Diffusing-wave spectroscopy from head-like tissue phantoms: influence of a non-scattering layer.

We investigate the influence of a non-scattering layer on the temporal field autocorrelation function of multiple scattered light from a multilayer turbid medium such as the human head. Data from Monte Carlo simulations show very good agreement with the predictions of the correlation-diffusion equation with boundary conditions taking into account non-diffusive light transport within the non-scattering layer. Field autocorrelation functions measured at the surface of a multilayer phantom including a non-scattering layer agree well with theory and simulations when the source-receiver distance is significantly larger than the depth and the thickness of the non-scattering layer. Our results show that for source-receiver distances large enough to probe the dynamics in the human cortex, the cortical diffusion coefficient obtained by analyzing field autocorrelation functions neglecting the presence of the non-scattering cerebrospinal fluid layer is underestimated by about~$40\,\%$ in situations representative of the human head.