The internal deformation of the brain is far more complex than the rigid motion of the skull. An ultrasound imaging technique that we have developed has a combination of penetration, frame-rate, and motion detection accuracy required to directly observe, for the first time, the formation and evolution of shear shock waves in the brain. Experiments at low impacts on the traumatic brain injury scale demonstrate that they are spontaneously generated and propagate within the porcine brain. Compared to the initially smooth impact, the acceleration at the shock front is amplified up to a factor of 8.5. This highly localized increase in acceleration suggests that shear shock waves are a fundamental mechanism for traumatic injuries in soft tissue.PACS numbers: Valid PACS appear here Traumatic brain injuries (TBI's) are a major source of death and disability worldwide. Falls and motor vehicle related accidents are the largest contributors. Current biomechanical predictive criteria for TBI are based on measurements of skull motion such as linear and rotational acceleration [1,2]. Although the relationship between skull motion and injury has been extensively tested [2][3][4][5][6][7], mechanisms relating the two have not been conclusively established [8], due to the complexity of the deformation of the brain [9][10][11] which behaves as a nonlinear viscoelastic medium. In situ measurements of the rapid transient motion of the whole brain during a traumatic event may establish a more accurate biomechanical description of injury.Shear vibration experiments on small brain samples have shown that the stress-strain relationship behaves nonlinearly for amplitudes as low as 1% [12,13], which is well below the strain threshold for injury [7,14,15]. Therefore, even mild injuries typically occur within the nonlinear elastic regime. Furthermore, the brain's shear wave speed (c t ∼2 m/s) is three orders of magnitude smaller than the compressional wave speed (c p ∼1500 m/s) therefore the deformation from an impact is almost entirely in shear mode.Outside of ultrasound, current methods that measure brain motion cannot capture this nonlinear behavior of shear waves due to limitations in either, frame rate, penetration, or motion detection accuracy. For instance, magnetic resonance elastography (MRE) has been used to investigate shear wave propagation in the brain, typically at frame rates on the order of tens of images/second. Furthermore the temporal sampling of MRE is fundamentally limited to a few ms by the finite integration time over the spin relaxation [16,17]. The harmonics generated by nonlinear shear wave propagation exceed these sampling capabilities. Optical methods have larger frame rates and they can measure motion in optically trans- * david.espindola@unc.edu † gfp@unc.edu parent materials [18][19][20]. However, they are limited to shallow penetration depths in soft tissue (∼ 2 mm) [21]. The only experimental corroboration of nonlinear shear waves and its characteristic harmonic signature was made over a decade ago in a homo...