Functional Ultrasound Imaging from Scratch

Functional ultrasound imaging (fUSI) is presented as a high-resolution, high-sensitivity brain imaging modality that tracks hemodynamic changes by imaging rapid fluctuations in ultrasound speckle caused by moving red blood cells. It achieves ~100 µm linear resolution at clinical frequencies, yields percent signal changes up to ~20% (far above typical fMRI BOLD), requires no magnet, and pairs naturally with low-intensity focused ultrasound neuromodulation. Skull penetration limits noninvasive use, but intraoperative applications and cases with acoustically transparent cranial implants demonstrate human feasibility. Extensions with contrast agents or genetic reporters promise super-resolution vasculature mapping or direct neural readouts.

The technical pipeline centers on ultrafast plane-wave transmissions that produce whole-image echoes at kHz rates, reconstructed with delay-and-sum beamforming or linear inverse methods (sensitivity matrix well conditioned). Functional contrast is typically extracted as Power Doppler: a moving-window standard deviation of complex images that primarily reflects the number of moving scatterers (blood volume) rather than velocity. Major practical problems are physiological and probe motion; common SVD “clutter” filters discard >99% variance and are blunt. Better practice is piecewise rigid motion correction (e.g., NoRMCorre) before milder filtering, and adopting autocorrelation-based analyses to recover velocity (flow) as an orthogonal signal. Python notebooks and tutorials accompany these recommendations to enable broader computational work on fUSI.