Indocyanine Green Angiography

Abstract

Indocyanine green angiography (ICGA) is an imaging technique used to evaluate choroidal vasculature and circulation in which indocyanine green (ICG) dye is injected intravenously. ICG was developed to study cardiac output and hepatic circulation, applied to the field of ophthalmology, and has helped overcome the limitations of sodium fluorescein, the dye used in fluorescein angiography (FA), in the study of choroidal vessels. FA provides great resolution of retinal circulation, but it minimally penetrates the retinal pigment epithelium (RPE), and choroidal circulation is not clearly visualized with this technique. ICG emits light in the near-infrared spectrum, improving considerably the amount of light that is able to go through the RPE and choroidal pigment, fluid, blood, or lipids. In addition, the majority of the dye binds to plasma proteins, and this property prevents the diffusion of ICG through the fenestrated choriocapillaris, so the dye can accurately delineate choroidal vessels. Also, it is considered a very safe technique, with a very low incidence of adverse reactions. All of these chemical and biophysical properties of ICG make it a very useful tool for the study of choroidal vasculature, especially in those cases in which fluid or blood obscure choroidal details in fluorescein angiogram. The choroid is a thick and multilayered structure, which constitutes the major blood supply of the eye and receives 90% of the ocular blood flow. The visualization of this complex structure by ICGA can help us expand our current knowledge and understanding of the choroid in the normal eye and also in pathologic conditions. For these reasons, ICGA is a very useful imaging technique in clinical practice.