A small amount of sodium fluorescein dye is injected into a vein in the patients arm. A special fundus camera is used to view the retina of the eye. The blue colored light coming from the camera causes the sodium fluorescein dye to glow in a yellow-green color. A special barrier filter in the camera permits only the fluorescent light to be viewed.
As this sequence begins, the image is black because no sodium fluorescein is present in the vessels. If you watch carefully, or stop the movie and watch it a frame at a time, you will notice that the background becomes visible before the retinal vessels. The background is a layer of the eye called the choroid which has it's own circulation and fills with dye slightly before the retinal vessels.
A second or so after reaching the choroid, the dye begins to fill the retinal vessels. The arteries become visible first. The dye then travels across the network of fine vessels called capillaries where nutrients and oxygen are transferred to tissue. The dye continues it's journey through the retina by filling the viens and exiting the eye.
This very early stage of an angiogram, where the dye enters the eye, crosses the capillary bed and fills the veins is called the transit phase. Photographs are usually taken at intervals up to about 10 minutes after the injection of dye.
An ophthalmologist examines the way the fluorescein dye travels through the vessels during the transit phase and the way in which the dye pools or stains tissue in later phases as a part of the diagnostic process for determining causes of decreased vision.