Digital angiography in ophthalmology
Fluorescein angiography is used in the routine clinical examination of patients with suspected retinopathy. Conventional angiography involves the photography of a subject's retina after injection with a small quantity of fluorescein dye. In this study, we have developed a technique to measure the movement of this dye through the retinal circulation. Two implementations of angiography have been investigated - conventional photography and capture of the angiographic frames using the scanning laser ophthalmoscope (SLO). After pre-processing to remove any noise present, the images are registered via cross-correlation to remove the effects of patient movement. A gamma variate plus 2nd order polynomial dye-dilution model is then fitted to the temporal change in fluorescein intensity at each point in the retina. Parameters are extracted from these fitted curves and their values combined to form functional images of the retinal circulation. A total of 16 angiograms of varying quality have been analysed using this technique. A distinctive filling pattern has been detected in the parametric images of the normal angiograms analysed, indicating a preferential flow of fluorescein, and hence blood, to the macula region of the retina. A number of abnormal angiograms were also analysed and the time to maximum images of these patients showed good agreement with their various pathologies. This included identifying occluded vessels, areas of ischaemia and oedema, as well as more subtle features such as leakage and microaneurysms. The generation of functional parametric images from fluorescein angiograms enables various aspects of the retinal circulation to be quantified. The technique offers a potential aid to diagnosis enabling the onset of retinopathy to be detected and its progression closely monitored.