A Comparison of Cup and Disc Measurements: Digital versus Film Images

Abstract

Abstract: : Purpose: To compare and evaluate cup and disc measurements of the optic nerve head using 45° color non–stereoscopic digital fundus images with measurements using color 30° stereoscopic film photographs. Methods: Retinal images of the disc and the macula were taken in both eyes of 62 people through pharmacologically dilated pupils using a 45° digital camera (6.3 mega pixels) and 30° stereoscopic retinal film images. For the present study, only right eyes centered on the optic disc (DRS Field 1) were graded. Vertical disc and cup measurements were made. Cup to disc ratios were subsequently computed from these measurements. The cup and disc measurements for the digital images were performed using Digital Healthcare’s Eye Q software. Measurements were taken in pixel units using the program’s measuring tool. The cup and disc measurements for film–based images were taken in inches using a template of graded circles following the Wisconsin Protocol for Optic Disc and Optic Cup Measurements. Similarity between digital and film measures was assessed through correlations. All measurements were performed by a single grader at different times for the digital and film images. The grader was masked to subject identity. Results: Among the 62 eyes, 6 digital images were ungradable. There were gradable film images corresponding to four of these. Correlations between the digital and film measurements were 0.87, 0.87 and 0.85 for disc and cup diameters and cup and disc ratio respectively. The mean (+/– SD) cup and disc ratio was 0.39 (+/– 0.17) for film measurements and 0.36 (+/– 0.15) for digital images. Conclusions: Despite a lack of stereopsis in the digital images, cup and disc measurements performed on digital and film images were highly correlated. The greater resolution of film as opposed to digital images may account for those cases in which an image was gradable from film but not digital images.

Keywords: optic disc • imaging/image analysis: clinical • clinical (human) or epidemiologic studies: systems/equipment/techniques