Progress in Optical Imaging of Brain Function

Measurement of multichannel continuous-wave near-infrared spectroscopy (CW-NIRS) is dependent on the modified Beer-Lambert law, which includes optical path length (PL) as an essential parameter. PLs are known to differ across different head regions and between individuals, but the distribution of PLs for the whole head has not been evaluated so far. Thus, using time-resolved near-infrared spectroscopy (TR-NIRS), we measured the optical characteristics, including PL, scattering coefficients (μls), and absorption coefficients (μa) at three wavelengths (760, 800, and 830 nm). We then constructed maps of these parameters on the surface of the subject′s head. While the PLs in nearby channels are similar, they differ depending on the regions of the head. The PLs in the region above the Sylvian fissure tended to be shorter than those in the other regions at all of the wavelengths. The difference in the distribution of PLs may be attributed to differences in tissue absorption and scattering properties. The current study suggests the importance of considering PL differences in interpreting functional data obtained by CW-NIRS.