Shear shock waves observed in the ex-vivo brain

For the past 50 years head injury biomechanics has been guided by measurements of accelerometers attached to the skull. These measurements provide a partial and indirect estimate of brain motion because the internal deformation of the brain is far more complex than the rigid motion of the skull. In this context, brain tissue behaves nonlinearly under conditions that are generally met by injurious impacts. Attempts to measure the in situ nonlinear brain mechanics with imaging methods (MRI, CT) have lacked the penetration, frame rate, or motion detection accuracy to capture the nonlinear transient events during traumatic injury. Here we present a high framerate (6200 images/second) ultrasound imaging method that can accurately measure the internal brain motion during the rapid transient events associated with a mild impact in an ex-vivo porcine brain. Our method relies on two main advancements 1) A flash focus ultrasound sequence which reduces the side lobes by 19dB and increases the SNR deep in the brain compared with a conventional plane wave compounding sequence. 2) An adaptive tracking algorithm which uses a quality weighted median filter to iteratively optimize correlation estimates. By imaging brain motion directly we were able to observe the formation of shear shock waves within the brain. The measured shock waves have a specific odd harmonic signature predicted by theory describing a cubically nonlinear elastic soft solid. Measurements of the frequency dependent attenuation and dispersion were used to fit this nonlinear theoretical model to our data. This yielded the first estimates of the cubic nonlinear parameter for brain tissue. This previously unobserved shear shock wave phenomenology dramatically amplifies the acceleration at the shock front, deep in the brain, compared with the acceleration imposed at the brain surface (up to a factor 8.5). A 30 g acceleration at the brain surface therefore develops into a 255 g shock wave deep inside the brain. The highly localized increase in acceleration suggests that the shear shock wave is a primary mechanism for traumatic injuries.