Measurement of retinal branching angles and vascular tortuosity using color fundus photographs.

For measurement of retinal branching angle, the lines denoting the direction of the branches were produced by SIVA software that tracked down each vessel in a specified area (defined as the region from 0.5 to 2 disc diameters away from the disc margin). Then the angle subtended by the daughter branches was automatically calculated by the software using the cosine rule of the angle at the bifurcation of the vessels. For assessment of retinal vascular tortuosity, the centerline of the vessel was automatically traced by the SIVA software. The retinal vascular tortuosity was derived from the integral of the curvature square along the path of the vessel, normalized by the total path length measured in a specified area (defined as the region from 0.5 to 2 disc diameters away from the disc margin). A straight vessel has lower tortuosity value while a tortuous vessel has a higher tortuosity value.