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 30(sic) confocal scanning laser ophthalmoscope (cSLO) and 50(sic) 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.