E. Y. Chew

Image Scaling Difference Between a Confocal Scanning Laser Ophthalmoscope and a Flash Fundus Camera.

BACKGROUND AND OBJECTIVE To evaluate scaling and measurement differences between flash and scanning laser fundus images. PATIENTS AND METHODS The authors analyzed fundus autofluorescence images of patients with geographic atrophy secondary to age-related macular degeneration imaged with both 30º confocal scanning laser ophthalmoscope (cSLO) and 50º flash fundus camera (FFC). Multiple vessel-crossing points served as landmarks. RESULTS The mean (±SD; range) scaling factor between cSLO and FFC images (by GRADOR) for the horizontal dimension was 1.217 (±0.0487; 1.0474-1.272) versus 1.138 (±0.0311; 1.0841-1.193) for the vertical dimension. The mean percentage difference between horizontal and vertical scaling factors was 7.48 (±2.29; 2.30-10.70). Refractive error (focus) and aperture size (or field of view of the image) were positively correlated and aspect ratio was negatively correlated with landmark pair measurements. CONCLUSION Inherent image-scaling differences between fundus autofluorescence imaging systems are not restricted to simple pixel-to-millimeter calibration variances, but appear to vary depending on measurement orientation. Differences should be considered when comparing measurements obtained using different imaging systems, particularly for clinical trials.