Suboptimal Image Focus Broadens Retinal Vessel Caliber Measurement

Abstract

Studies have used central retinal arteriolar (CRAE) and central retinal venular (CRVE) calibers, measured from images produced with computerized image analysis, to detect risk factors for systemic diseases. The authors explored suboptimal image focus as a possible contributing factor to artificially larger vascular caliber measurements. From the reading center image collections, 30 digital retinal images were selected for optimum quality. Image analysis software was used to derive nine progressively blurred versions of the originals. IVAN measurement software was used to measure CRAE and CRVE in the original and the blurred series derived from them. To check the adequacy of the simulation, progressively defocused series of images were taken of several volunteers. For CRAE, each level of simulated blurring produced a statically significant increase in apparent vessel caliber from the original (P<0.01, Wilcoxon signed rank test). For an average CRAE of 160 μm, mean broadening with minimal/moderate/severe blurring was 3 μm/12 μm/33 μm. For CRVE, every blurring level beyond the first was found to be significant (P<0.01). From an average CRVE of 200 μm, mean broadening ranged from 0 to 11 μm with minimal to severe blurring. Analysis of the progressively defocused series taken of volunteers yielded similar results overall. Suboptimal focus can result in erroneously larger vessel caliber measurements. Slight blurring has a minimal effect, but more severe blurring has a progressively greater effect.