The evaluation of OCT tomograms depends on the ability of the observer to identify both differences in the relative reflectivity of different tissue layers and morphological changes in tissue structures . In some cases, because of the high axial resolution of the OCT images, small changes in morphology may be difficult to assess by direct observation of the images. In these cases, automated computer image processing tools may be used to extract precise quantitative measurements from the images.
Retinal thickness is an important consideration in the assessment of many macular diseases. The high axial resolution of OCT combined with the well-defined contrasts in reflectivity at the anterior and posterior boundaries of the retina make OCT uniquely suited for measurement of this parameter
Retinal thickness may be increased with edema. The accumulation of intraretinal fluid will lead to both an increased retinal thickness, and also a change in the scattering properties of the tissue. An important location to measure retinal thickening is directly in the fovea, where edema can have a profound effect on visual acuity. This type of measurement can be particularly useful in tracking patients with macular edema due to diabetic retinopathy (Figure 2-9), or for screening and following patients with cystoid macular edema following cataract surgery ,
Retinal edema may be differentiated from retinal traction by the identification of cystic spaces within the retina, indicative cystoid macular edema, or by visualization of the posterior hyaloid or an epiretinal membrane, which may be causing retinal traction.
Decreased retinal thickness often occurs with atrophy or scarring and may be either focal or diffuse.
Changes in retinal architecture or cellular morphology result in changes in optical properties which may be observed on the OCT tomograms. The apparent reflectivity measured by OCT is a combination of the actual reflectivity and the scattering and absorption characteristics of the overlying media. Thus, the apparent reflectivity for retinal imaging may be af
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