Foveal Avascular Zone Analysis

We analyzed FAZ with respect to size, perimeter, and circularity index. OCT-A scans of the superficial and deep retinal vasculature were exported and independently assessed by two trained graders (J.W.S. and J.K.) using ImageJ (version 1.49, National Institutes of Health, Bethesda, MD, USA). The image scale was set using known image size of 452 x 452 pixels. Because each scan length is 3 mm horizontally and vertically, a pixel aspect ratio of 1.0 was set, resulting in a scale of 150.67 pixels per mm. The whole edge points were manually connected to each other along the borderline of the identifiable capillary network in the parafoveal area. FAZ size and perimeter were calculated by the software from the comparison of the 9 mm² total image size.
Circularity index is a measure of compactness of a shape relative to a circle. The circularity index of a circle is 1.0. Thus, a ratio closer to 0 indicates an irregular shape, and that closer to 1.0 indicates a circular shape. The circularity index is calculated as a function of the perimeter and the area of a shape.
Our hypothesis for integrating FAZ circularity index into the analysis is that if disruption of parafoveal capillary network progresses, FAZ shape would be less likely that of a perfect circular shape, which may lead to a decrease in FAZ circularity index. Circularity index has been applied in other fields of ophthalmology to describe the perfectness of capsulorhexis in cataract surgery or the prognosis of geographic atrophy.[34 (link), 35 (link)]