Transcranial Functional Ultrasound Imaging

Transcranial functional ultrasound (fUS) imaging leverages ultrafast plane‐wave emissions and Power Doppler processing to map cerebral blood volume (CBV) changes linked to neuronal activity. By emitting plane waves at pulse repetition frequencies of several kilohertz and coherently compounding backscattered echoes, fUS can achieve high spatial resolution and temporal resolution.

The strongest intact-skull demonstrations to date are preclinical, including non-invasive rat imaging. In humans, current published functional studies use acoustically transparent cranial windows or sonolucent skull implants rather than the intact adult skull. After beamforming, the sequence of raw echoes is Doppler-filtered to isolate moving scatterers (red blood cells) and extract microvascular flow dynamics.

Key equations:

$$f_D = \frac{2\,v\,f_0\,\cos\theta}{c}$$

where $f_0\approx 2\,\text{MHz}$, $c\approx1540\,\text{m/s}$, and flow velocities $v\sim1\,\text{mm/s}$ yield Doppler shifts $f_D\approx2.6\,\text{Hz}$.
The Power Doppler signal at pixel $(x,y)$ is computed as

$$P_{\mathrm{PD}}(x,y) = \sum_{k=1}^{N} \lvert s_k(x,y)\rvert^2$$

with $s_k$ the beamformed echoes over $N$ frames, yielding hemodynamic fluctuations of $\Delta P/P_0\approx2\text{–}5\%$.

Functional resolution & depth (note): Caveats: published human results to date use an acoustically-transparent cranial window/implant (Rabut 2024; Soloukey 2025) or are in rodents (Jones 2024) — fully non-invasive imaging through the intact adult skull is hard because thick frontal/parietal bone is nearly opaque at ≥2 MHz (scattering destroys phase coherence); only the thin temporal window (~2 cm aperture) transmits well. Through that window lateral resolution is aperture-diffraction-limited (~λz/D) and degrades with depth (~0.4 mm near the window → several mm deep). The kHz frame rate buys Doppler sensitivity, but the neural-information temporal resolution is hemodynamic (~1 s), and functional resolution is floored at ~0.3–0.5 mm by neurovascular coupling.