Figure 2-14, Circular tomogram representing a cylindrical section around the optic disc at a diameter of 3,4 mm. White and blue lines indicate the computer generated profiles of the anterior and posterior retinal boundaries, and the posterior margin of the retinal nerve fiber layer. Thicknesses are displayed averaged over quadrant and clock hour.
Figure 2-15, A vertical linear tomogram showing the cross-section of the optic nerve head. Disc parameters such as cup and disc diameter, neuroretinal rim area, and cup-to-disc ratio may be estimated from the image as shown.
ary between the vitreous and retina. Significant back-scatter signal may be observed from the vitreous with inflammatory infiltrate, vitreous condensations, or hemorrhage. An operculum or pseudo-operculum may also present as a focal, thin area of backscatter anterior to the retinal surface. The posterior hyaloid membrane is nominally indistinguishable from the superficial retina on the CX image, but becomes apparent when the posterior vitreous is detached . The reflection from the posterior hyaloid is typically weak and often appears patchy because the vitreous gel and the intervening fluid have similar indices of refraction.
An epiretinal membrane may be observed easily if it is separated from the neurosensory retina. Often, however, the membrane will be tightly adherent to the retinal surface, in which case the distinction between it and the retina depends on differences in optical reflectivity, and on the presence of secondary features, such as a macular pseudohole, or traction detachment of the retina. The reflection from an epiretinal membrane is usually stronger than that observed from the underlying retinal tissue.
An epiretinal membrane which is separated from the retina is distinguished from the posterior hyaloid based on the membrane s higher reflectivity, greater thickness, and differences in contour A contracted epiretinal membrane usually has a flatter contour than a detached posterior hyaloid, indicating greater tension.
Alterations in the thickness of the retinal nerve fiber layer may be a powerful indicator of the onset of neurodegenerative diseases such as glaucoma. The NFL appears in the OC images as a highly backscat-tering layer in the superficial retina and exhibits increased reflectivity compared to the deeper retinal layers. NFL thickness may be assessed at individual points on a cylindrical or linear tomogram in the peripapillary region. Alternatively a computer algorithm can be used to evaluate both retinal and NFL thickness. :igure 2-14 shows a clockwise circular tomogram around a normal optic disc. Computer generated profiles of the NFL and retinal boundaries are displayed, and the NFL thicknesses are reported as either averages over each quadrant or clock hour ,
Careful inspection of the circular tomograms is required to identify focal NFL defects, which must be distinguished from normal variations in reflectivity and NFL thickness. The observation of depressions from both the anterior and posterior margins of the NFL is a helpful indicator of actual thinning.
Linear tomograms through the optic disc are useful for assessing disc and cup parameters (Figure 215). The points at which the choriocapillaris terminates at the lamina cribosa may be used to determine the boundaries of the disc. Extrapolation of these points to the retinal surface defines a line segment which measures the disc diameter. Total retinal and retinal nerve fiber layer thickness can also be assessed at these points. The cup diameter may be estimated by choosing a line segment parallel to the line segment defining the disc diameter, but taken at a certain depth into the cup. Throughout this text, we have used a depth of 140 jam to define the cup diameter. The disc and cup diameters can provide an estimate of disc, cup, and neuroretinal rim areas.
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