Brain Multiphysics (Jan 2021)
Super-resolved shear shock focusing in the human head
Abstract
Shear shocks, which exist in a completely different regime from compressional shocks, were recently observed in the brain. These low phase speed (≈ 2 m/s) high Mach number (≈ 1) waves could be the primary mechanism behind diffuse axonal injury due to a very high local acceleration at the shock front. The extreme nonlinearity of these waves results in unique behaviors that are different from more commonly studied nonlinear compressional waves. Here we show the first observation of super-resolved shear shock wave focusing. Shear shock wave imaging and numerical simulations in a human head phantom over a range of frequencies/amplitudes shows the super-resolution of shock waves in the low strain and high strain-rate regime. These results suggest that even for mild accelerations injuries as small as a grain of rice on the scale of mm2 can be easily created deep inside the brain. Statement of Significance: The relationship between brain motion and traumatic brain injury remains poorly understood despite many decades of investigation. We have developed high frame-rate ultrasound imaging techniques combined with motion tracking sequences that can capture a previously unobtainable high strain and high strain-rate regime. This quantitative imaging method has led to the discovery that shear waves can develop into shear shocks. To the best of our knowledge, we are the only group in the world that has observed these shear shocks in soft tissue. In this manuscript we demonstrate that shear waves are focused into destructive shocks deep inside the human head where rate-dependent metrics, such as acceleration and strain-rate, are amplified by an order of magnitude. Furthermore, it is shown that the destructive power of these shear shocks is superresolved into tiny areas about the size of a grain of rice. To achieve these results, we have made technical innovations in the field of ultrasound by designing shock-capturing imaging sequences, and simulations tools that can model shear shocks. There is an overwhelming amount of evidence that shear shock wave physics is a necessary and primary component of brain biomechanics and, we hypothesize, brain injury. Local measurements and simulations of this shock wave behavior, which are absent from current biomechanical models of the brain, may fundamentally change the way we approach the design of protective equipment in transportation, sports, playground safety, falls and our understanding of the extreme biomechanical environment to which our brains can be subjected.
