11/10/2023 0 Comments Dark spot on retina scanTherefore, it is practically impossible to differentiate the contribution from melanin in the RPE alone by using simple fluorescent intensity detection. 16, 17 However, overall NIA emission intensity is contributed by the melanin distribution in the RPE as well as in choroid. Near-infrared autofluorescence (NIA) has been used to visualize the melanin distribution in human outer retina. 13 – 15 A noninvasive in vivo assessment of melanin changes in the RPE could be a valuable adjunct in the clinical diagnosis of aging and disease progression. 10 – 12 Ultrastructural studies have shown that RPE melanosomes undergo significant changes with age or disease. The role of RPE melanin as a major protective agent in the RPE cells sheltering the RPE from exposure to radiation, oxidative stress, and light damage is well documented. PRC-NVU plays a key role in the process of the vision-sustaining visual cycle and contributes to photoreceptor pigment regeneration. The evaluation of the RPE in vivo has great significance in both visual science and clinical studies, since RPE health will directly affect photoreceptor-RPE-choroid neurovascular unit (PRC-NVU) function. An OCT-based assessment of the reflectivity of the photoreceptors layer is reported to study SCE in human retina. 7, 8 The directional reflectance property shown by the photoreceptor bands, known as the optical Stiles–Crawford effect (SCE), provides the information on the structural integrity of the photoreceptors. Therefore, directional OCT (dOCT) imaging of human retina enables the delineation of HFL from the ONL and accurate measurement of macular photoreceptor nuclear layer thickness, which are important biomarkers for retinal degenerations. 6 The directional scattering properties exhibited by the Henle’s fiber layer (HFL) in human retina account for their oblique orientation with respect to the outer nuclear layer (ONL). 3 – 5 The reflection from the RFNL is found to be highly directional, and the implications of the variability caused by the directional reflectance on clinical measurements are reported. The reduction in light scattered by the retinal nerve fiber layer (RFNL) near the optic nerve head is reported to be an early indicator of axonal degeneration and a sensitive way to detect glaucomatous damage. However, this information is not utilized in current clinical OCT systems. Furthermore, measuring changes in the angular (directional) scattering of retina layers might further enhance our ability to sense microscopic changes in cellular and subcellular morphology that might precede and/or follow disease progression. The amount of light backscattered/reflected from the retinal layers can provide critical information regarding tissue properties and underlying changes. 1, 2 OCT extracts depth-resolved information about the intensity of backscattered light from retinal layers using low coherence interferometry. Over the past several years, optical coherence tomography (OCT) has evolved into a powerful tool for the in vivo investigation of cross-sectional retinal anatomy and pathology. The differences in directional scattering between strains shows that directionality decreases with an increase in melanin concentrations in RPE, suggesting increasing contribution of Mie scattering by melanosomes. The reflections from the RPE are largely insensitive in highly pigmented mice. The intensity of light backscattered from these layers was found highly sensitive to the angle of illumination, whereas the inner/outer segment (IS/OS) junctions showed a reduced sensitivity. The directional reflectivity of other retinal melanin-free layers, including the internal and external limiting membranes, and Bruch’s membrane (albinos) were also measured and compared between the strains. The backscattering signal strength was measured with a directional OCT system in which the pupil entry position of the narrow OCT beam can be varied across the dilated pupil of the eyes of the mice. The directional scattering of the RPE was studied in three mice strains with three distinct melanin concentrations: albino (BALB/c), agouti (129S1/SvlmJ), and strongly pigmented (C57BL/6J). The directional (angular) reflectivity of the retinal layers was investigated with focus on the scattering from retinal pigment epithelium (RPE). Optical coherence tomography (OCT) is a powerful tool in ophthalmology that provides in vivo morphology of the retinal layers and their light scattering properties.
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