Understanding Mechanical Wave Propagation in Anisotropic Samples through Magnetic Resonance Elastography

The presence and progression of neuromuscular pathology, including spasticity, Duchenne's muscular dystrophy and hyperthyroidism, has been correlated with changes in the intrinsic mechanical properties of skeletal muscle tissue. Tools for noninvasively measuring these properties, such as Magnetic Resonance Elastography, could benefit basic research into understanding neuromuscular pathologies, as well as translational research to develop therapies, by providing a means of assessing and tracking their efficacy. Magnetic Resonance based Elastography, has shown promising results for the detection and monitoring of many diseases affecting isotropic soft tissues, thanks to its ability to offer deep tissue penetration approaching sub-millimeter resolution. Research is still at the early stage for anisotropic tissues where materials properties are direction-dependent, such as striated (skeletal and cardiac) muscle. In these cases, MRE evaluations become more complex and there is the need of extracting more information. This dissertation details the research of Magnetic Resonance Elastography applied specifically to anisotropic tissues such as skeletal muscle with the goal of using directional variations in the mechanical properties of anisotropic soft tissues to detect alterations caused by pathological conditions. Innovative approaches to design anisotropic and viscoelastic composite fibrous phantoms mimicking skeletal muscle tissue through 3D printing are proposed. These can be used as a means of validation of future developments in the MRE technique, but also of other techniques based on the propagation of mechanical waves. A novel analytical theory for shear wave propagation in anisotropic fibrous tissues is also presented in the current work, stemming from the fusion of transformation acoustics and relativistic mechanics areas, with the idea to simplify the mathematical description making the problem isotropic via an appropriate transformation of the coordinate system. Analytical solutions can potentially provide higher solution speed and ease in parametric and optimization studies, compared to computationally heavy numerical methods. A way of excite fast and slow shear waves in anisotropic sample in order to fully characterize their properties is also implemented with an interchangeable MRE set-up for a low field table-top MRI system allowing for both axial and torsional actuations. Its application to anisotropic samples, such as skeletal muscle, enables the measurement of direction-dependent properties.