  Using high frame-rate ultrasound and high sensitivity motion tracking, we
recently showed that shear waves sent to the ex vivo porcine brain develop into
shear shock waves with destructive local accelerations inside the brain which
may be a key mechanism behind deep traumatic brain injuries. Direct measurement
of brain motion at an adequate frame-rate during impacts has been a persistent
challenge. Here we present the ultrasound observation of shear shock waves in
the acoustically challenging environment of the in situ porcine brain during a
single-shot impact. The brain was attached to a plate source which was vibrated
at a moderate amplitude of 25g, to propagate a 40 Hz shear wave into the brain.
Simultaneously, images of the moving brain were acquired at 2193 images/s,
using a custom imaging sequence with 8 interleaved ultrasound transmit-receive
events, designed to accurately track shear shocks. To achieve a long
field-of-view, wide-beam emissions were designed using time-reversal ultrasound
simulations and no compounding was used to avoid motion blurring. A peak
acceleration of 102g was measured at the shock-front, 7.1 mm deep inside the
brain. It is also shown that experimental shear velocity, acceleration, and
strain-rate waveforms in brain are in excellent agreement with theoretical
predictions from a custom higher-order finite volume method hence demonstrating
the capabilities to measure rapid brain motion even in the presence of strong
acoustical reverberations from the porcine skull.
