QUANTIFYING CHANGES IN RETINAL CIRCULATION - THE GENERATION OF PARAMETRIC IMAGES FROM FLUORESCEIN ANGIOGRAMS

Fluorescein angiography is an established technique for examining the functional integrity of the retinal Circulation. The ability to quanti fy this function offers the possibility of early detection of changes due to retinopathy. We have developed a technique to generate function al, parametric images of the retinal circulation. A given angiogram is first registered to align consecutive frames. At each point in the re tina, a graph of fluorescein intensity versus time is then constructed and fitted with a gamma variate curve. Parameters are extracted from these curves and formed into parametric images showing the variation i n fluorescein passage across the entire area of the angiogram. Paramet ers examined to date include rime to maximum intensity, time of arriva l and rise time. The technique has been demonstrated using photographi c and scanning laser ophthalmoscopic angiograms of both normal subject s and patients With a variety of retinopathies. The time to maximum im ages of the normal subjects reveals a similar filling pattern in each case, whilst the pathologies present in the abnormal angiograms are cl early identified. The generation of functional time to maximum images enables the health of the retinal circulation to be quantified with re spect to the rate at which the vasculature fills with fluorescein, Thi s offers a potential tool for detecting the onset of retinopathy and m onitoring its progression.