Figure 2-4 shows a large field-of-view OCT tomogram of a healthy retina including both the macula and peripapillary region . Large scale anatomic features, such as the fovea, optic disc, and retinal profile are evident in the tomogram and are identifiable by their characteristic morphology. The vit-reoretinal interface is demarcated by the contrast between the non-reflective vitreous and the backscatter-ing surface of the retina. The fovea appears to the left in the tomogram as a characteristic thinning of the retina and shows the lateral displacement of the retina anterior to the photoreceptors. The optic disc appears to the right in the tomogram, showing the optic nerve head contour and evidence of normal cupping.
A highly reflective red layer delineates the posterior boundary of the retina in the tomogram, and corresponds to the retinal pigment epithelium (RPE) and choriocapillaris. This posterior ia ver terminates at the margin of the optic disc consistent with the termination of choroidal circulation at the lamina cribosa. Below the choriocapillaris, relatively weak scattering returns from the deep choroid and sclera, due to attenuation of the signal after passing through the neurosensory retina, RPE, and choriocapillaris . A dark layer indicative of minimal reflectivity appears just anterior to the choriocapillaris layer, and represents the outer segments of the retinal photoreceptors. The intermediate layers of the retina anterior to these segments exhibit moderate backscattering. The inner margin of the retina shows another area of bright back-scattering, a red layer that corresponds in location and in anatomical variation to the retinal nerve fiber layer (NFL). According to the tomogram, the NFL increases in thickness from the macula to the optic disc as expected from normal anatomy.
A narrow field-of-view image (Pigure 2-5) of a normal fovea further delineates the contrast between different retinal layers. The anterior and posterior margins of the retina are again defined by highly reflec tive layers corresponding to the NFL and RPE / choriocapillaris, The RPE appears distinct from the choriocapillaris directly beneath the fovea, where the pigmentation is the heaviest. Above the minimally reflective photoreceptors, alternating layers of moderate and low reflectivity reveal the stratified structure of the retina. Moderate backscattering is observed from the inner and outer plexiform layers (IPL, OPL), which like the NFL, consist of a fibrous structures running perpendicular to the incident beam. In contrast, minimal backscattering is noted from the nuclear layers, in which, like the photoreceptors, the cell bodies are oriented parallel to the incident light. Retinal blood vessels are identified by their increased backscatter and by their shadowing of the reflections from the RPE and choriocapillaris. The larger choroidal vessels also appear in the image and have minimally reflective, dark lumens.
In analogy to X-ray computed tomography (CT) or magnetic resonance imaging (MRI), information on three-dimensional structure may be obtained by using the optical sectioning capability of OCT to acquire serial images of consecutive slices through the retina. As an example, six sagittal tomograms were obtained consecutively from the macula of a human subject with a lateral displacement of 225 |im between each image (Figure 2-6), The locations of each tomogram on the retina are labeled on a corresponding iundus photograph.
Characteristic features of the retina appear consistently in the serial sections. The anterior and posterior surfaces of the neural retina are demarcated by backscattering at the NFL and vitreoretinal interface, and tile highly backscattering red layer representing the RPE and choriocapillaris. The sequence of tomograms shows the development and resolution of the foveal depression, which reaches its maximum depth at the fovea centralis. Retinal blood vessels derived from the superior and inferior branches of the central retinal artery are evident in the tomograms from the partial shadowing of the deep retinal structure beneath the vessels.
Figure 2-7 displays OCT sections taken through different radial planes, each passing through the center of the optic nerve head, which are useful in com-
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