Magnetic Resonance Elastography Of Anisotropic Materials Under Pre-Strained Boundary Conditions
thesisposted on 01.08.2020, 00:00 authored by Luca Leoni
Magnetic Resonance Elastography (MRE) is a noninvasive diagnostic imaging technique. Developed recently, it is capable to assess tissues’ viscoelastic properties mechanically stimulating them and exploiting Magnetic Resonance Imaging (MRI) motion-sensitive gradients. MRE is promising for applications on a wide range of organs and tissues, including skeletal muscles. Variations in viscoelastic properties of tissues might be sign of an ongoing pathology in a tissue. In the muscular case, diseases affecting those properties could be spasticity, Duchenne muscular dystrophy, and hyperthyroidism. Not only MRE is practical to detect early stages of the specified syndromes, but we hypothesize that it also can be employed for a deep understanding of how active and passive tensions influence apparent muscle viscoelastic features. The present work aims at investigating how passive pre-tension determines alterations of apparent shear stiffness in an Ecoflex™ phantom mimicking skeletal muscle fibrous architecture and transversely isotropic properties at different axial elongation values. Such analysis is performed both experimentally and computationally. Experimental MRE in pre-strained boundary conditions requires the design on SolidWorks and the machining of a MR suitable setup simultaneously capable to axially elongate the cited phantom, and to provide it with mechanical shear excitation. Magnetic Resonance Elastography acquisitions are made with an Agilent 9.4 T imaging system for which the setup is specifically designed. Finite Element Analysis (FEA) is implemented on COMSOL Multiphysics, after having built a phantom model in SolidWorks. The FE virtual experiment, by means of a direct problem, simulates the practical trials, imposing progressively larger pre-strain boundary conditions to the phantom model. An analysis of the wave images obtained enables to understand how frequency dependent viscoelastic parameters and axial pre-strain influence propagation of shear waves.