Fluorescence Properties and Metabolic Features of Indocyanine Green (ICG) as Related to Angiography

Indocyanine green (ICG) is a fluorescent dye that has been used for the imaging of retinal and choroidal vasculatures for more than 30 years. Its high molecular weight, specific metabolic features, and its infrared spectra make the specificity of the images obtained with this dye in ophthalmology. The focus of this paper is to review the basic properties of ICG and to show how some clinical features related to basic properties also depend on the instrumentation used to perform ICG angiography. Indocyanine green has a complex molecular structure that leads to amphiphilic properties, that is, both hydrophilic and lipophilic properties. These properties explain that a specific interaction with phospholipids influences the emission spectrum and the fluorescence yield of ICG. The composition of cell membranes mainly composed of phospholipid bilayers is consistent with a binding and/or a diffusion of ICG molecules observed on angiograms. Likewise, ICG can bind to the lipid component of miliary drusen, explaining their hyperfluorescence. A knowledge of ICG basic properties and interactions may allow a better understanding of angiograms performed with this dye.