Simulation of wave fields observed in brain MR elastography by 3D finite element analysis

Introduction: Image contrast in magnetic resonance elastography (MRE) is based on mechanical properties of tissues [1]. Recently, there has been an increasing interest in the area of MRE of the brain. So far, MRE has shown to be the only suitable noninvasive method for determining elastic [2-4] and even viscoelastic [5-7] properties of the human brain parenchyma in vivo. However, data reported in several studies varies substantially, e.g. the shear modulus varies between about 1 and 15 kPa. Among other reasons these variations are most likely caused by (i) the use of different elasticity reconstruction techniques and (ii) various experimental settings used for the challenging problem of mechanical wave generation in the brain and motion encoding in the phase of the MR signal. The latter is influenced by the type of mechanical excitation devices and the driving frequencies. Two different excitation modes are commonly used to induce mechanical shear waves in the brain tissue by generating either a vibrating nod [6,7] or nay [4,5,8] motion of the head. The objective of this study was to investigate whether it is possible to simulate the wave field characteristics of these different excitation modes with a 3D finite element analysis to get a better understanding of the transfer mechanisms of mechanical head vibrations into shear wave propagation inside the brain.