Ultrasound Brain Elastography

Ultrasound Brain Elastography applies shear waves to the cranium—either via time-harmonic vibration (20–60 Hz) or acoustic radiation force bursts (~160–200 Hz)—and tracks the resulting tissue displacements with high-frame-rate ultrasound imaging. The measured wave propagation speed is directly inverted to yield the local shear modulus of brain parenchyma, exploiting the relation $c_s=\sqrt{\mu/\rho}$. Typical shear wave speeds in healthy adult brain are 1–2 m/s, corresponding to shear moduli of 1–4 kPa (ρ≈1000 kg/m³).

In a time-harmonic protocol, a multifrequency drive (27–56 Hz) generates continuous sinusoidal shear waves. The displacement field $u(x,t)=A\,e^{i(\omega t - kx)}$ is measured, and the complex wave number $k=\omega/c_s + i\alpha$ encodes both stiffness and attenuation. The inversion uses

$$\mu = \rho \biggl(\frac{\omega}{k}\biggr)^2,\quad \alpha\sim0.5\text{–}1.5\ \mathrm{dB/cm\ at\ 50\,Hz}.$$

Wavelengths $\lambda=c_s/f$ range 20–60 mm, setting a spatial resolution floor of ~2–3 mm after smoothing.

Current implementations achieve 80 fps (12.5 ms) for 6-tone THE and up to 200 fps (5 ms) for single-frequency SWE, with fields-of-view of 8–12 cm through the temporal bone. Skull attenuation (>25 dB at depth >10 cm) limits penetration and degrades SNR, leading groups to report reproducibility metrics (ICC < 0.92, CV ≤ 11 %) rather than direct CNR values.