Functional ultrasound (fUS)

Optical & ultrasound (in vivo) · reads

Functional ultrasound is an imaging method that sends ultrasound plane waves into the brain 1,000 times a second; the Power Doppler signal this yields is proportional to the volume of moving blood in small vessels. What it reads is cerebral blood volume, and because blood volume follows neural activity through neurovascular coupling, the activity is inferred indirectly. In the intraoperative setup only vessels with axial flow above 4 mm/s contributed. In humans it reaches the brain during brain surgery, at 250 µm, and in newborns through the anterior fontanel.

Spatial resolution: 100–250 µm
Temporal resolution: 300 ms (sample interval)
Field of view: 55.6 cm³ (acquired volume)
Reads: power-Doppler signal intensity; cerebral blood volume
Destructive?: non-destructive
Prerequisites: other: Neonatal fontanelle: place and stabilize a miniaturized probe in a headset over the anterior fontanelle; no invasive preparation is described.; surgery: Intraoperative route: open the skull and dura before fUS imaging.; surgery: Prosthesis/window route: surgically place a sonolucent cranial prosthesis or window for subsequent imaging.
Theoretical spatial resolution: 100 µm
Theoretical field of view: 55.6–228 cm³ (reachable envelope)

Description

Functional ultrasound (fUS) is an imaging method that maps brain activity through its blood supply with ultrafast ultrasound. In the intraoperative setup plane waves are transmitted at a frame rate of 1 kHz; the Power Doppler signal is proportional to the fractional volume of moving blood in small vessels; in the intraoperative setup, only vessels with axial flow above 4 mm/s contributed. The image tracks cerebral blood volume (CBV) over time. CBV changes follow neuronal activity through neurovascular coupling, so the activity is inferred indirectly. Functional connectivity is derived from temporal correlations between CBV changes in distant regions. The maps reach deep within the cortex, including the sulci. In adults, fUS reaches the brain during brain surgery, imaging transient CBV changes at 250 µm; each Doppler acquisition spans one heart cycle (1 s) at the 1 kHz frame rate and is repeated every 3 s. In newborns, a miniaturized linear probe anchored in a 3D-printed headset images through the anterior fontanel, where the imaging depth reached 49 mm after optimization, covering more than 40% of the neonatal brain volume. In rodents, rats are imaged at 100 µm × 100 µm in-plane resolution with 200 µm slices, and mice through a cranial window in which an ultrasound-permeable polymer film replaces a dorsal section of skull (about 8 mm × 5 mm), with a 15 MHz linear array producing a power Doppler image every 300 ms.

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