Observation of direction-dependent mechanical properties in the human brain with multi-excitation MR elastography

Abstract

Magnetic resonance elastography (MRE) has shown promise in noninvasively capturing changes in mechanical properties of the human brain caused by neurodegenerative conditions. MRE involves vibrating the brain to generate shear waves, imaging those waves with MRI, and solving an inverse problem to determine mechanical properties. Despite the known anisotropic nature of brain tissue, the inverse problem in brain MRE is based on an isotropic mechanical model. In this study, distinct wave patterns are generated in the brain through the use of multiple excitation directions in order to characterize the potential impact of anisotropic tissue mechanics on isotropic inversion methods. Isotropic inversions of two unique displacement fields result in mechanical property maps that vary locally in areas of highly aligned white matter. Investigation of the corpus callosum, corona radiata, and superior longitudinal fasciculus, three highly ordered white matter tracts, revealed differences in estimated properties between excitations of up to 33%. Using diffusion tensor imaging to identify dominant fiber orientation of bundles, relationships between estimated isotropic properties and shear asymmetry are revealed. This study has implications for future isotropic and anisotropic MRE studies of white matter tracts in the human brain.

Keywords: Anisotropic soft tissue; Human brain; Magnetic resonance elastography; Nonlinear inversion; Stiffness; White matter.

Figure 1

The real component of the complex displacement fields (ũ = {ũ, ṽ, w̃}) generated by excitation from two different actuator setups: (a) anterior-posterior (AP) and (b) left-right (LR). The two excitations produce distinctly different patterns, particularly for the out-of-plane direction. AP excitation results in patterns with left-right symmetry, while the LR excitation exhibits anterior-posterior symmetry, as expected.

Figure 2

The RMS shear strain components of the strain tensor from both the anterior-posterior (AP) and left-right (LR) excitations. There are similar high magnitude regions for each direction but, also, distinct differences where the excitation was applied to the subject’s head.

Figure 3

(a) Fractional anisotropy (FA, color intensity) from DTI (from top-left) coronal, axial and sagittal, where colors describe the fiber-tract’s dominant direction: red indicates left-right, blue indicates inferior-superior, and green indicates anterior-posterior; bottom-right is a sagittal magnitude image from MRE data. The reconstructed (b) G′ and (c) G″ material maps for anterior-posterior (AP, top) and left-right (LR, bottom) excitations. Significant differences in reconstructed values are easily identified.

Figure 4

Comparison of reconstructed isotropic material properties, G′ and G″, with the strain ratio of shear parallel to shear perpendicular to fibers in the fiber-tract reference frame ${\epsilon }_{\Vert /\perp }^f$ (Eq. (9)) for (a) corpus callosum, CC, (b) corona radiata, CR, and (c) superior longitudinal fasciculus, SLF. The numerical values of mean and standard deviations across experimental repeats are presented in Tables 1, 2, and 3.