Wave propagation in the human brain and skull imaged in vivo by MR elastography

Traumatic brain injuries (TBI) are common, and often lead to permanent physical, cognitive, and/or behavioral impairment. TBI arises in vehicle accidents, assaults, athletic competition, and in battle (due to both impact and blast). Despite the prevalence and severity of TBI, the condition remains poorly understood and difficult to diagnose. Computer simulations of injury mechanics offer enormous potential for the study of TBI; however, computer models require accurate descriptions of tissue constitutive behavior and brain-skull boundary conditions. Lacking such data, numerical predictions of brain deformation remain uncertain. Brain tissue is heterogeneous, anisotropic, nonlinear, and viscoelastic. The viscoelastic properties are particularly important for TBI, which usually involves rapid deformation due to impact. Magnetic resonance elastography (MRE) is a non-invasive imaging modality that provides quantitative spatial maps of biologic tissue stiffness in vivo. MRE is performed by inducing micron-amplitude propagating shear waves into tissue with a surface actuator at steady state while images of the wave motion are acquired using a standard clinical MRI scanner. A custom synchronized MRI pulse sequence, with "motion-sensitizing gradients", is used to encode wave displacements, and at various time points. Elastograms, or images with contrast corresponding to complex shear modulus (storage and loss modulus), can be computed from the raw spatial-temporal displacement data by inverting the governing equations of motion. Wave images and elastograms can provide fundamental insight into the dynamics of human brain and skull under rapidly time-varying loads. In this study, we aim to understand in vivo brain motion as the cranium is exposed to acoustic frequency pressure waves (45 Hz). This loading approximates some physical features of blast, albeit at very low levels.