In Vivo Evaluation of Skeletal Muscle Morphological and Mechanical Properties using Medical Imaging

Sammanfattning: Skeletal muscles are soft tissues that play an important role in maintaining body posture and enabling movements through force generation. The force-generating capacity is associated with the morphological and mechanical properties of the muscle. Abnormal muscle force-generating capacity may result in or be the outcome of abnormal muscle morphological and mechanical properties. Medical imaging is a widely-used and powerful in vivo measurement technique that could provide valuable information for diagnosis, treatment planning, outcome evaluation, and research of muscle-related pathologies. The overall objectives of this thesis were to quantify skeletal muscle morphological and mechanical properties in vivo using different medical imaging techniques and to evaluate the performance of the proposed medical imaging techniques with respect to different aspects of reliability. This thesis was based on two papers that focused on the morphological and mechanical properties of skeletal muscles, respectively.In the first study, skeletal muscle morphological parameters, i.e., volume, fascicle length, and pennation angle of the tibialis anterior and gastrocnemius medialis, were measured in vivo using three-dimensional freehand ultrasound (3DfUS) and magnetic resonance imaging (MRI), respectively. In total, sixteen able-bodied subjects were recruited in this study. Seven of them received both 3DfUS and MRI measurements, while the remaining subjects received 3DfUS measurements twice at a one-week interval. Good to excellent intra-rater reliability and inter-session repeatability was found in muscle morphological parameters quantified by 3DfUS measurements, as indicated by the high intra-class correlation coefficient values and low standard error of measurement. The only statistically significant difference found between 3DfUS and MRI measurements was the pennation angle of the tibialis anterior, although the actual discrepancy was small. This study suggested that 3DfUS could be considered as an alternative to MRI for measuring 3D skeletal muscle morphological parameters in vivo.In the second study, a direct inversion approach based on magnetic resonance elastography (MRE) and diffusion tensor imaging (DTI) for skeletal muscle’s anisotropic mechanical properties quantification was proposed. DTI was used to identify the muscle fascicle orientation. An incompressible transversely isotropic material model was adopted to describe the material property of the skeletal muscles. The proposed approach was evaluated with a multifrequency MRE setup on gastrocnemius and soleus in five able-bodied subjects. The existence of anisotropy and frequency dependence was observed in all muscle sub-compartments using the proposed inversion approach. The necessity of using DTI to identify muscle fascicle orientation was also illustrated. This study suggested that the proposed direct inversion approach had the potential to quantify the anisotropic mechanical properties of skeletal muscles in both healthy and pathological conditions.