Monte Carlo diffusion hybrid model for photon migration in a two-layer turbid medium in the frequency domain

We propose a hybrid Monte Carlo (MC) diffusion model for calculating the sp atially resolved reflectance amplitude and phase delay resulting from an in tensity-modulated pencil beam vertically incident on a two-layer turbid med ium. The model combines the accuracy of MC at radial distances near the inc ident beam with the computational efficiency afforded by a diffusion calcul ation at further distances. This results in a single forward calculation se veral hundred times faster than pure MC, depending primarily on model param eters. Moder predictions are compared with MC data for two cases that span the extremes of physiologically relevant optical properties: skin overlying fat and skin overlying muscle, both in the presence of an exogenous absorb er. It is shown that goad agreement can be achieved for radial distances fr om 0.5 to 20 mm in both cases. However, in the skin-on-muscle case the choi ce of model parameters and the definition of the diffusion coefficient can lead to some interesting discrepancies. (C) 2000 Optical Society of America .