The Shape of the Foveal Avascular Zone: When a Circle Isn't Round

Numerous metrics are used to analyze the morphometry of the foveal avascular zone (FAZ). Two such metrics, roundness and circularity, have recently shown a marked growth in their use. However, there have been inconsistencies across studies with respect to how these metrics are defined, as well as the algorithms used to calculate them. In some cases, the exact definition or algorithm is not disclosed. These issues significantly limit the translational utility of these biomarkers. We simulated FAZ shapes as circles or ellipses of differing aspect ratios and evaluated roundness and circularity with two commonly used approaches for FAZ analyses. Differing shape analysis algorithms produced conflicting results even with identical equations, and differing metric definitions produced incongruent results. Therefore caution should be used when comparing FAZ circularity and roundness metrics across studies, especially in the absence of detailed information about the algorithms used. TRANSLATIONAL RELEVANCE: Quantitative assessment of OCT-A images includes evaluating circularity and roundness of the FAZ. Inconsistent or inaccurate mathematical definitions of these metrics impacts their utility as biomarkers and impairs the ability to combine and compare results across studies.