Pulsation-resolved deep tissue dynamics measured with diffusing-wave spectroscopy

We present a technique for measuring transient microscopic dynamics within deep tissue with sub-second temporal resolution, using diffusing-wave spectroscopy with gated single-photon avalanche photodiodes (APDs) combined with standard ungated multi-tau correlators. Using the temporal autocorrelation function of a reference signal allows to correct the temporal intensity autocorrelation function of the sample signal for the distortions induced by the non-constant average photon count rate. We apply this technique to pulsation-synchronized measurements of tissue dynamics in humans. Measurements on the forearm show no dependence on the pulsation phase. In contrast, the decay rate of the DWS signal measured on the wrist over the radial artery shows a pulsation-induced modulation of 60-90% consistent with pulsatile variations of arterial erythrocyte flow velocity. This might make time-resolved DWS interesting as a sensitive and fast method for investigating deep tissue perfusion, e.g. in intensive care.