Andrew A. Badachhape 1 Biomedical Engineering, Washington University in St. Louis, St. Louis, MO 63105 e-mail: abadachhape@wustl.edu Ruth J. Okamoto Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, MO 63105 Ramona S. Durham Biomedical Engineering, Washington University in St. Louis, St. Louis, MO 63105 Brent D. Efron Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, MO 63105 Sam J. Nadell Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, MO 63105 Curtis L. Johnson Biomedical Engineering, University of Delaware, Newark, DE 19716 Philip V. Bayly Biomedical Engineering, Washington University in St. Louis, St. Louis, MO 63105; Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, MO 63105 The Relationship of Three- Dimensional Human Skull Motion to Brain Tissue Deformation in Magnetic Resonance Elastography Studies In traumatic brain injury (TBI), membranes such as the dura mater, arachnoid mater, and pia mater play a vital role in transmitting motion from the skull to brain tissue. Mag- netic resonance elastography (MRE) is an imaging technique developed for noninvasive estimation of soft tissue material parameters. In MRE, dynamic deformation of brain tis- sue is induced by skull vibrations during magnetic resonance imaging (MRI); however, skull motion and its mode of transmission to the brain remain largely uncharacterized. In this study, displacements of points in the skull, reconstructed using data from an array of MRI-safe accelerometers, were compared to displacements of neighboring material points in brain tissue, estimated from MRE measurements. Comparison of the relative amplitudes, directions, and temporal phases of harmonic motion in the skulls and brains of six human subjects shows that the skull–brain interface significantly attenuates and delays transmission of motion from skull to brain. In contrast, in a cylindrical gelatin “phantom,” displacements of the rigid case (reconstructed from accelerometer data) were transmitted to the gelatin inside (estimated from MRE data) with little attenuation or phase lag. This quantitative characterization of the skull–brain interface will be valua- ble in the parameterization and validation of computer models of TBI. [DOI: 10.1115/ 1.4036146] Keywords: magnetic resonance elastography, mechanical characterization, brain defor- mation, tissue 1 Introduction Traumatic brain injury (TBI) is a prevalent neurological disor- der where brain tissue is damaged by an external force [1]. Diffuse axonal injury, or shearing and stretching of axonal fibers, is a common type of TBI often caused by sudden linear or angular accelerations of the head [2]. While the effects of TBI are well documented, the mechanisms linking mechanical insult and neu- rological injury are not well understood. Mathematical modeling and computer simulation of TBI can provide insight into these mechanisms, and guide the development of new methods for injury prediction, prevention, diagnosis, and management. How- ever, to be useful, computer models require accurate descriptions of material behavior, boundary conditions, and loading. Recent research has largely focused on the material properties of brain tissue; however, quantitative characterization of the skull–brain interface in vivo remains an important need. Magnetic resonance elastography (MRE) is a prominent tool for assessing the properties of brain tissue in vivo. MRE is a non- invasive MRI technique that enables assessment of the dynamic mechanical properties of living biological tissues [3,4]. In MRE of the brain, vibrations are applied to the skull in order to induce shear waves throughout the brain [5–7]. These shear waves are observed using MRE imaging sequences with motion-encoding gradients that oscillate at the vibration frequency, producing phase contrast images proportional to displacement [3,4]. The speeds of shear waves in soft tissue are determined by its mechanical prop- erties [5–7]. Previous studies have demonstrated the ability of MRE to provide local estimates of the mechanical properties of healthy brain tissue [8–11], as well as changes in brain material properties due to development, aging, injury, or disease [12–15]. MRE is also a valuable tool for parameterizing and validating computational models of TBI [16,17]. A number of methods exist to estimate material properties in soft tissue, such as local direct inversion (LDI) [18], nonlinear inversion (NLI) [19,20], or local frequency estimation (LFE) [21]. In contrast to the substantial progress on estimation of brain material properties, the transmission of skull motion to brain tis- sue motion has not been extensively characterized. Inside the skull, the brain is encased in multiple membranes including the dura mater, arachnoid mater, and pia mater. As the skull moves, these membranes, together with the cerebrospinal fluid (CSF) and blood contained between the various layers, cushion the brain and limit its displacement [22,23]. Together, these membranes and fluid layers comprise the skull–brain interface and play an impor- tant role in injury mechanics. Modeling the characteristics of the skull–brain interface is challenging, however [24]. While experi- mental studies of ex vivo tissue samples have provided some 1 Corresponding author. Manuscript received September 1, 2016; final manuscript received February 15, 2017; published online March 21, 2017. Assoc. Editor: Barclay Morrison. Journal of Biomechanical Engineering MAY 2017, Vol. 139 / 051002-1 Copyright V C 2017 by ASME Downloaded From: https://biomechanical.asmedigitalcollection.asme.org on 11/19/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use