Modelling of light propagation in the adult head for optical brain function mesurement

Near infrared (NIR) spectroscopy and imaging are now established techniques for measurements of change in oxygenation and haemo-dynamics within the brain. However, scattering of the light in biological tissue causes ambiguity in the spatial sensitivity profiles and the partial optical path lenght in the brain. The light propagation in the adult head model of which optical properties are chosen from the reported data of in vitro experiments is validated by time-resolved measurements of the human foreheads. The apparent optical properties estimated by a least squares fit of the diffusion model to the temporal point spread function for the adult head model agree with the experimental results. The heterogeneity of the head scarcely affects the total optical path lenght. However the partial optical path lenght in the brain and the spatial sensitivity profiles which can not be directly obtained by experiment are considerably affected by the heterogeneity of tissue, especially the presence of low scattering subarachnoid space surrounding the brain.