Characteristics of Foveal Autofluorescence Images of Normal Retinas

Abstract

Abstract: : Purpose:The status of the retinal pigment epithelium (RPE) is an important marker in age related macular degeneration (ARMD). Its abnormal state exhibits lesions such as drusen and geographic atrophy, and hypo– and hyperpigmentation. RPE can be monitored by the scanning laser ophthalmoscope (SLO) using laser light to elicit autofluorescence (AF) of lipofuscin molecules. Characterization of AF is critical for accurate segmentation of ARMD pathology, but is complicated by the non–uniform AF background. We developed mathematical and geometric models of this background for normal (non–diseased) foveas and determined their common geometric features. Methods:AF fundus images were obtained from 14 normal maculas of 10 patients using the SLO. They were filtered and contrast–enhanced to obtain gray level elliptical patterns in the form of isobars, one for each gray level scale value. From these we determined the eccentricities and angles of orientation, and compared the horizontal and vertical line scans in a zone 1,500 microns in diameter centered on the point of lowest foveal fluorescence. The image data were fit with a mathematical model of elliptic quadratic polynomials in two equal zones, the center and the remaining annulus. The model was also fit to grid subsets of image data comprising 1.7% of the center and 1.1% of the annulus, and the accuracy of all fits was determined. Results: Foveal AF images for the 14 retinas segmented into nested concentric gray level isobars. The gray levels became lighter radially in all directions and the mean isobar resolution was 31 ± 7 microns. The model fits to the complete image data had mean absolute normalized errors ranging from 3.6 ± 3.7% to 7.3 ± 7.1%. Additional analysis on 10 left eyes showed that the eccentricity of the ellipses increased from 0.42 ± 0.12 centrally to 0.52 ± 0.14 peripherally (p = .0005), with mean axes of orientation 97.12 ± 15.46 degrees. The vertical and horizontal line scans showed significantly higher gray levels temporally relative to other quadrants at radii greater than 450 microns (p = 0.05 or less). The model fits to the grid data had mean absolute errors ranging from 3.7 ± 3.8% to 7.3 ± 7.2%. There was no significant difference in any parameters between males and females or as function of age. Conclusions: Normal AF fundus images show finely resolved concentric elliptical foveal patterns that are consistent with anatomic structures. A two–zone elliptic quadratic polynomial model is an accurate representation of foveal data, and can reconstruct foveal data from small subsets of data, making it useful for mathematical image analysis of normal and pathologic maculas.

Keywords: age–related macular degeneration • imaging/image analysis: clinical • drusen