Brain response to extracranial pressure excitation imaged in vivo by MR elastography

Traumatic brain injuries (TBI) are common, and often lead to permanent cognitive impairment. Despite the prevalence and severity of TBI, the condition remains poorly understood. 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. Magnetic resonance elastography (MRE) is a non-invasive imaging modality that provides quantitative spatial maps of tissue stiffness in vivo. MRE is performed by inducing micron-amplitude propagating shear waves into tissue and imaging the resulting motion with a specialized "motion- sensitive" MRI pulse sequence. Invoking a restricted form of Navier's equation these data can be inverted to estimate material stiffness. As such, clinical interest in MRE has largely been driven by the direct empirical relationship between tissue stiffness and health. However, the so-called "raw" MRE data themselves (3-D displacement measurements) and calculated strains can elucidate loading paths, anatomic boundaries and the dynamic response of the intact human head. In this study, we use the MRE imaging technique to measure in vivo displacement fields of brain motion as the cranium is exposed to acoustic frequency pressure excitation (45, 60, 80 Hz) and calculate the resulting shear-strain fields (2-D).