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R. S. Rajpara, K. Whipple, A. G. Smith, G. Gregori, R. W. Knighton, R. K. Lee, D. R. Anderson; In vivo Retinal Histology of Shields Gray Crescents by Spectral Domain Optical Coherence Tomography (SD-OCT). Invest. Ophthalmol. Vis. Sci. 2008;49(13):1866.
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Shields characterized temporal optic nerve head (ONH) "gray crescents" (AJO 89:238 (1980), a dark region that makes it difficult to judge ONH cupping. Subsequently, it became necessary to clarify that this structure was different from a different type of dark tissue at the curved disc boundary, the former designated type A and the latter, type B. The histological characteristics of these are here characterized from images obtained by spectral domain ocular coherence tomography (SD-OCT).
Digital color photographs were taken with a Nidek simultaneous stereo camera and images recorded by SD-OCT (prototype of Cirrus HD-OCT, Carl Zeiss Meditec, Inc.). The en face SD-OCT images were registered with the ONH photographs. Cross-sectional B-scan images across the areas of gray crescent were examined to explore the anatomical characteristics of the two types of crescents.
In B-scan images across the areas of gray crescent, type A gray crescents are seen to represent regions where the retinal pigmented epithelium (RPE) is absent and the dark underlying choroid is directly visible. In contrast, Type B gray crescents represent regions of the retina where the RPE is clumped or thickened, typically appearing to be folded upon itself, revealing the embryonic continuity of the RPE and neuro-retina. Regions that were gray or black on color photographs were bright in the en face SD-OCT images. With Type A crescents, the gray regions on the color photographs are bright in the en face OCT image because of greater illumination of deeper tissue where RPE is absent. In contrast, the Type B crescents are bright in the en face OCT because of the higher reflected signal from the thickened, folded RPE.
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