An Image Processing Approach to Characterizing Choroidal Blood Flow

Indocyanine green (ICG) dye angiography has made possible routine visualization of choroidal blood flow in the human eye; however, to date, its clinical utility has been limited. An overlying layer of densely pigmented tissue and the complex, multilayered vascular structure of the choroid combine to produce angiographic images of low contrast which are difficult to interpret. Conventional image processing can enhance individual images of the blood vessels, but this approach contributes no information about the dynamics of blood flow. Using relatively inexpensive, commercially available personal computer hardware, angiographic image processing algorithms were developed which appear to characterize uniquely a subject choroid in terms of various blood flow parameters. We believe this to be the first successfully demonstrated approach to routinely characterizing the human choroidal circulation in a way that conserves spatial distribution of blood flow dynamics across the entire observed choroidal area. The computer system allows acquisition of digital images from photographic film negatives; alternatively, real-time direct digitization of images from a high-resolution video camera is possible. Once acquired, the digitized data are manipulated according to various algorithms that employ time-sequence analysis to generate two-dimensional curves or three-dimensional surfaces which characterize the choroidal circulation. The unique correspondence of each three-dimensional surface to the subject choroidal circulation from which it was derived is demonstrated. Grouping the characteristic three-dimensional surfaces according to various topographic features in common may provide a basis for discriminating between normal and abnormal choroidal circulations. Invest Ophthalmol Vis Sci 31:629-637,1990 The importance of choroidal blood flow to maintenance of the sensory retina-especially the foveahas long been recognized; however, its role in the etiology of retinal diseases is not well understood. In large part this has resulted from inability to visualize routinely the choroidal vasculature and to differentiate between normal and abnormal blood flow through it. Fluorescein angiography of the choroidal vasculature is hampered by the presence of macular xanthophyll, retinal pigment epithelium, and choroidal pigment-all of which block the visible light wavelengths absorbed and emitted by the dye-and by the rapid extravasation of unbound fluorescein molecules which stain choroidal tissue. Although indocyanine green (ICG) dye angiography permits visualFrom the "Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland; the tWilmer Ophthalmological Institute, Baltimore, Maryland; and the |St. Gallen Eye Clinic, St. Gallen, Switzerland