Fig. 1

Illustration of the subdivision used to estimate vessel tortuosity (a) and vascular width (b). Magnified (3×) fundus photo images were used for the analysis, and images showing the subdivided superior central retinal vein are presented. With imageJ software (Version 1.47, NIH, Maryland, USA), image was processed twice: sharpening to enhance the vessel cords for tortuosity analysis (a) and reversal to highlight the vessel walls for width analysis (b). Vessel tortuosity was estimated with relative length variation (refer to equation of top) by using straight and freehand line selection tools (a). Vascular width was measured at three points: at disc margin at 1.0 disc diameter from the margin, and at 2.0 disc diameter from the margin by using straight line selection tool (b). L = curvilinear length of the vessel; l = linear length of the chord; vw = vascular width