Distinguishing shear and tensile myocardial wall stiffness using ex vivo anisotropic Magnetic Resonance Elastography.

The organized myofiber structure within the myocardium indicates its mechanical anisotropy. By projecting the MR Elastography (MRE) stiffness matrix along either the myocardial fiber or sheet orientations determined by Diffusion Tensor Imaging (DTI), anisotropic MRE (aMRE) maps axial and transverse shear and Young's moduli into three tensile and six shear deformation modes. Ten healthy ex vivo swine hearts were imaged three times at 3T using MRE and DTI sequences. aMRE results showed a within-subject coefficient of variation at 19 % for the fiber model and 28 % for the sheet model across specimens and metrics, with coefficients lower than 15 % for seven of the ten specimens across models. Repeatability coefficient of ±0.5 kPa for Young's moduli and ±0.17 kPa for shear's moduli, demonstrating repeatability within the 95 % agreement limit. Isotropic MRE underestimated stiffnesses by 31 % compared to aMRE, where anisotropic moduli aligned more closely with prior finite element studies and some mechanical loading tests. The myocardium's anisotropic elasticity reflects with its helicoidal myofiber microstructure, with mid-wall circumferential fibers requiring twice the force to deform as longitudinal fibers at the epicardium or endocardium. At the mid-wall, fiber model values were μ