Somewhat analogous to coronary angiography (used to determine the patency of arteries supplying the heart), FFA images the retina before and after the intravenous injection of a special dye, Fluorescein Sodium. It shows the perfusion status of the retina and is invaluable in diagnosing and determining the prognosis and treatment of a variety of diseases of the retina and the choroid, most prominent being diabetic retinopathy, retinal vein occlusions, etc.