Functional ultrasound (fUS)

Functional ultrasound imaging maps neural activity through its haemodynamic correlate, like fMRI, but using ultrafast plane-wave ultrasound and power-Doppler processing to detect changes in cerebral blood volume at high spatiotemporal resolution (roughly 100 micrometre, sub-second) and deep into the brain. Its sensitivity comes from acquiring thousands of images per second and using spatiotemporal filtering to separate slow-moving blood in small vessels from tissue motion.

fUS is minimally invasive rather than fully non-invasive: the adult skull strongly attenuates and aberrates ultrasound, so most functional work is done through a thinned skull, an acoustic window, or a cranial prosthesis, or in neonates through the fontanelle. It has produced real-time decoding of movement intention in animals and is entering human intraoperative and implant-window studies, but transcranial whole-brain fUS in intact adults remains an open challenge, and like all haemodynamic methods it is limited by neurovascular coupling.