ACCURACY OF THE DIFFUSION-APPROXIMATION IN DETERMINING THE OPTICAL-PROPERTIES OF A 2-LAYER TURBID MEDIUM

We have have examined the possibility of determining the optical prope rties of a two-layer medium by using a diffusion approximation radiati on transport model [Appl. Opt. 37, 779 (1998)]. Continuous-wave and fr equency-domain (FD) low-noise Monte Carlo (NIC) data were fitted to th e model. Marquardt-Levenberg and a simulated annealing algorithm were used and compared as optimization techniques. Our particular choice of optical properties for the two-layer model was consistent with skin a nd underlying fat in the presence of an exogenous chromophore [Appl. O pt. 37, 1958 (1998)]. The results are therefore specific to this set o f optical properties. It was found that the cw diffusion solution coul d never be used to estimate all optical properties reliably. The combi ned cw and FD solutions could not be used to estimate some of the top- layer optical properties to an accuracy of better than 10%, although t he absorption and the transport scattering coefficients of the bottom layer could be estimated to within 7% and 0.5%, respectively. No impro vement was found from simultaneously fitting MC data at three differen t modulation frequencies. These results point to the need for a more a ccurate radiation transfer model at small source-detector separations. (C) 1998 Optical Society of America OCIS codes: 170.3660, 170.4090, 2 90.1990, 290.7050.