Reevaluation of near-infrared light propagation in the adult human head: implications for functional near-infrared spectroscopy

Abstract

Using both experimental and theoretical methods, we examine the contributionof different parts of the head to near-IR (NIR) signal. Time-resolvedspectroscopy is employed to measure the mean optical path length(PL), and the absorption (mu(a)) and reduced scattering (mu(s)')coefficients in multiple positions of the human head. Monte Carlosimulations are performed on four-layered head models based on anindividual magnetic resonance imaging ({MRI}) scan to determine mu(a)and mu(s)' in each layer of the head by solving inverse problems,and to estimate the partial path length in the brain (p-PL) and thespatial sensitivity to regions in the brain at the source-detectorseparation of 30 mm. The PL is closely related to the thickness ofthe scalp, but not to that of other layers of the head. The p-PLis negatively related to the PL and its contribution ratio to thePL is 5 to 22% when the differential path length factor is 6. Mostof the signal attributed to the brain comes from the upper 1 to 2mm of the cortical surface. These results indicate that the {NIR}signal is very sensitive to hemodynamic changes associated with functionalbrain activation in the case that changes in the extracerebral tissueare ignorable.