Diffusing-wave spectroscopy of flows

The technique of diffusing-wave spectroscopy (DWS) consists in deriving properties of random multiple-scattering media from measurements of the temporal autocorrelation function of scattered intensity. Both the characteristics of light- scattering particles (size, absorption and scattering coefficients, etc.) and the macroscopic dynamic structure of the sample may be studied by means of DWS. The technique has a considerable potential for application in medical diagnostics. We report theoretical and experimental results that allow the method of DWS to be extended to random media with hidden, spatially localized flows of scatterers. Our model experiments were performed with a flow of an aqueous suspension of polystyrene beads confined within a rigid sample made of titanium dioxide particles suspended in resin. We show that the method of DWS allows the noninvasive diagnostics of the flow. As a possible application of the method, we discuss the use of DWS for noninvasive in vivo optical monitoring of blood flow in humans and animals.