[plastic_enjoyer]

>The fUSI technique estimates changes in neural activity by measuring changes in blood flow. Neurons, like all cells, need blood to function. Increases in neural activity require increases in blood flow, which fUSI techniques measure by projecting ultrasound at a brain region of interest and recording the waves that bounce back. When sound bounces off of a mass of flowing blood, the returning sound waves wiggle at a different frequency than those emitted. fUSI uses this Doppler shift phenomenon to estimate changes in blood flow and, by proxy, the electrochemical chatter of neurons.

So, it seems this works similar to fNIRS which measure changes in the blood oxygen level via near-infrared spectroscopy. The downside of this is, that changes of the blood oxygen level happen significantly slower than the generation of action potentials thus making this technology not really feasible for real-time applications.