Twenty-seven patients were scanned with an FD-OCT instrument (RTVue, v. 3.0; Optovue, Inc., Fremont, CA), used for image acquisition. The GCC and ONH scan patterns were used (
Fig. 1). The GCC scan covered a 7 × 7 mm rectangular area of the macula centered 0.75 mm temporal to the fixation point.
10 The ONH scan was a combination of radial and circular scans and covered the optic disc and surrounding region.
11 Each eye was scanned three times for the ONH scan and once for the GCC scan. The OCT images were exported and reviewed by coauthor Tan. Images with signal strength index (SSI) less than 42 were excluded. Images with inaccurate fixation or the retina out of view were also excluded. The images were then analyzed by automated image-processing software developed by coauthor Tan to obtain GCC and NFL maps. The image-processing software is similar to the software (RTVue, v. 4.0) derived from coauthor Tan's software. Details of the software were described in our previous publication.
8,11 Briefly, the maximum gradient of intensity was used to detect the boundaries of retinal layers. Neighbor constraint and a knowledge model were used to classify the boundaries. The inner limiting membrane (ILM) and outer NFL boundary were detected for the ONH scan. The outer limit of the inner plexiform layer (IPL) boundary and ILM were detected for the GCC scan. We used our custom software rather than commercial software to directly access point-by-point map data and perform efficient batch processing. Because the algorithm in the commercial software was adapted from our custom software, the same results can be obtained using commercial software.