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 $\Delta P/P_0\approx2\text{–}5\%$.