The effect of inhomogeneities within a highly scattering medium on the reflectance and on the transmittance was studied both with numerical simulations and with measurements on phantoms. A slab of diffusing medium with optical properties similar to those of a compressed breast was condidered. With an elementary Monto Carlo code the temporal point spread functions corresponding to photons emerging at different distances from the thin collimated light source were evaluated. For some detector positions, the coordinates of all the scattering points for each received photon were stored. Numerical simulations were carried out first for a homogeneous medium, requiring a few days for calculations. Using the stored trajectories it was possible to obtain in a short time, the corresponding temporal point spread functions when an absorbing inhomogeneity was considered within the slab. It was possible to obtain the temporal point spread function for many different positions, sizes, and shapes of the inhomogeneity inside the diffusing medium. The accuracy of the numerical results was checked by a comparison to experimental results obtained from measurements on suspensions of calibrated spheres having known optical properties. Measurements were carried out both with a picosecond system and with a continuous wave system.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.