Fig. 1.

Conceptual illustration of time-resolved diffuse correlation spectroscopy. TD-DCS uses trains of long-coherence-length laser pulses to measure both TOFs and path length-dependent autocorrelation functions at the detector for improved determination of blood flow. The figure illustrates two different photon paths (purple and green) in highly scattering media for the light pulse at time t (dashed line) and at time t + τ (solid line). On average, light traveling in more superficial layers (purple paths) has a shorter TOF than light traversing deeper (green paths) into the tissue (time differences on the 10–100 ps time scale). The difference in the paths at t and t + τ is due to the motion of the red blood cells, which causes flow-dependent fluctuations in the detected intensity on the 10–1000 ns timescale. Time-tagging photons’ TOF (ps) and absolute arrival (ns) allows multiple analyses to be performed from the same data stream. In particular, photons can be separated by TOF into those traveling shorter (purple) or longer (green) paths through the tissue, enabling the calculation of the DCS correlation function from photons traveling to different depths in the tissue.