Usefulness of Flash Fundus Camera for Taking Images of Fundus Auto fluorescence

Asma Kiran, Muhammad Suhail Sarwar, Asad Aslam Khan

Abstract


The fluorescent emission from the crystalline lens varies with the excitation light used, the age of the patient, the amount of nuclear sclerosis, and the concurrent diseases that may be present, such as diabetes. The lens fluorescence has a broad peak ranging from 500 to about 550 nm for the commonly used wavelengths for autofluorescence photography of the fundus. Therefore, autofluorescence of the crystalline lens overlaps the fluorescence produced by fluorescein. To produce useful autofluorescence images, we need to be able to either reject or bypass the fluorescence of the lens. Scanning laser ophthalmoscopes have a confocal capability in which only conjugate points on the fundus are imaged. Points not lying on the conjugate planes are rejected. This allows confocal scanning laser ophthalmoscopes (cSLOs) to use excitation and barrier wavelengths similar to those used in fluorescein angiography to obtain autofluorescence photographs. The camera system used a charge-coupled device camera cooled to −20°C and a restricted field of view of 13° “to minimize the loss in contrast caused by light scattering and fluorescence from the crystalline lens.” This system was capable of imaging autofluorescence, but the published images had low contrast, and a 13° field of view is not acceptable for clinical practice. Because the lens fluorescence occurs with wavelengths shorter than the upper cutoff of the barrier filter, lens autofluorescence is usually not much of a problem unless the patient has severe degrees of nuclear sclerosis. The limitations of this system for autofluorescence include all of those that a typical fundus camera would face, particularly for patients with small pupils. Initial autofluorescent photography was possible with this system, but the signal was low, resulting in dark images. Increasing the gain, along with increasing the brightness of the photographs by normalizing the picture, increased the noise. The barrier filter was placed in the near-infrared region, which was on the far, declining edge of the fluorescence spectrum of lipofuscin. The optical performance of the camera and the eye is better adapted to visible wavelengths.