Brain Response to Extracranial Acoustic Loads: Shear Wave Propagation Characterized by Vector Fields

Traumatic brain injuries (TBI) due to blast are common in modern combat situations, and often lead to permanent cognitive impairment. Despite the prevalence and severity of blast-induced TBI, the condition remains poorly understood. Computer simulations of blast and blast injury mechanics offer enormous potential; however, computer models require accurate descriptions of tissue mechanics and boundary conditions in vivo. To gain insight into the mechanisms of blast injury, we applied direct (light) oscillatory pressure loading to the skulls of human volunteers, and measured displacement and strain fields using the methodology of magnetic resonance elastography (MRE). MRE is a non-invasive imaging modality that provides quantitative spatial maps of tissue stiffness. MRE is performed by inducing micron-amplitude propagating shear waves into tissue and imaging the resulting harmonic motion with standard clinical MRI hardware. Shear waves are initiated by an MR-compatible actuator and detected by a specialized “motion-sensitive” MRI pulse sequence (software). Motion sensitized MR images provide displacement field data which can be inverted to estimate material stiffness by invoking a restricted form of Navier’s equation. Clinical interest in MRE has largely been driven by the empirical relationship between tissue stiffness and health. However, the “raw” MRE data (3-D displacement measurements) themselves 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 we calculate the resulting shear-strain fields (2-D). We estimate the Poynting vector (energy flux) field to illuminate the directions of internal wave propagation, and to identify the energy absorbing and reflecting regions within the brain.