1.1) (Table 1.2).1 DRCR.NET Image Acquisition Protocol Optical Coherence Tomography Using: Heidelberg Spectralis OCT 1. The fovea is recognized on a cross-sectional image by its characteristic depression, due to the thinning of the retina with the absence of the inner layers at the macula (Fig. The choriocapillaris and the choroid are visible outer to the RPE-Bruch’s zone. The outermost layer imaged is the hyperreflective RPE-Bruch’s complex. Due to the increased length of outer cone segments in the central fovea, this line is slightly elevated in the foveal region. The hyperreflective layer of photoreceptor junction of inner and outer segments, currently termed as the photoreceptor inner segment ellipsoid zone, lies beneath this layer. A thin hyperreflective line underneath the outer nuclear layer corresponds to the location of the external limiting membrane (ELM). The relatively thick hyporeflective outer nuclear layer is visible next. Subsequently, hyperreflective plexiform layers are imaged with an inner nuclear layer situated between them. Outer to this is a hyporeflective ganglion cell layer. The hyperreflective retinal nerve fiber layer lies next to this layer. The first layer visible on OCT images is the hyperreflective ILM line at the vitreoretinal interface. The light scattering potential of nuclear layers is lower. The axonal layers, nerve fiber layer, and plexiform layers are capable of potent light scatter and hence are hyperreflective. Histologically, the retina consists of ten layers, four of them are cellular and two are neuronal junctions. The slab image represents an average signal intensity value for each A-scan location through the selected depth of the slab. A slab of selective thickness, the slab thickness of interest, can be studied by selecting and adjusting the anterior and posterior boundaries represented by two separable same-color dashed lines. Blood vessels of the choroid can be seen by changing the thickness of sections. Sections with thickness varying from 2 to 20 μm can be obtained with thinner sections known as slices and thicker sections known as slabs. The software adapts to concavity of RPE/choriocapillaris (Wei et al. RPE fit software identifies RPE/choriocapillaris complex to display it as a curved 3D section plane. En face images that follow the curvature of posterior pole can be obtained. It provides C-scan images adapted to retinal curvature. The RPE fit feature of the Cirrus HD-OCT (Carl Zeiss Meditec Inc., CA, USA) is an important advancement to understand various retinal pathology (Lumbroso et al. Raster and radial scans are also available to scan the macula. In RTVue 3D macular scans have a 4 mm × 4 mm macular cube scan with 101 B-scans each consisting of 512 A-scans and a MM5 protocol. The Topcon 3D OCT has a 512 × 128 and a 256 × 256 scanning protocol. In Heidelberg Spectralis volume scan uses a scanning protocol with fast 25 B-scans, each of which consists of 512 A-scans or a dense 1024 × 49 default scanning protocol. As the average foveal thickness is 160–180 microns, the resolution of A-scan is less than 4 microns theoretically. It takes 1.6 s to acquire 200 horizontal scans (200 A-scans per B-scan). The 200 × 200 protocol achieves faster imaging at the cost of resolution. The 512 × 128 protocol analyzes 128 horizontal scans at high resolution (512 A-scans per B-scan). The Cirrus HD-OCT (Carl Zeiss Meditec Inc., CA, USA) provides two macular cube scan protocols. Similarly, optic nerve topographic scans are cube scans centered on the optic nerve. Manual centering of the cube scans other areas of interest besides the fovea. Rapid series B-scans are acquired, generally in a 6 mm × 6 mm square area centered on the fovea. They are volume scans similar to computed tomography or magnetic resonance scans that acquire volumetric cubes of data.
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