Accuracy of the diffusion approximation in determining the optical properties of a two-layer turbid medium

The purpose of this study was to examine the possibility of determining the optical properties of a two-layer turbid medium using a diffusion approximation radiation transport model. Continuous wave (CW) and frequency domain (FD) low noise Monte Carlo (M.C.) data were fitted to the 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 and underlying fat in the presence of an exogenous chromophore (table l). It was found that the CW diffusion solution could never estimate all optical properties reliably. The combined CW and FD solutions could not estimate some of the top layer optical properties to better than 10%, although the absorption and transport scattering coefficients of the bottom layer could be estimated to within 0.5 and 7% respectively. No improvement was found from simultaneously fitting M.C. data at three different modulation frequencies. These results point to the need for a more accurate radiation transfer model at small source-detector separations.