Functional ultrasound · Grey Matter

Functional ultrasound images brain activity by measuring how much blood flows through the smallest vessels of each spot of tissue, and it offers fMRI-like maps at much finer resolution from a device the size of a hand, provided the skull is out of the way.

Functional ultrasound. Functional ultrasound images brain activity by measuring how much blood flows through the smallest vessels of each spot of tissue, and it offers fMRI-like maps at much finer resolution from a device the size of a hand, provided the skull is out of the way.

It works by sending plane ultrasound waves into the brain thousands of times a second and comparing the echoes. Tissue barely moves between frames while red blood cells do, so filtering out the still part leaves a Doppler image of blood volume in vessels down to tens of micrometres. Active neurons call for more blood within a second or two (the same neurovascular coupling that fMRI relies on), so the image becomes a map of activity. The first demonstration, in rats in 2011, reached about 100 µm at 15 MHz.

The skull is the obstacle. Bone absorbs and distorts ultrasound at these frequencies, so recordings need a thin skull (young animals), a removed piece of bone, or an acoustically transparent window; in 2024 an adult human was imaged at about 200 µm through a plastic window that replaced part of the skull after an injury.

It can decode intentions. In monkeys recorded through an opening in the skull above the posterior parietal cortex, single trials of fUS predicted where the animal was about to move its eyes or hand, which makes it a candidate for less invasive brain-computer interfaces.

It reads blood. Like fMRI it sees a slow consequence of activity, so it maps where and roughly when, never which neuron fired.

Functional ultrasound has the resolution and lacks the way in.

Its physics would read the whole depth of the brain at a tenth of a millimetre, and the skull is what keeps it in the operating room.

Questions: Can functional ultrasound decode what someone is about to do, and what would a wearable version mean for privacy? In monkeys recorded through an opening in the skull over the posterior parietal cortex, functional ultrasound predicted from single trials the direction and the effector (eye or hand) of a movement the animal was planning, at about 100 µm resolution. In people it has so far worked only where bone was removed or replaced, as through a transparent skull implant after an injury. A version that worked through intact bone would read intentions in depth without surgery, which is why it is watched as a likely future case for mental privacy rules written for neural data.