Computer-assisted methods to evaluate retinal vascular caliber: what are they measuring?

PURPOSE Computer-assisted methods to measure retinal vessel diameters have been incorporated into research, but it is not clear which component of the vessels they are measuring. This study was conducted to compare measurements of retinal vessel diameter by using imaging-processing software on color fundus photographs (FPs) and fluorescein angiographs (FAs). METHODS FP and FA images were taken simultaneously in 52 eyes of 31 patients referred for angiography for diagnosis of retinal disease. Arteriolar and venular calibers were measured in two concentric zones around the optic disc center. Pearson correlation coefficients and Bland-Altman plots were used to evaluate the agreement between the measurements made by FP and FA. RESULTS The differences between the diameters measured by the microdensitometric method from FP and FA were 2.59 ± 8.67 μm in the inner arteriola, 4.93 ± 7.47 μm in the outer arteriola, -1,58 ± 8.49 μm in the inner venula, and -1.80 ± 7.28 μm in the outer venula. The differences plotted by the Bland-Altman method were slight. The Pearson correlation coefficients of measurements by FP and FA were 0.84 for inner zone and 0.87 for outer zone arterioles and 0.93 and 0.94 for the inner and outer zone venules, respectively. CONCLUSIONS The very slight differences between measurements of retinal vessel diameter by the two methods demonstrate that the microdensitometric method mostly measures the vessel lumen. Differences in vessel diameters measured by the microdensitometric method observed in clinical conditions may therefore be ascribed to variation in wall thickness or vasoconstriction.