Mapping of Optical Pathlength of Human Adult Head at Multi-Wavelengths in Near Infrared Spectroscopy Akihisa Katagiri, Ippeita Dan, Daisuke Tuzuki, Masako Okamoto, Noriaki Yokose, Kouji Igarashi, Tatsuya Hoshino, Tokuo Fujiwara, Youichi Katayama, Yui Yamaguchi, and Kaoru Sakatani Abstract Measurement of multichannel continuous-wave near-infrared spec- troscopy (CW-NIRS) is dependent on the modified Beer-Lambert law, which includes optical pathlength (PL) as an essential parameter. PLs are known to differ across different head regions and different individuals, but the distribu- tion 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 char- acteristics including PL, scattering coefficients (m 0 s ), and absorption coefficients (m a ) at three wavelengths (760, 800, 830 nm). Then, we constructed maps of these parameters on the subjects’ head surface. While the PLs in nearby chan- nels 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 inter- preting functional data obtained by CW-NIRS. 1 Introduction Multichannel continuous-wave near-infrared spectroscopy (CW-NIRS), also known as functional NIRS (fNIRS), or Optical topography (OT), is an emer- ging neuroimaging technique that allows the noninvasive monitoring of hemo- dynamic changes associated with human brain activity [1]. CW-NIRS has several advantages with respect to other neuroimaging methods: it is more tolerant of physical movements, allows more freedom in choice of experimental settings and is less expensive. Thus, CW-NIRS has been applied in various K. Sakatani (*) Division of Optical Brain Engineering, Department of Neurological Surgery; Division of Applied System Neuroscience, Department of Advanced Medical Science, Nihon University School of Medicine, Tokyo, 173-8610, Japan e-mail: sakatani@med.nihon-u.ac.jp E. Takahashi, D.F. Bruley (eds.), Oxygen Transport to Tissue XXXI, Advances in Experimental Medicine and Biology 662, DOI 10.1007/978-1-4419-1241-1_29, Ó Springer ScienceþBusiness Media, LLC 2010 205