Accurate estimation of the radiation distribution in the adult human head requires realistic head models generated from magnetic resonance imaging (MRI) scans with true optical properties of each layer of the head. In this study, a complex three-dimensional structural data obtained by MRI are introduced in a three-dimensional Monte Carlo code, with varying optical properties and arbitrary boundary condition, to calculate the spatial sensitivity profile of photon in head, so-called banana-shaped. It is therefore a better model to incorporate the contribution of cerebrospinal fluid (CSF) when modeling the head. The spatial sensitivity of near-infrared spectroscopy measurement to regions in the brain, as well as the effect of optical fiber arrangement on the regions of measurement are investigated. It is shown that the detected signal in brain imaging measurements is greatly affected by the heterogeneity of the head tissue and its scattering properties.