Measuring the Thickness of Macular RGC+ Layer With Spectral-Domain Optical Coherence Tomography (SD-OCT)

Macular retinal layer thickness was measured with Spectralis SD-OCT (Heidelberg Engineering GmbH, Heidelberg, Germany). The images were generated using high resolution volume scan mode with automatic real-time mean value of 15. Measurements were taken for each subject's tested eye. Macular raster scans were acquired with 49 B-scans consisting of 1024 A-scans. This scan resulted in an imaging area of approximately 6 × 6 mm centered at the fovea, corresponding to the central 20° visual field (Fig. 3A). Any scan with a quality score less than 20 dB was excluded from analysis. The thickness of each layer was read from the automatic segmentation algorithms provided by the onboard SD-OCT software (version 6.3.1.0). The RGC+ layer thickness was the sum of the ganglion cell layer and inner plexiform layer thickness.

(A) Examples of images acquired from SD-OCT. Top: An overlay of RGC layer thickness map on a fundus photo. Macular RGC layer thickness map (the heat map) is centered at the fovea. The diameter of the green circle corresponds to the central 20º visual field. Bottom: A B-scan image, that is a cross-sectional view of the retina. The green lines are the boundaries of ganglion cell layer and inner plexiform layer. (B) Correlation between estimated macular RGC counts. The OCT-aided RGC counts are pitted against the SAP-based RGC counts. Each black dot represents the estimated RGC counts of each subject's tested eye. The solid line is the identity line, where y = x. (C) A Bland-Altman plot. The difference between the two estimations is plotted as a function of the average of the two estimations. Each black dot indicates a data point from each subject. The mean difference is the estimated bias of the two estimations indicated by the horizontal solid black line. The horizontal dotted red lines represent 95% limits of agreement, defined as the mean difference ± 1.96 standard deviation.