ICG is used to acquire an angiogram of the choroid. The choroid is the layer of blood vessels and connective tissue between the sclera (white of the eye) and retina. It supplies nutrients to the inner parts of the eye.
A procedure similar to fluorescein angiography, but ICG angiography uses indocyanine green dye, which fluoresces in the infra-red (non-visible) light.
The infra-red wavelengths have the ability to penetrate the retinal layers making the circulation in deeper layers visible when photographed with an infra-red sensitive camera.