Phase shift of spatially modulated light in turbid media

Modulated Imaging is an experimental method that measures the amplitude and phase of diffusely reflected light from a sinusoidally incident illumination. When periodic light is incident on a highly diffusive media, it undergoes amplitude attenuation and phase shift which depend on the optical properties of the tissue and the angle of incidence. We have previously reported the capability of this method for quantifying the absorption and scattering from amplitude-only measurements. The measure of the phase shift, however, has additional potential for simultaneous determination of the absorption, scattering, and anisotropy optical properties, as well as for diffuse optical tomography. In this work we experimentally investigate the behavior of the phase shift for varying optical properties. Measures have been performed on liquid phantoms varying the scattering (from 0.02 to 9mm-1) and the absorption (from to 0.0004 to 0.1mm-1). The frequency of the spatially modulated light has been varied from 0 to 0.35mm-1 in all the experiments. The phase shift is measured using a simple phase-shifting method used in optical topography. At the selected optical properties, the phase shift increases non-linearly with spatial frequency. An increase in the phase shift as the scattering is increased and a decrease with increasing absorption is shown. The dependence of the phase shift on the incidence angle of light is also reported. The development of a model to simulate the experimental data is in progress. We will discuss forward modeling efforts for analytic prediction of the phase shift for comparison with the experimental data.