Diffuse correlation spectroscopy (DCS)

Diffuse correlation spectroscopy measures cerebral blood flow rather than blood oxygenation. It shines coherent near-infrared light into tissue and records the rapid intensity fluctuations (speckle) of the diffusely scattered light; moving red blood cells decorrelate the speckle, and the decay rate of the intensity autocorrelation function yields a blood-flow index. It is thus complementary to fNIRS and DOT, which report haemoglobin concentration.

DCS is attractive because flow is a more direct correlate of metabolism than oxygenation and can be monitored continuously at the bedside. Its limitations mirror the optical family: sensitivity to superficial (scalp) flow requires multi-distance correction, signal-to-noise falls with source-detector separation limiting depth, and the temporal resolution and interpretation depend on tissue optical properties that are hard to measure independently.