It is helpful to look at a normal example of an angiogram to understand the abnormal examples listed below. The normal image accompanying this text shows the vessels as white. You can see two sets of vessels on the image. The superficial retinal vessels are labelled with an arrow. The deeper network of choroidal vessels is not labelled. The choroidal vessels are much more abundant. The fluorescein angiogram does not show choroidal vessels for two reasons. The main one is that the wavelength of the light emitted from fluorescein is blocked by the overlying retinal pigment epithelium. Therefore only the retinal vessels are seen which are superficial to the retinal pigment epithelium. The underlying choroidal vessels are not seen. Secondly, the ICG dye does not leak from the intact choroidal capillaries so the vessles remain visible. The fluorescein dye does leak from intact choroidal capillaries and that leakage also obscures individual choroidal vessels.
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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.
ICG is injected intravenously and flows through the body to reach the choroidal and retinal circulation. Due to its nature ICG stays in the retinal and choroidal vessels, this allows the distinct outlines of the vessels of the choroid to be seen and identified. ICG is sometimes used to complement fluorescein angiography (FA). FA is often referred to retinal angiography while ICG angiography is referred to choroidal angiography.
ICG was first used in [1] but was not brought into the practical clinical setting until .[2]
In the case of a patient with 20/100 vision, the fluorescein angiogram demonstrated leakage of fluorescein dye over a large area near the fovea. Traditional treatment would dictate that the entire area of leakage be treated with laser surgery. The treatment of this lesion would cause an instant decline in vision to 20/400.
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The ICG angiography, performed on the same day, reveals a pinpoint leak not visible with fluorescein angiography. Focal treatment based on the ICG angiogram caused an increase in vision from 20/100 to 20/80.
After treatment the laser scar is visible on the color fundus photograph. Both the fluorescein and the ICG angiograms show no leakage of dye demonstrating that the focal treatment was effective.
Photographs by UIHC Opthalmic Imaging Staff
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