transient intrinsic optical signal response before phosphodiesterase activation in retinal photoreceptors

Transient intrinsic optical signal (IOS) has been observed in stimulus-evoked retinal photoreceptors. This study is to compare IOS changes in wild-type and retinal degeneration 10 (rd10) mouse retinas, to evaluate the effect of cyclic guanosine monophosphate phosphodiesterase on photoreceptor-IOS. Time-lapse near-infrared light microscopy was employed to monitor the spatiotemporal dynamics of the IOS responses in freshly isolated retinas activated by visible light stimulation. Comparative IOS recordings were conducted at postnatal days 14 (P14) and P16. At P14, intrinsic optical signal magnitudes and spatiotemporal dynamics in wild-type and rd10 retinas were similar, indicating that the phosphodiesterase deficiency in rd10 did not affect the formation of photoreceptor-IOS. At P16, IOS magnitude in rd10 significantly decreased compared to that in wild-type, suggesting the IOS sensitivity to the photoreceptor degeneration in rd10. Our experimental results and theoretical analysis indicate that early disc-based stages of the phototransduction cascade before the activation of phosphodiesterase may contribute to the formation of the photoreceptor-IOS responses; and the IOS can be a sensitive biomarker for objective assessment of retinal function.

Impact statement

Comparative study of wild-type and rd10 mice was implemented to reveal that transient intrinsic optical signal (IOS) was initiated before the phosphodiesterase activation in stimulus-activated photoreceptors and the IOS magnitude was sensitive to photoreceptor degeneration. The photoreceptor-IOS promises a noninvasive biomarker for objective assessment of age-related macular degeneration, retinitis pigmentosa, and other eye diseases that can produce photoreceptor dysfunctions.

Simultaneous functional imaging of neuronal and photoreceptor layers in living human retina

Functional retinal imaging, especially of neuronal activity non-invasively in humans, is of tremendous interest. Although the activation of photoreceptor cells (PRCs) could be detected in humans, imaging the function of other retinal neurons had been so far hardly possible. Here, using phase-sensitive full-field swept-source optical coherence tomography (FF-SS-OCT), we show simultaneous imaging of the activation in the photoreceptor and ganglion cell layer/inner plexiform layer (GCL/IPL). The signals from the GCL/IPL are 10-fold smaller than those from the PRC and were detectable only using algorithms for suppression of motion artifacts and pulsatile blood flow in the retinal vessels. FF-SS-OCT with improved phase evaluation algorithms, therefore, allowed us to map functional connections between PRC and GCL/IPL, confirming previous ex vivo results. The demonstrated functional imaging of retinal neuronal layers can be a valuable tool in diagnostics and basic research.

Light-induced changes of the subretinal space of the temporal retina observed via optical coherence tomography

Photoreceptor function is impaired in many retinal diseases like age-related macular degeneration. Currently, assessment of the photoreceptor function for the early diagnosis and monitoring of these diseases is either subjective, as in visual field testing, requires contact with the eye, like in electroretinography, or relies on research prototypes with acquisition speeds unattained by conventional imaging systems. We developed an objective, noncontact method to monitor photoreceptor function using a standard optical coherence tomography system. This method can be used with various white light sources for stimulation. The technique was applied in five volunteers and detected a decrease of volume of the subretinal space associated with light adaptation processes of the retina.

Depth-Resolved Physiological Response of Retina to Transcorneal Electrical Stimulation Measured With Optical Coherence Tomography

Transcorneal electrical stimulation (TES) has become an effective strategy to modulate retinal neural activities and partially restore visual function in ophthalmic diseases. However, the exact responses in different retinal layers still need to be clarified. This paper's goal was to evaluate the depth-resolved retinal physiological responses evoked by TES by using optical coherence tomography (OCT). A custom-built spectral-domain OCT system was used to record the intrinsic optical signals (IOSs) in different retinal layers. TES and flickers were used to stimulate the retina electrically and visually. Tetrodotoxin was used to inhibit the retinal neural activity for confirming the origin of TES-induced IOSs. We found both positive and negative IOSs could be evoked by TES in three segmented retinal layers, especially in the inner retina and subretinal space. The TES-induced IOSs correlated with the TES intensity. After tetrodotoxin injection, the IOSs evoked by TES were significantly declined, peculiarly in the inner retina. The IOSs elicited by flickers kept increasing during the stimulation, while those evoked by TES kept at a stable level. In conclusion, TES could elicit IOSs that originated from retinal neural activity in all segmented layers. The TES-induced IOSs were highly synchronized to the electrical field in the retina.

Functional optical coherence tomography of neurovascular coupling interactions in the retina

Quantitative evaluation of retinal neurovascular coupling is essential for a better understanding of visual function and early detection of eye diseases. However, there is no established method to monitor coherent interactions between stimulus-evoked neural activity and hemodynamic responses at high resolution. Here, we report a multimodal functional optical coherence tomography (OCT) imaging methodology to enable concurrent intrinsic optical signal (IOS) imaging of stimulus-evoked neural activity and hemodynamic responses at capillary resolution. OCT angiography guided IOS analysis was used to separate neural-IOS and hemodynamic-IOS changes in the same retinal image sequence. Frequency flicker stimuli evoked neural-IOS changes in the outer retina; that is, photoreceptor layer, first and then in the inner retina, including outer plexus layer (OPL), inner plexiform layer (IPL), and ganglion cell layer (GCL), which were followed by hemodynamic-IOS changes primarily in the inner retina; that is, OPL, IPL, and GCL. Different time courses and signal magnitudes of hemodynamic-IOS responses were observed in blood vessels with various diameters.

Functional optical coherence tomography of retinal photoreceptors

IMPACT STATEMENT

Retinal photoreceptors are the primary target of age-related macular degeneration (AMD) which is the leading cause of severe vision loss and legal blindness. An objective method for functional assessment of photoreceptor physiology can benefit early detection and better treatment evaluation of AMD and other eye diseases that are known to cause photoreceptor dysfunctions. This article summarizes in vitro study of IOS mechanisms and in vivo demonstration of IOS imaging of intact animals. Further development of the functional IOS imaging may provide a revolutionary solution to achieve objective assessment of human photoreceptors.

Light-Dependent OCT Structure Changes in Photoreceptor Degenerative rd 10 Mouse Retina

Purpose

Using optical coherence tomography (OCT) to analyze the effects of light/dark adaptation in a mouse model of inherited photoreceptor degeneration (rd10), and to study dynamics of subretinal fluid during the progress of retinal degeneration.

Methods

rd10 and wild-type (WT) C57BL/6J mice were reared in cyclic light or darkness and imaged with Bioptigen UHR-OCT or Spectralis HRA+OCT after adaptation to either light or darkness.

Results

OCT images from rd10 mice were analyzed at three progressive stages of degeneration. After light-adaptation, stage I (postnatal age [P]26–29) eyes demonstrated no apparent subretinal fluid. At stage II (P32–38), subretinal fluid was present and restricted to parapapillary area, while at stage III (P44–45) extensive subretinal fluid was present across many retinal areas. Following overnight dark-adaptation, WT eyes showed a large reduction in outer retinal thickness (4.6 ± 1.4 μm, n = 16), whereas this change was significantly smaller in stage I rd10 eyes (1.5 ± 0.5 μm, n = 14). In stage II rd10 eyes, dark-adaptation significantly reduced the extent of subretinal fluid, with the amount of reduction correlating with the amount of fluid pre-existing in the light-adapted state. However, dark-adaptation did not significantly alter the amount of subretinal fluid observed in stage III rd10 mice. In addition, dark-rearing of rd10 mice from P6 to P30 slowed retinal degeneration.

Conclusions

Visual experience in the form of light/dark adaptation exerts a significant effect on outer retinal structure in the context of photoreceptor degeneration. This effect may arise from light-dependent alterations in fluid transport across the RPE monolayer, and promote photoreceptor survival as induced by dark-rearing.

BEST1 gene therapy corrects a diffuse retina-wide microdetachment modulated by light exposure

One of the most common forms of monogenic macular degeneration worldwide is caused by dominant or recessive bestrophinopathies associated with mutations in the BEST1 gene. Disease expression is known to start with a retina-wide electrophysiological defect leading to localized vitelliform and atrophic lesions and vision loss. To develop lasting therapies for this incurable disease, there is a need for greater understanding of the early pathophysiology before lesion formation. Here we find that the loss of retinal pigment epithelium apical microvilli and resulting microdetachment of the retina represent the earliest features of canine bestrophinopathies. We show that retinal light exposure expands, and dark adaptation contracts, the microdetachments. Subretinal adeno-associated virus-based gene therapy corrects both the vitelliform lesions and the light-modulated microdetachments.

Mutations in the BEST1 gene cause detachment of the retina and degeneration of photoreceptor (PR) cells due to a primary channelopathy in the neighboring retinal pigment epithelium (RPE) cells. The pathophysiology of the interaction between RPE and PR cells preceding the formation of retinal detachment remains not well-understood. Our studies of molecular pathology in the canine BEST1 disease model revealed retina-wide abnormalities at the RPE-PR interface associated with defects in the RPE microvillar ensheathment and a cone PR-associated insoluble interphotoreceptor matrix. In vivo imaging demonstrated a retina-wide RPE–PR microdetachment, which contracted with dark adaptation and expanded upon exposure to a moderate intensity of light. Subretinal BEST1 gene augmentation therapy using adeno-associated virus 2 reversed not only clinically detectable subretinal lesions but also the diffuse microdetachments. Immunohistochemical analyses showed correction of the structural alterations at the RPE–PR interface in areas with BEST1 transgene expression. Successful treatment effects were demonstrated in three different canine BEST1 genotypes with vector titers in the 0.1-to-5E11 vector genomes per mL range. Patients with biallelic BEST1 mutations exhibited large regions of retinal lamination defects, severe PR sensitivity loss, and slowing of the retinoid cycle. Human translation of canine BEST1 gene therapy success in reversal of macro- and microdetachments through restoration of cytoarchitecture at the RPE–PR interface has promise to result in improved visual function and prevent disease progression in patients affected with bestrophinopathies.

Optophysiological Characterisation of Inner Retina Responses with High-Resolution Optical Coherence Tomography

Low coherence laser interferometry has revolutionised quantitative biomedical imaging of optically transparent structures at cellular resolutions. We report the first optical recording of neuronal excitation at cellular resolution in the inner retina by quantifying optically recorded stimulus-evoked responses from the retinal ganglion cell layer and comparing them with an electrophysiological standard. We imaged anaesthetised paralysed tree shrews, gated image acquisition, and used numerical filters to eliminate noise arising from retinal movements during respiratory and cardiac cycles. We observed increases in contrast variability in the retinal ganglion cell layer and nerve fibre layer with flash stimuli and gratings. Regions of interest were subdivided into three-dimensional patches (up to 5–15 μm in diameter) based on response similarity. We hypothesise that these patches correspond to individual cells, or segments of blood vessels within the inner retina. We observed a close correlation between the patch optical responses and mean electrical activity of the visual neurons in afferent pathway. While our data suggest that optical imaging of retinal activity is possible with high resolution OCT, the technical challenges are not trivial.

Super-Resolution Scanning Laser Microscopy Based on Virtually Structured Detection

Light microscopy plays a key role in biological studies and medical diagnosis. The spatial resolution of conventional optical microscopes is limited to approximately half the wavelength of the illumination light as a result of the diffraction limit. Several approaches-including confocal microscopy, stimulated emission depletion microscopy, stochastic optical reconstruction microscopy, photoactivated localization microscopy, and structured illumination microscopy-have been established to achieve super-resolution imaging. However, none of these methods is suitable for the super-resolution ophthalmoscopy of retinal structures because of laser safety issues and inevitable eye movements. We recently experimentally validated virtually structured detection (VSD) as an alternative strategy to extend the diffraction limit. Without the complexity of structured illumination, VSD provides an easy, low-cost, and phase artifact-free strategy to achieve super-resolution in scanning laser microscopy. In this article we summarize the basic principles of the VSD method, review our demonstrated single-point and line-scan super-resolution systems, and discuss both technical challenges and the potential of VSD-based instrumentation for super-resolution ophthalmoscopy of the retina.

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

PURPOSE

Over the past 25 years, optical coherence tomography (OCT) and adaptive optics (AO) ophthalmoscopy have revolutionised our ability to non-invasively observe the living retina. The purpose of this review is to highlight the techniques and human clinical applications of recent advances in OCT and adaptive optics scanning laser/light ophthalmoscopy (AOSLO) ophthalmic imaging.

RECENT FINDINGS

Optical coherence tomography retinal and optic nerve head (ONH) imaging technology allows high resolution in the axial direction resulting in cross-sectional visualisation of retinal and ONH lamination. Complementary AO ophthalmoscopy gives high resolution in the transverse direction resulting in en face visualisation of retinal cell mosaics. Innovative detection schemes applied to OCT and AOSLO technologies (such as spectral domain OCT, OCT angiography, confocal and non-confocal AOSLO, fluorescence, and AO-OCT) have enabled high contrast between retinal and ONH structures in three dimensions and have allowed in vivo retinal imaging to approach that of histological quality. In addition, both OCT and AOSLO have shown the capability to detect retinal reflectance changes in response to visual stimuli, paving the way for future studies to investigate objective biomarkers of visual function at the cellular level. Increasingly, these imaging techniques are being applied to clinical studies of the normal and diseased visual system.

SUMMARY

Optical coherence tomography and AOSLO technologies are capable of elucidating the structure and function of the retina and ONH noninvasively with unprecedented resolution and contrast. The techniques have proven their worth in both basic science and clinical applications and each will continue to be utilised in future studies for many years to come.

A Review of Adaptive Optics Optical Coherence Tomography: Technical Advances, Scientific Applications, and the Future

Purpose

Optical coherence tomography (OCT) has enabled “virtual biopsy” of the living human retina, revolutionizing both basic retina research and clinical practice over the past 25 years. For most of those years, in parallel, adaptive optics (AO) has been used to improve the transverse resolution of ophthalmoscopes to foster in vivo study of the retina at the microscopic level. Here, we review work done over the last 15 years to combine the microscopic transverse resolution of AO with the microscopic axial resolution of OCT, building AO-OCT systems with the highest three-dimensional resolution of any existing retinal imaging modality.

Methods

We surveyed the literature to identify the most influential antecedent work, important milestones in the development of AO-OCT technology, its applications that have yielded new knowledge, research areas into which it may productively expand, and nascent applications that have the potential to grow.

Results

Initial efforts focused on demonstrating three-dimensional resolution. Since then, many improvements have been made in resolution and speed, as well as other enhancements of acquisition and postprocessing techniques. Progress on these fronts has produced numerous discoveries about the anatomy, function, and optical properties of the retina.

Conclusions

Adaptive optics OCT continues to evolve technically and to contribute to our basic and clinical knowledge of the retina. Due to its capacity to reveal cellular and microscopic detail invisible to clinical OCT systems, it is an ideal companion to those instruments and has the demonstrable potential to produce images that can guide the interpretation of clinical findings.

Light-Induced Thickening of Photoreceptor Outer Segment Layer Detected by Ultra-High Resolution OCT Imaging

PURPOSE

We examined if light induces changes in the retinal structure that can be observed using optical coherence tomography (OCT).

METHODS

Normal C57BL/6J mice (age 3-6 months) adapted to either room light (15 minutes to ∼5 hours, 50-500 lux) or darkness (overnight) were imaged using a Bioptigen UHR-OCT system. Confocal histologic images were obtained from mice killed under light- or dark-adapted conditions.

RESULTS

The OCT image of eyes adapted to room light exhibited significant increases (6.1 ± 0.8 μm, n = 13) in total retina thickness compared to the same eyes after overnight dark adaptation. These light-adapted retinal thickness changes occurred mainly in the outer retina, with the development of a hyporeflective band between the RPE and photoreceptor-tip layers. Histologic analysis revealed a light-evoked elongation between the outer limiting membrane and Bruch's membrane from 45.8 ± 1.7 μm in the dark (n = 5) to 52.1 ± 3.7 μm (n = 5) in the light. Light-adapted retinas showed an increase of actin staining in RPE apical microvilli at the same location as the hyporeflective band observed in OCT images. Elongation of the outer retina could be detected even with brief light exposures, increasing 2.1 ± 0.3 μm after 15 minutes (n = 9), and 4.1 ± 1.0 μm after 2 hours (n = 6). Conversely, dark-adaptation caused outer retinal shortening of 1.4 ± 0.4 μm (n = 7) and 3.0 ± 0.5 μm (n = 8) after 15 minutes and 2 hours, respectively.

CONCLUSIONS

Light-adaption induces an increase in the thickness of the outer retina and the appearance of a hyporeflective band in the OCT image. This is consistent with previous reports of light-induced fluid accumulation in the subretinal space.

Cell-based therapeutic strategies for replacement and preservation in retinal degenerative diseases

Cell-based therapeutics offer diverse options for treating retinal degenerative diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). AMD is characterized by both genetic and environmental risks factors, whereas RP is mainly a monogenic disorder. Though treatments exist for some patients with neovascular AMD, a majority of retinal degenerative patients have no effective therapeutics, thus indicating a need for universal therapies to target diverse patient populations. Two main cell-based mechanistic approaches are being tested in clinical trials. Replacement therapies utilize cell-derived retinal pigment epithelial (RPE) cells to supplant lost or defective host RPE cells. These cells are similar in morphology and function to native RPE cells and can potentially supplant the responsibilities of RPE in vivo. Preservation therapies utilize supportive cells to aid in visual function and photoreceptor preservation partially by neurotrophic mechanisms. The goal of preservation strategies is to halt or slow the progression of disease and maintain remaining visual function. A number of clinical trials are testing the safety of replacement and preservation cell therapies in patients; however, measures of efficacy will need to be further evaluated. In addition, a number of prevailing concerns with regards to the immune-related response, longevity, and functionality of the grafted cells will need to be addressed in future trials. This review will summarize the current status of cell-based preclinical and clinical studies with a focus on replacement and preservation strategies and the obstacles that remain regarding these types of treatments.

Enhancement of intrinsic optical signal recording with split spectrum optical coherence tomography

Functional optical coherence tomography (OCT) of stimulus-evoked intrinsic optical signal (IOS) promises to be a new methodology for high-resolution mapping of retinal neural dysfunctions. However, its practical applications for non-invasive examination of retinal function have been hindered by the low signal-to-noise ratio (SNR) and small magnitude of IOSs. Split spectrum amplitude-decorrelation has been demonstrated to improve the image quality of OCT angiography. In this study, we exploited split spectrum strategy to improve the sensitivity of IOS recording. The full OCT spectrum was split into multiple spectral bands and IOSs from each sub-band were calculated separately and then combined to generate a single IOS image sequence. The algorithm was tested on in vivo images of frog retinas. It significantly improved both IOS magnitude and SNR, which are essential for practical applications of functional IOS imaging.