Toshiyuki HAYASHI, Yoshihiko KASHIO and Eiji OKADA

Department of Electronics and Electrical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

(Received February 17, 2003; Accepted May 16, 2003)

The light propagation in the brain is highly affected by the superficial tissues such as skin, skull and cerebrospinal fluid (CSF). The development of an adequate model to calculate light propagation in the head is very important for optical brain activity measurement. We developed a novel approach, the direct hybrid Monte Carlo-diffusion method (DHMDM), which can calculate light propagation in the three-dimensional head models including low scattering regions in which the light propagation does not obey either the diffusion approximation or the radiosity theory. The effect of thickness of the superficial tissue upon the spatial sensitivity profiles for optical brain activity measurement is evaluated by the DHMDM. The sensitivity to the brain activity decreases with increasing thickness of the superficial layer whilst the spatial decay of sensitivity on the brain surface depends little upon the thickness of the superficial layer.

Key words: biomedical optics, light propagation, diffusion equation, finite element method, brain activity