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R. Cappelli, R. Keshavamurthy, V. S. Brar, K. V. Chalam, S. Grover; Evaluation of Thickness of Individual Layers of the Retina With Spectral-domain OCT for Establishment of Normal Clinical Values. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3788.
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Spectral-domain OCT (SD-OCT) acquires a large number of A-scans (40,000 A-scans/second) in a short period of time and provides high-resolution cross-sectional images with delineation of all the layers of the retina. Data on the thickness of individual layers of the retina, as measured by the SD-OCT, is lacking. We measured the thickness of the retinal pigment epithelium (RPE), photoreceptor layer (PRL) and outer nuclear layer (ONL) using the SD-OCT system.
Twenty eyes of normal, healthy volunteers (ages ranging from 22 to 66) were imaged using a SD-OCT system, (Spectralis OCT; Heidelberg Engineering, Germany). The scanning speed was 40,000 A-scans/second and the axial resolution was 6 µm. The fovea was scanned using the 6 mm radial scan mode with an internal fixation source. The operator independently monitored fixation. Retinal thickness could be subdivided into three separate complexes (RPE-Bruch’s membrane, PRL and ONL thickness) using a reference line that was moved manually to measure the thickness. The thickness of each of these complexes was measured at the fovea and at four extra-foveal points 500 µm away from the center of the fovea.
The mean RPE, PRL and ONL thickness at the fovea were 64 +/- 5.9 µm, 37 +/- 5.7 µm, and 123 +/- 22.7 µm, respectively. The mean RPE, PRL, and ONL thickness at the extra-foveal points were 46 +/- 0.9 µm, 34 +/- 0.7 µm, and 81+/- 3.7 µm, respectively. A paired t-test revealed a statically significant difference (P<0.05) between the foveal RPE measurement and each of the four extra-foveal measurements (superior, inferior, temporal, and nasal). Similarly, a statically significant difference (P<0.05) was found between the foveal OPL measurement and each of the four extra-foveal measurements. A statistically significant difference P<0.05) between the foveal PRL measurement and the temporal PRL measurement was also observed.
Automatic segmentation allowed for detailed quantitative and topographic analysis of the RPE and the overlying retina. Accurate measurement of different segments of the retina is relevant and useful in the diagnosis of retinal disorders and in the evaluation of therapeutic responses to pharmacological agents. Establishment of a normative database for the SD-OCT will further serve to aid clinicians in the diagnosis of retinal diseases.
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