How does indocyanine green (ICG) fluorescence work?

Indocyanine green (ICG) is a cyanine dye used in medical diagnostics. ICG becomes fluorescent on excitation either with a laser beam or by near infra-red (NIR) light. The absorption and fluorescence spectrum of ICG is in the near infrared region. Both depend largely on the solvent used and the concentration. ICG absorbs mainly between 600 nm and 900 nm and emits fluorescence between 750 nm and 950 nm. The large overlapping of the absorption and fluorescence spectra leads to a marked reabsorption of the fluorescence by ICG itself. The maximum fluorescence values of ICG are approx. 810 nm in water and approx. 830 nm in blood. For medical applications based on absorption, the maximum absorption at approx. 800 nm (in blood plasma at low concentrations) is important. The fluorescence released by ICG can be detected using specific scopes and cameras and then transmitted to a standard monitor where they facilitate the identification of anatomical structures such as lymph nodes, biliary ducts, and vessels where the dye is present.