Indocyanine green angiography (ICGA) is used to image the choroidal circulation and its abnormalities. Even though fundus fluorescein angiography (FFA) is a good tool for imaging retinal circulation in great detail, its ability to image the choroidal circulation is limited by the poor transmission of fluorescence through retinal pigment epithelium (RPE), media opacities, and retinal exudates. The physical characteristics of indocyanine green (ICG) allow its visualization through RPE, lipid exudates, and serosanguineous fluid. Indocyanine green has an absorption peak at 790 to 805 nm and a peak emission spectrum at 835 nm.
As its absorption and emission spectrum is of a higher wavelength than that of fluorescein, the infrared rays to and from ICG can penetrate better through the RPE, macular xanthophyll pigments, and media opacities. Also, 98% of ICG in serum is protein-bound, allowing only limited diffusion through the fenestrations of the choriocapillaris, whereas FA diffuses quickly, blurring the anatomy of the choroid.
ICG was approved for human use by Food and Drug Administration (FDA), the USA, in 1956. ICG angiography (ICGA) of the human choroid was first performed by R W Flower in 1972. Initially, the clarity of images was poor as the ICG molecule has poor fluorescence efficiency compared to FA. But, technological advances in imaging systems (scanning laser ophthalmoscope or SL- based systems) led to the development of high-resolution cameras that could capture ICGA images with great clarity. In the era of anti-vascular endothelial growth factor (anti-VEGF) agents and optical coherence tomography (OCT), the monitoring of the choroidal neovascular membrane (CNVM) has become easier.
Still, ICGA continues to be an important imaging modality in clinical practice in evaluating various disorders, including idiopathic polypoidal choroidal vasculopathy (IPCV), retinal angiomatous proliferation (RAP), central serous chorioretinopathy (CSCR), ocular inflammatory conditions including sympathetic ophthalmia (SO) and Vogt Koyanagi Harada syndrome (VKH); and ocular tumors.
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