Digital area measurement of fundus landmarks

To present and validate an image analysis technique for the calculation of the true projection area of fundus landmarks from a digital fundus image. The authors present the description, technical details and results of the study. Experimental study using a calibrated Gullstrand-type model eye with a scale of half circles in the centre of the artificial fundus. The axial length of the model eye was set to different values ranging from 21 mm to 29 mm, and the area of a half circle was calculated from digital fundus images with a magnification-calibrated image analysis program (UTHSCSA Image Tool). The calibration was based on Littmann's formula for correcting the magnification of fundus images. The maximum deviation of the calculated area size from the true fundus object area was -3.7% under myopic and +3.4% under hyperopic conditions. The 95% confidence interval for repeated area measurements was from +1.83 to -1.46%. The magnification-calibrated image analysis technique based on Littmann's formula is a non-invasive, accurate and reproducible tool for true area measurement of fundus landmarks.