An abnormal change in muscle mechanical properties is usually indicative of disease or injury. In this thesis, the in vivo anisotropic mechanical properties of skeletal muscles were investigated non-invasively using a combined medical imaging modality - magnetic resonance elastography (MRE) combined with diffusion tensor imaging (DTI). By assuming a transversely isotropic model, the shear modulus parallel (μ_║) and perpendicular (μ_┴) to the local fibre orientation, as well as the anisotropic ratio (μ_║/μ_┴), were calculated. DTI provided the local fibre direction in anisotropic materials based on the diffusion of water molecules. Experiments were conducted in ex vivo phantoms and bovine muscle samples, an in vivo mouse model, and in vivo human subjects. Results from these studies showed that this combined MRE/DTI imaging method was capable of visualizing and quantifying the anisotropic shear moduli (µ_║, μ_┴) and detecting changes in the mechanical anisotropic ratio (µ_║/μ_┴) in ex vivo phantoms and in vivo skeletal muscles.

Specifically, the phantom results showed that the MRE/DTI technique was able to differentiate various levels of anisotropy, and the results correlated well with the “gold standard” rheometry tests. Moreover, in vivo animal results indicate that this technique can detect changes in muscle mechanical anisotropy due to necrosis, consistent with histological findings. Finally, in an in vivo human study, it was demonstrated that the mechanical anisotropy ratio can be used to track the physiological response of skeletal muscle after eccentric exercise.

Overall, these results have shown that the mechanical anisotropy ratio (μ_║/μ_┴) has potential as an imaging biomarker for detecting pathological and physiological changes in skeletal muscle in vivo, and the MRE/DTI technique can be used to accurately measure this tissue anisotropic mechanical property non-invasively.