Optical windows for head tissues in near-infrared and short-wave infrared regions: Approaching transcranial light applications

Optical properties of the rat head tissues (brain cortex, cranial bone and scalp skin) are assessed, aiming at transcranial light applications such as optical imaging and phototherapy. The spectral measurements are carried out over the wide spectral range of 350 to 2800 nm, involving visible, nearinfrared (NIR) and short-wave infrared (SWIR) regions. Four tissue transparency windows are considered: similar to 700 to 1000 nm (NIR-I), similar to 1000 to 1350 nm (NIRII), similar to 1550 to 1870 nm (NIR-III or SWIR) and similar to 2100 to 2300 nm (SWIR-II). The values of attenuation coefficient and total attenuation length are determined for all windows and tissue types. The spectra indicate transmittance peaks in NIR, NIR-II and SWIR-II, with maximum tissue permeability for SWIR light. The use of SWIR-II window for the transcranial light applications is substantiated. Furthermore, absorbance of the head tissues is investigated in details, by defining and describing the characteristic absorption peaks in NIR-SWIR.