Autofluorescence characteristics of normal foveas and reconstruction of foveal autofluorescence from limited data subsets.

There is increasingly strong interest in noninvasive analysis of retinal disease, especially as related to age-related macular degeneration (AMD), the leading cause of vision impairment and blindness in the Western world.1,2 One of the major methods of such analysis is through the use of the confocal laser scanning ophthalmoscope (cSLO), which in its autofluorescence (AF) mode can record the complex signal predominantly from autofluorescent lipofuscin granules in the RPE.3-9 These fluoresce strongly in response to 488-nm laser light, and abnormalities of their distribution are leading indicators of the severity of AMD.10-13 However, these images are not determined by the AF of lipofuscin alone, a fact that complicates their interpretation. One complicating factor is the absorption of the 488-nm light by the macular pigment (MP), especially the carotenoids lutein and zeaxanthin.14,15 This absorption is greatest in the center of the macula, which on fundus photographs and under visual observation is characterized by strong yellow color.16 There is similar absorption of the 488-nm light by melanin granules in the RPE,17 which also have greater density centrally. Finally, photopigments, particularly cone photopigments in the fovea, will attenuate the signal if they are not completely bleached. AF images obtained with the SLO from normal retinas display characteristic nonuniform, elliptical patterns centered on the macula. These patterns are darker in the center, indicating lower fluorescence (hypofluorescence) resulting from greater light absorption by the attenuating pigments, and are lighter (hyperfluorescent) peripherally. Depending on their appearance, they have been classified by one group of investigators,18 as belonging to four subtypes (phenotypes) based on relative size and intensities of the hypo- and hyperfluorescent regions. To complicate the matter further, the overall patterns are not permanent, but change with age. AF increases in older individuals, with increased lipofuscin at the posterior pole, at least to age 70,19,20 whereas the measured ratio of foveal AF to maximum AF intensity has been reported not to vary with age.3 Quantification of these patterns has so far been lacking and is needed to make progress in assessment of retinal disease. At present, in the clinical setting, lesions of AMD are manually graded on fundus photographs by trained graders who use semiquantitative scales.21-26 Clinical evaluations of increased AF and decreased AF lesions are likewise qualitative in character.27 Manual grading has several drawbacks, such as poor repeatability and extensive and time-consuming training necessary for adequate proficiency. Because of these drawbacks, there has been a great deal of interest in developing automated digital techniques for quantification of macular disease.28-33 Accurate mathematical models of normal images could form the basis for such techniques. As an initial step in this direction, the two-dimensional structure of normal foveal AF images was examined in two distinctly different but related ways. In the first, a pattern of gray-level (GL) isobars in the actual image data was determined by applying standard image-processing techniques of filtering and contrast enhancement. The resultant patterns, which were found to be approximately concentric and elliptical, were then analyzed to determine geometric properties of eccentricity, axis of orientation, and resolution. This was followed by the second step, the development of a mathematical model that could accurately represent the image data. Because the first analysis showed approximate elliptical contours in the data, the model chosen was based on quadratic polynomial functions, which mathematically generate precise elliptical contours in two dimensions. The mathematical model was then tested for accuracy of fit to real image data as well as data reconstruction from small data subsets.