Early detection of Alzheimer's disease (AD) is crucial for prompt interventions to preserve brain function. Neuronal hyperexcitability promises a biomarker of early-stage AD. However, in vivo, detection of neuronal hyperexcitability is technically challenging. With a custom-designed optical coherence tomography (OCT), this study demonstrated the feasibility of using retinal intrinsic optical signal (IOS) imaging, also known as optoretinography (ORG), for detecting photoreceptor hyperexcitability associated with early-stage AD. Morphological OCT analysis revealed detectable thinning of retinal thickness in 3xTg-AD mice from 5 months of age. In contrast, functional ORG disclosed increased photoreceptor hyperexcitability of 3xTg-AD mice from 4 months of age.
The wall-to-lumen ratio (WLR) of retinal blood vessels promises a sensitive marker for functional assessment of eye conditions. However, in vivo measurement of vessel wall thickness and lumen diameter is still technically challenging, hindering the wide application of WLR in research and clinical settings. In this study, we demonstrate the feasibility of using optical coherence tomography (OCT) as one practical method for in vivo quantification of WLR in the retina. Based on three-dimensional vessel tracing, lateral en face and axial B-scan profiles of individual vessels were constructed. By employing adaptive depth segmentation that traces each blood vessel for en face OCT projection, the vessel wall thickness and lumen diameter could be reliably quantified. A comparative study of control and 5xFAD mice confirmed WLR as a sensitive marker of the eye condition.
As one modality extension of optical coherence tomography (OCT), OCT angiography (OCTA) provides unrivaled capability for depth-resolved visualization of retinal vasculature at the microcapillary level resolution. For OCTA image construction, repeated OCT scans from one location are required to identify blood vessels with active blood flow. The requirement for multi-scan-volumetric OCT can reduce OCTA imaging speed, which will induce eye movements and limit the image field-of-view. In principle, the blood flow should also affect the reflectance brightness profile along the vessel direction in a single-scan-volumetric OCT. In this article, we report a retinal vascular connectivity network (RVC-Net) for deep learning OCTA construction from single-scan-volumetric OCT. We compare the effects of RVC with three adjacent B-scans and a single B-scan input models into RVC-Net. The structural-similarity index measure (SSIM) loss function was selected to optimize deep learning contrast enhancement of microstructures, i.e., microcapillaries, in OCT. This was confirmed by comparing RVC-Net performances with SSIM and mean-squared-error (MSE) loss functions. The involvement of RVC and SSIM loss function enabled microcapillary resolution OCTA construction from singlescan- volumetric OCT.
The retina is a light-sensing neural network, converting light energy to bioelectric signals for visual information processing. The retina is often damaged by neurodegeneration, which leads to severe vision loss. The initial symptoms of retinal neurodegenerative diseases are manifested by functional abnormalities. Delayed dark adaptation is one of the earliest functional symptoms of retinal neurodegeneration. Therefore, objective measurement of dark adaptation promises the early diagnosis of various retinal diseases. In this study, we demonstrate the feasibility of functional optical coherence tomography (OCT) imaging of the retina during dark adaptation in vivo. A custom-designed spectral-domain OCT was used in dark adaptation measurement, and two-month-old C57BL/6J mice were used in this study. We observed three image biomarkers. First, the outer retinal thickness became thinner during dark adaption. Second, OCT intensity of the inner segment ellipsoid zone was gradually decreased over time under darkness. Third, during dark adaptation, there was a rearrangement of the interdigitation zone between the outer segment and retinal pigment epithelium. Functional OCT enabled the concurrent measurement of retinal thickness changes and intrinsic optical signal (IOS) changes in the retina during dark adaptation. These functional biomarkers may help assess the early dysfunction of outer retinal neurons.
This study is to compare the optical coherence tomography (OCT) amplitude intrinsic optical signal (IOS) and phase IOS change of the human retina after light stimulation. A custom constructed OCT was employed for functional optoretinography imaging. A white LED was used as the retinal stimulator. OCT amplitude and phase IOS were computed by comparing the amplitude and phase before and after light stimulation. Both amplitude IOS and phase IOS were observed right after the stimulus onset, predominantly in the outer retina. The phase IOS is more sensitive to the layer boundaries.
The purpose of this study is to use optical coherence tomography (OCT) to characterize the reflectance profiles of retinal blood vessels and to use these features for artery-vein classification in OCT angiography (OCTA). The retinal arteries and veins show unique features in the depth-resolved OCT. Both the upper and lower side of the retinal arteries have hyperreflective boundaries. However, retinal veins reveal only hyper-reflective boundary at the upper side. In both small and large arteries, relatively uniform lumen intensity was observed. On the other hand, the vein lumen intensity was dependent on the vessel size; the bottom half of the lumen of small veins show a hyper-reflective zone while the bottom half of the lumen of big veins a hypo-reflective zone.
Delayed dark adaptation due to homeostatic imbalances in the retina is a subclinical symptom of retinal neurodegenerative diseases, such as age-related macular degeneration (AMD), diabetic retinopathy (DR), and retinitis pigmentosa (RP), the leading cause of irreversible vision loss. Thus, objective measurement of dark adaptation can facilitate the early diagnosis of various retinal diseases. However, there is a lack of noninvasive methods capable of simultaneous assessment of both anatomical arrangement and functional interactions among retinal cells. Here we demonstrate intrinsic signal optoretinography (ORG) of dark adaptation in the C57BL/6J mouse retina. Optical coherence tomography (OCT) imaging for ORG was continuously performed during dark adaptation at every 5 min interval for 30 minutes, and the spatiotemporal relationship of retinal activities was examined. We observed three imaging features in the retina under different light conditions. First, it was notable that dark adaptation caused a reduction in outer retinal thickness. Second, in the light-adapted retina, the 3rd band of the outer retina was distinguished, which was absent in the dark-adapted retina, and a hypo-reflective band between the 3rd and 4th band was only observed in the light-adapted retina. Third, OCT intensity of the 2nd outer retinal band markedly decreased in the dark-adapted retina. A strong positive correlation between morphophysiological activities was also confirmed. OCT-based ORG enables the measurement of dynamic progress of dark adaptation in a quantitative way with layer-specificity, which can aid in diagnosing early-stage retinal diseases.
Significance: As one part of the central nervous system, the retina manifests neurovascular defects in Alzheimer’s disease (AD). Quantitative imaging of retinal neurovascular abnormalities may promise a new method for early diagnosis and treatment assessment of AD. Previous imaging studies of transgenic AD mouse models have been limited to the central part of the retina. Given that the pathological hallmarks of AD frequently appear in different peripheral quadrants, a comprehensive regional investigation is needed for a better understanding of the retinal degeneration associated with AD-like pathology.
Aim: We aim to demonstrate concurrent optical coherence tomography (OCT) and OCT angiography (OCTA) of retinal neuronal and vascular abnormalities in the 5XFAD mouse model and to investigate region-specific retinal degeneration.
Approach: A custom-built OCT system was used for retinal imaging. Retinal thickness, vessel width, and vessel density were quantitatively measured. The artery and vein (AV) were classified for differential AV analysis, and trilaminar vascular plexuses were segmented for depth-resolved density measurement.
Results: It was observed that inner and outer retinal thicknesses were explicitly reduced in the dorsal and temporal quadrants, respectively, in 5XFAD mice. A significant arterial narrowing in 5XFAD mice was also observed. Moreover, overall capillary density consistently showed a decreasing trend in 5XFAD mice, but regional specificity was not identified.
Conclusions: Quadrant- and layer-specific neurovascular degeneration was observed in 5XFAD mice. Concurrent OCT and OCTA promise a noninvasive method for quantitative monitoring of AD progression and treatment assessment.
This study is to characterize intrinsic optical signal (IOS) changes in the photoreceptor outer segment (OS) and inner segment (IS). Functional optical coherence tomography (OCT) was used for in vivo IOS imaging of wild-type mice (C57BL/6J). Depth-resolved OCT revealed stimulus-evoked IOSs at individual retinal layers, with sub-photoreceptor resolution. Rapid IOS response occurred in the OS region immediately after the stimulus onset. In contrast, transient IOS showed a time delay and gradually increased in the IS region. We anticipate that the OS-IOS and IS-IOS can work as objective biomarkers to reflect photoreceptor phototransduction and metabolism property, respectively.
High-resolution ophthalmic imaging is imperative for detecting subtle changes of photoreceptor abnormality at the early stage of retinal diseases. However, optical resolution in retinal imaging is inherently limited by the low numerical aperture of the ocular optics. Virtually structured detection (VSD) has been demonstrated to break the diffraction limit of imaging systems by shifting the high-frequency components to the passing bandwidth of the imaging system. However, its implementation for human subjects remains a challenge due to the uncertain cut-off frequency of the modulation transfer function (MTF) required for VSD processing. This study demonstrates an objective method to derive the MTF from spectral profiles, enabling quantitative estimation of the optimal cut-off frequency. A custom-built line-scan scanning laser ophthalmoscopy was developed, and two-dimensional line-profile patterns were acquired at a 25 kHz frame rate. We found that the MTF profiles exhibited significant differences between subjects as well as view fields. VSD-based super-resolution images exhibited improved resolution and contrast to differentiate individual photoreceptors compared to the equivalent wide-field imaging. Besides, the motility process on the VSD image further improved the image quality as the photoreceptors revealed clear boundaries and more integrated shape, compared to that in the VSD image. We anticipate that the VSD-based imaging will provide a simple, low-cost, and phase-artifact-free strategy to achieve super-resolution retinal ophthalmoscopy.
KEYWORDS: Intrinsic optical signal imaging, Information operations, In vivo imaging, Signal attenuation, Optical coherence tomography, Retina, Physiology, Optical imaging, Intrinsic optical signal, Image segmentation
Early detection of photoreceptor dysfunction is essential for preventing vision loss due to retinal degenerative diseases, such as age-related macular degeneration (AMD) and inherited retinal degenerations (IRDs). Functional intrinsic optical signal (IOS) imaging promises a high-resolution method for objective optoretinography (ORG). Stimulus-evoked photoreceptor-IOS has been recently demonstrated in healthy animal and human retinas. The fast photoreceptor-IOS response was found to occur at the photoreceptor outer segment (OS) right after the onset of retinal stimulation. However, in vivo IOS response of photoreceptor dysfunctions is not yet validated, which is essential to measure the clinical usability of ORG measurement. In this study, we report in vivo IOS imaging of rod photoreceptor dysfunction in retinal degeneration 10 (rd10) mice. A custom-designed optical coherence tomography (OCT) was used for photoreceptor-IOS imaging. A significant attenuation of the photoreceptor-IOS was found in rd10 mice due to disorganized ultrastructure of the photoreceptor OSs, which appeared ahead of progressive rod cell death. Our experiments demonstrate that fast photoreceptor-IOS is highly sensitive to ultrastructural integrity of the photoreceptor OSs. We anticipate that quantitative imaging of fast photoreceptor-IOS will provide objective ORG measurement to advance the study and diagnosis of AMD, IRDs, and other retinal diseases that can cause photoreceptor dysfunctions.
Physiological dysfunction of diseased cells might occur prior to detectable morphological abnormalities such as retinal cell damage and thickness change. Functional assessment of photoreceptor physiology is essential for the early detection of eye diseases. It is desirable to develop a high-resolution method for objective assessment of photoreceptor physiology. Functional intrinsic optical signal (IOS) imaging, also known as optoretinography (ORG) or optophysiology, measures transient light changes correlated with retinal neural activities. The photoreceptor-IOS arises promptly after the beginning of stimulation, which ensures a unique biomarker for objective ORG measurement of physiological conditions of retinal photoreceptors. In this study, the feasibility of functional optical coherence tomography (OCT) imaging of fast photoreceptor-IOS in human photoreceptors has been demonstrated. The fast photoreceptor-IOS occurred before stimulus-evoked pupillary response and thus allows nonmydriatic ORG of human photoreceptors. The outer segment (OS) was confirmed as the source of fast photoreceptor-IOS by depth-resolved OCT. The active IOS changes were found at both OS boundaries, which connected to the inner segment and retinal pigment epithelium. This supports that the mechanism of the fast photoreceptor-IOS can be explained by transient OS shrinkage due to phototransduction.
The hyaloid vascular system (HVS) is a transient capillary network nourishing developing eye. Currently, there is a lack of noninvasive imaging techniques for functional investigation of the HVS with high spatial and temporal resolutions. In this study, we demonstrated the feasibility of longitudinal optical coherence tomography angiography (OCTA) observation of the HVS regression in C57BL/6J mice. Longitudinal 3D OCT/OCTA measurements were conducted at 2- week, 3-week, and 4-week age by using a custom-designed OCT system. Three-dimensional OCT volume was acquired over 1.2 mm x 1.2 mm x 1.05 mm, containing a part of the lens tip, vitreous chamber, and the retina. Both OCT and OCTA successfully detected the hyaloid vessels, but OCTA benefits visualizing functional preservation of the blood vessels. OCTA images at 4-week age revealed functional loss of hyaloid vessels; while the OCT images still showed vascular remnants. We anticipate that longitudinal OCT/OCTA observation will be helpful to unravel the complex mechanism of the involution of the HVS correlated with eye development.
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