Fluorescein and Indocyanine Green Angiography

ANGIOGRAPHY

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,2,3, Because the inflammation may be either generalized or localized to a specific region of the chorioretinal complex, depending on the nature of the disease process, the fluorescein angiogram may yield valuable information not only with regard to severity of the breakdown of the barrier such as fluorophotometry but also with regard to localization of the site of inflammation. In instances in which inflammation coexists with ischemia or neovascularization, fluorescein angiography can be particularly valuable in distinguishing between these elements. Additionally, fluorescein angiography can show alterations to the retinal pigment epithelium during acute inflammatory phases and also show chronic changes in the pigment epithelium that often remain after the inflammation has subsided.

The rationale for the use of sodium fluorescein for diagnostic imaging of the fundus is covered more completely in other chapters. By absorbing photic energy in the wavelength range of 465 to 490 nm and emitting at a different wavelength range of 520 to 530 nm, sodium fluorescein yields a dynamic high-contrast image of the retinal structures, emphasizing the discrete intravascular compartments silhouetted against their adjacent structures, when photographed with appropriate barrier and excitation filters in place. In some instances, the presence of the monochromatic filters alone is sufficient to decrease light scattering and consequently improve resolution of fine detail in eyes with hazy intraocular media. In most instances, however, the luminescence provided by the excited fluorescein molecule is responsible for the apparent image enhancement available by this technique. With the recent availability of more sophisticated video and digital signal processing techniques, the capacity for image enhancement has been further increased. Because of its relatively low molecular weight of 376 Daltons, sodium fluorescein freely traverses the fenestrated capillaries and larger vessels comprising the choroid. However, the tight junctions between adjacent retinal pigment epithelial cells and healthy mature retinal capillary endothelial cells effectively block the ingress of fluorescein from the choroid to the subretinal space, or from the retinal vessels into the neurosensory retina or vitreous, respectively, under normal conditions. The loss of this selective barrier effect as a result of disease underlies the value of fluorescein angiography as an imaging modality. In addition to ischemia, mechanical or thermal trauma and inflammation result in breakdown of the barrier effect of tight junctions and allow passage of fluorescein, generally in proportion to the degree of the inflammation. 1 4 Because the inflammation may be either generalized or localized to a specific region of the chorioretinal complex, depending on the nature of the disease process, the fluorescein angiogram may yield valuable information not only with regard to severity of the breakdown of the barrier such as fluorophotometry but also with regard to localization of the site of inflammation. In instances in which inflammation coexists with ischemia or neovascularization, fluorescein angiography can be particularly valuable in distinguishing between these elements. Additionally, fluorescein angiography can show alterations to the retinal pigment epithelium during acute inflammatory phases and also show chronic changes in the pigment epithelium that often remain after the inflammation has subsided.

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 wavelenths 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]

Case Example:

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

References:

1. Kogure K, Choromokos E. Infrared absorption angiography. J Appl Physiol. ;26(1):154-7.
2. Yannuzzi LA, Slakter JS, Sorenson JA, Guyer DR, Orlock DA. Digital indocyanine green videoangiography and choroidal neovascularization. Retina.;12(3):191-223.