Effect of roughness in nondiffusive regions within diffusive media

Recently it has been shown that clear regions within diffusive media can be accurately modeled within the diffusion approximation by means of a novel boundary condition [J. Opt. Sec. Am. A 17, 1671 (2000)] or by an approximat ion to it [Phys. Med. Biol. 41, 767 (1996); Med. Phys. 27, 252 (2000)]. Thi s can be directly applied to the study of light propagation in brain tissue , in which there exist clear regions, and in particular in the cerebrospina l fluid (CSF) layer under the skull. In this work we present the effect tha t roughness in the boundary of nondiffusive regions has on the measured ave rage intensity, since, in practice, the CSF layer is quite rough. The same conclusions can be extended to any diffusive medium that encloses rough non diffusive regions. We will demonstrate with numerical calculations that the roughness statistics of the interfaces (although not their actual profiles ) must be known a priori to correctly predict the shape of the average inte nsity. We show that as the roughness increases, the effect of the nondiffus ive region diminishes until it disappears, thus yielding data similar to th ose of a fully diffusive region. We also present a numerical study of the d iffuse light scattered in the presence of both diffusive and nondiffusive r egions and the interaction between the two, showing that when the nondiffus ive region is rough, the spatial-intensity distribution produced by the two regions can he very similar. (C) 2001 Optical Society of America.