We introduce, validate and demonstrate a new software correlator for high-speed measurement of blood flow in deep tissues based on diffuse correlation spectroscopy (DCS). The software correlator scheme employs standard PC-based data acquisition boards to measure temporal intensity autocorrelation functions continuously at 50 - 100 Hz, the fastest blood flow measurements reported with DCS to date. The data streams, obtained in vivo for typical source-detector separations of 2.5 cm, easily resolve pulsatile heart-beat fluctuations in blood flow which were previously considered to be noise. We employ the device to separate tissue blood flow from tissue absorption/scattering dynamics and thereby show that the origin of the pulsatile DCS signal is primarily flow, and we monitor cerebral autoregulation dynamics in healthy volunteers more accurately than with traditional instrumentation as a result of increased data acquisition rates. Finally, we characterize measurement signal-to-noise ratio and identify count rate and averaging parameters needed for optimal performance.
Keywords: (030.0030) Coherence and statistical optics; (030.5260) Photon counting; (030.5290) Photon statistics; (110.4153) Motion estimation and optical flow; (170.1470) Blood or tissue constituent monitoring; (170.1610) Clinical applications; (170.2655) Functional monitoring and imaging; (170.3880) Medical and biological imaging; (170.5270) Photon density waves; (170.6480) Spectroscopy, speckle; (170.6510) Spectroscopy, tissue diagnostics; (290.4210) Multiple scattering.