Study for usefulness of Indocyanine green as an infrared marker

Indocyanine green (ICG) is widely used as a tracer for the non-invasive estimation of liver function. ICG has properties of binding with plasma protein, and has a large absorption peak at 805 nm. There were no reports, however, about the IR absorption peak of ICG at 7.1 mum, which absorption coefficient amounts to similar to 13000 cm(-1). In this study, ICG was exposed to free electron lasers (FELs) with wavelength of 7.1 mum and usefulness of ICG as an IR-marker was discussed. ICG film sample was formed on IR-transparent BaF2 crystal substrate and exposed to FELs with the wavelength of 7.1 mum. After exposure, the sample was analyzed by FT-IR and film thickness measurements. As results, ICG ablated with the FEL of the power density of more than 5 W/cm(2) (=P-dth), and that the molecular structure of ICG was still stable for the power density of less than P-dth, 3 W/cm(2). Therefore, ICG can be considered as a novel infrared marker (IR marker) to the living tissue which absorbs FEL photon energy without changing the IR absorption peak.