Functional ultrasound imaging (fUS)

Functional ultrasound imaging maps changes in cerebral blood volume as a proxy for neural activity, using ultrafast plane-wave (or diverging-wave) insonification at frame rates of several kilohertz followed by power-Doppler processing to detect the faint blood-flow signal in small vessels. Unlike conventional Doppler it images the whole field of view at once, giving it the sensitivity to resolve activity-driven microvascular changes at roughly 100-micrometre spatial and sub-second temporal scales, deep into the brain and without contrast agents.

fUS occupies a niche between fMRI and electrophysiology: better spatial resolution and portability than blood-oxygenation (BOLD) fMRI, deeper reach than optical imaging, but it measures a hemodynamic (neurovascular) signal, not spikes, so it is slow and indirect. It has been demonstrated in rodents, ferrets, and awake behaving non-human primates, transfontanelle in human neonates, and intraoperatively in adults; fully non-invasive transcranial adult imaging remains limited by skull aberration and attenuation.