Abstract Text: Introduction: Despite the prevalence and severity of traumatic brain injury (TBI), the brain’s response to trauma and the mechanisms involved with injury remain unclear. Quantifying the relative motion between the brain and skull at this interface can help researchers better understand how skull acceleration contributes to brain injury. Data on skull-brain coupling can be used to improve computational TBI models that will elucidate the injury mechanisms and aid in the design of protective equipment [1]. Magnetic resonance elastography (MRE) can be used to measure the relative motion of the skull and brain in vivo [2,3]. Previous work introduced a method to separately measure the relative motion of the skull and brain by isolating signal from fat in the bone marrow [4]. In this study, we expand on that approach using a novel chemically encoded spiral in/out MRE sequence to simultaneously measure both skull and brain motion to ultimately quantify the transmission of motion from the skull to the brain.
Methods: MRE displacement data of the brain and skull was acquired using a 2D multishot spiral in/out MRE sequence [5]. A chemical shift encoding algorithm was incorporated into this protocol using multiple echo times to account for the phase difference between water and fat. All scanning was conducted using a Siemens 3T Prisma MRI scanner and a 20-channel head coil. Vibrations were generated at 50 Hz using a Resoundant pneumatic actuation system (Rochester, MN) and a passive pillow driver. Low motion encoding gradient strength (5 mT/m) was used to capture rigid body motion of both the skull and brain while avoiding temporal phase wrapping [2,4]. A single human subject (male, age 25) was scanned using this fat-water MRE sequence. Imaging data was processed through a joint fat-water reconstruction and resulting phase images were processed to obtain maps of complex motion with sensitization in the x (left-right), y (anterior-posterior), and z (superior- inferior) directions. A high-resolution dixon scan was collected to define regions of interest for skull and brain from the fat and water data, respectively, to determine displacement amplitudes. Points around the circumference of the skull were defined in terms of polar coordinates and described by their polar angle, θ. Displacement amplitude and relative phase of the harmonic motion was quantified for each point in both skull and brain.
RESULTS AND Discussion: Our in vivo analysis of brain and skull motion showed an overall lower displacement amplitude of the brain relative to the skull, indicating reduced transmission of motion to the brain in all three directions. The displacement fields show that the brain and skull predominately move in phase with each other with the scalp moving independently. While the highest displacement was observed in the anterior-posterior direction, the superior-inferior direction was characterized with a notable phase shift between the front and back of the head.
Conclusions: These preliminary results support the hypothesis that the brain moves out of phase with the skull as captured by our modified fat-water spiral MRE sequence. Further sequence developments will seek to optimize the skull signal and reduce artifacts caused by the scalp [3]. An increased range of actuation frequencies with multiple excitation directions will allow us to comprehensively quantify skull-brain transmission. Understanding this relationship will allow researchers to improve TBI models by accounting for the skull-brain interface and evaluate how this relative motion changes in patients with a history of mild TBI. Output from this work could be used to improve the development of protective equipment and safety standards.
ACKNOWLEDGEMENTS: This project was supported in part by NIH grant U01-NS112120, the Department of Defense in the Center for Neuroscience and Regenerative Medicine, the NIH Bench-to-Bedside Award, and Office of Naval Research Grant N00014-22-1-2198.
REFERENCES [1] Bayly, PV et al., Ann. Biomed. Eng., 2021. [2] Badachhape, AA et al., J. Biomech. Eng., 2017. [3] Badachhape, AA et al., J. Biomech., 2018. [4] Yin, Z et al., Magn. Reson. Med., 2018. [5] Johnson, CL et al., Magn. Reson. Med., 2013.