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Figure 2-10, OCT tomograms of a neurosensory retinal detachment (A), and a serous detachment of the retinal pigment epithelium (B). Both detachments exhibit an elevation of the retina over an optically clear space corresponding to serous fluid accumulation. In the pigment epithelial detachment, however, the red band defining the posterior sensory retina! boundary is also elevated and the detached RPE severely shadows the reflections from the choroid below.

fected by abnormalities in the cornea, aqueous, lens, vitreous, and anterior retinal layers.

Causes of increased or hyper-reflectivity include inflammatory infiltrate into any layer of the retina or choroid, fibrosis, such as in a disciform or other scar, hard exudate, and hemorrhage. Hard exudate is highly reflective, but completely blocks the reflections from the deeper retinal layers (Figure 2-9). Blood also has a high scattering coefficient, which has two effects. I he amount of backscattering from blood is great, leading to a bright reflection from hemorrhage; however, the scattering is also significant, leading to a rapid attenuation of the incident light as it propagates through the blood. Thus, blood vessels are most readily identified by their shadowing effect on the reflection from the RPE and choriocapillaris. The attenuation of the incident light depends on the thickness of the scattering medium. Thin hemorrhages appear as thin, highly reflective bands that have little effect on the underlying tissue. Thick hemorrhages, however, completely attenuate the probe light after more than approximately 200

Figure 2-11, OCT tomograms of a hemorrhagic (A) and a fibrovascular (B) detachment of the retinal pigment epithelium (RPE), The hemorrhagic detachment displays high reflectivity just beneath the RPE corresponding to the hemorrhage. The incident light is attenuated by the superficial blood, completely shadowing the reflections from the deeper hemorrhage and choroid. In contrast, the fibrovascular detachment shows moderate backscattering throughout the sub-RPE space. Minimal reflection from the choroid is also visible.

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Decreased, or hypo-reflectivity may be caused by retinal edema, in which fluid accumulation leads to a decreased density of microscopic scatterers and a corresponding reduction in the scattering coefficient (Figure 2-9). Alterations in cellular structure such as hypopigmentation of the RPE may also result in reduced reflectivity.

These morphological causes of reduced backscat-tering must be distinguished from alterations in the incident light caused by dense cataracts, cloudy media, astigmatism, poorly centered intraocular lens implants, or poor alignment of the OCT instrument while imaging. Abnormalities in the intervening structures typically cause a diffuse hypo-reflectivity throughout the OCT image, in all retinal layers. Focal decreases in reflectivity may be caused by shadowing from hyper-reflective tissues, such as hemorrhage, hard exudate, or a detached RPE, Local reductions in back-scattering may also occasionally occur through silhouetting of the scanning probe beam by a narrow pupil In this case, the reflectivity from all retinal layers is proportionately reduced, and the region of reduced reflectivity dynamically changes with alterations in the alignment of the delivery system.

The distinction between blood, serous fluid, and exudate is also made on the basis of reflectivity. The scattering coefficient, which along with anisotropy determines reflectivity, is essentially proportional to the concentration of particles in the fluid. Serous fluid, containing few cells, is optically transparent, so its accumulation is immediately recognized on OC as a region devoid of backscattering, Blood, in contrast, has a large number of cells and a very hig:i scattering coefficient, leading to both enhanced reflectivity and increased attenuation of the incident light, Cloudy exudate typically has an intermediate appearance between blood and serous fluid on OCT images.

Detachments of the Neurosensory Retina and Retinal Pigment Epithelium

OCT is extremely useful for evaluating detachments of the neurosensory retina and RPE [10]. Neurosensory detachments appear as a shallow elevation of the retina, with an optically clear space between the retina and RPE (Figure 2-10), The backscattering from the normally minimally reflective photoreceptors is increased, resulting in a well-defined fluid-retina boundary.

Serous detachments of the pigment epithelium have a distinctly different appearance (Figure 2-10) The reflective band corresponding to the RPE is focally elevated over an optically clear space. Like the photo receptors in the neurosensory detachment, the detached RPE is more highly reflective than normal, perhaps due to a refractive index difference between serous fluid and the choriocapillaris, or due to decompensation and morphological changes in the RPE cells themselves. The increased reflectivity from the RPE strongly shadows the backscattering signal from the choroid below the detachment, often even obscuring the reflective boundary between the serous fluid and the basement membrane. The angle of the detachment is also more acute in a pigment epithelial as compared to a neurosensory detachment because of the tight adherence of RPE cells to the basement membrane at the edge of the detachment, which supports an increased fluid pressure,

The increased reflectivity from the photoreceptors in a neurosensory detachment may mimic the high reflectivity from the pigment epithelium, but rarely does it significantly shadow the reflections from the RPE and choroid. Thus, the distinction between a neurosensory and a pigment epithelial detachment relies on assessing the strength of the reflection below the serous fluid collection, and evaluating the angle of the detachment.

Neurosensory detachments may occasionally be confused with severe retinal edema on OCT, since in many cases the fluid accumulation and reduced back-scattering which occurs with edema is preferentially seen in the outer retinal layers. It is important in these cases to identify a smooth and continuous fluid-retina boundary to establish the diagnosis of a sensory retinal detachment.

Hemorrhagic detachments of the RPE have all of the characteristics of a serous R PE detachment, except that an optical backscatter signal corresponding to blood is observed directly beneath the detached RPE (Figure 2-11), The blood usually appears moderately, rather than highly, backscattering because of the attenuation of the incident light through the detached RPE. The optical penetration through both the detached RPE and hemorrhage is usually less than 100 jam.

Fibrovascular pigment epithelial detachments also show some optical backscatter within the sub-RPE space (Figure 2-11), However, a lower scattering coefficient compared to blood results in reduced apparent reflectivity from the fibrovascular proliferation and increased penetration of the probe light, which often reaches the choroid. Other lesions, such as a vitelliform lesion, may have a similar appearance of constant mild to moderate reflectivity extending between an elevated RPE and the choroid.

Confluent soft drusen may be mistaken for small pigment epithelial detachments. However, no shad-

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