High frame-rate imaging and adaptive tracking of shear shock wave formation in the brain: A Fullwave and experimental study

Linearly polarized nonlinear shear waves have a cubic nonlinearity which generates shear shock waves. This behavior has been only observed in a homogeneous gelatin phantom with ultrafast plane wave ultrasound imaging and a correlation-based tracking algorithm to determine particle motion. However, in heterogeneous soft tissue, such as brain, clutter degrades motion tracking. We propose a high frame-rate ultrasound imaging sequence consisting of multiple focused emissions that reduce the side lobes and clutter artifacts to accurately track shear shock wave motion in the soft tissue of the human body. This imaging technique is applied to an ex-vivo porcine brain to demonstrate applications to traumatic brain injury. A previously developed adaptive tracking algorithm is optimized to determine and characterize the specific odd harmonic signature that is characteristic of shear shock waves. This optimization and validation was determined with a numerical description of shear shock waves using a Rusanov-Fourier scheme, that supports nonlinearity and an arbitrary frequency dependent attenuation law. Fullwave simulations were then used to generate ultrasound images of shear shock waves propagating through a medium displaced according to theoretical predictions. The performance of the adaptive tracking algorithm is compared to the Cramer Rao Lower Bound (CRLB) in terms of bias, and jitter. Experimentally, we show how the significant improvements in tracking performance from the proposed imaging sequence and tracking algorithm were necessary for this first experimental observation of shear shock waves in the brain.