Clinical Application of Matched Filter Processing of Digital Color Fundus Images

Abstract

Purpose: : The digital color fundus image with Matched Filter processing reported by Chaudhuri in 1989 is made up of 2.5x10⁵ pixels, but the resolution of digital color fundus images has improved to more than 1.0x10⁷ pixels. The higher resolution suggested that this technique might be used to study blood vessels and vascular diseases of the retina. However, the possibility also existed that it could also enhance the appearance of nerve fibers. The purpose of this study was to determine the usefulness of high-resolution digital color fundus imaging with matched filter processing.

Methods: : A digital camera (D80, Nikon) was attached to a no mydriasis-type of fundus camera (TRC-NW6S, Topcon) to take non-compressed photographs of the fundus. The non-compressed color fundus photographs were saved and processed by the Matched Filter program. Eyes with diabetes retinopathy, age-related macular degeneration, branch retinal vein occlusion, and glaucoma were studied. We compared the matched filter images with the fluorescein angiograms (FA) (TRC-50IX, Topcon), optical coherence tomograms (OCT), and results of the Humphrey field analyzer (HFA).

Results: : Indistinct blood vessels were enhanced by the matched filter processing, and leaked blood, such as hard exudates, was enhanced. However, the leakage and the circulation changes were not sufficient to be used for diagnosis. Structures considered to be reflections from nerve fibers were seen, and nerve fiber layer defects (NFLDs) were enhanced in eyes with glaucoma. In addition, a retinal brightness peripapillary profile of one disc diameter demonstrated that the brightness in the part corresponding with the NFLD was reduced. This profile corresponded with the nerve fiber thickness measured by OCT and was able to explain the results of HFA.

Conclusions: : The Matched Filter processing of retinal vascular lesions is useful for diagnosis only from a color fundus images, but does not substitute for FA. In addition, The matched filter processing enhanced not only the blood vessels but also the nerve fibers. This method is simple and easy and can be carried out with comparatively inexpensive equipment in comparison with OCT, and we conclude that it will be useful for the diagnosis of the glaucoma.

Keywords: image processing • retina • nerve fiber layer