New procedure aims to prevent vision loss
Vision loss in persons over 50 years old is commonly caused by the localized excessive growth of new blood vessels in the choroid, the layer of the eye between the retina and the sclera (the white outer layer of the eyeball). To locate these blood vessels by an established procedure, patients receive an intravenous injection of a fluorescent dye. The dye passes through the blood stream and internally illuminates the translucent blood vessels. A camera is used to record the movements of the fluorescent dye as it flows through the blood vessels in the choroid. Unfortunately, the glowing dye passes through all the blood vessels, and the fluorescence can therefore obscure the viewing of the choroid, thereby making identification and precise location difficult.
To overcome this problem, Dr. Ran Zeimer and his colleagues at The Johns Hopkins University (Baltimore, MD) have developed an experimental method of delineating the choroid structure using laser-targeted angiography (LTA). In this method, the fluorescent dye is first heat-encapsulated inside heat-sensitive fat cells (liposomes) and then injected into a peripheral vein. The dye is then released by means of a laser pulse directed at the choroid layer. Because the laser is aimed locally on a 0.6-mm area, only a small amount of dye is released and becomes visible. With the release of the visible dye, targeted vessels are illuminated while the background remains dark, enhancing the visualization of the choroid at a later time.
After the dye is released by the laser pulse, an angiographic sequence is acquired, usually for a duration of two seconds. To perform LTA, a fundus camera from Zeiss (Germany) is equipped with an MC-1134GN, 1134 ¥ 480-pixel camera from Texas Instruments (Dallas, TX). The camera output is fed into a video enhancer and then stored on magnetic tape.
For analysis, the tape is operated and the stored images are digitized with a Video Model-12 imaging board from Epix (Buffalo Grove, IL). The 16-Mbyte on-board image memory allows multiple images to be digitized at a video rate and permits the acquisition of a complete sequence of angiograms from any one test. Moreover, the presence of the whole sequence in memory permits a consistent and efficient analysis.