Quantification of Skeletal Muscle Morphology and Mechanical Properties Using Medical Imaging

QC 20250331

Abstract

Skeletal muscle is crucial for enabling movement, maintaining posture, and stabilising joints. These functions are largely related to the morphology and mechanical properties of skeletal muscle. To quantify these properties in vivo, medical imaging techniques have been widely used, with ultrasonography and magnetic resonance imaging (MRI) being the most commonly used imaging modalities. This thesis presents four studies using different imaging techniques to quantify the morphology and mechanical properties of skeletal muscle. 

In the first study, we used three-dimensional freehand ultrasound (3DFUS) and MRI to quantify 3D skeletal muscle morphological parameters, including muscle volume, fascicle length, and pennation angle. We demonstrated that 3DFUS provided reliable and repeatable measurements, with strong agreement with MRI-based measurements. Given its lower cost and better accessibility, we suggest that 3DFUS could serve as a viable alternative to MRI for quantifying skeletal muscle morphology. 

In the second and third studies, we used two elastography techniques, magnetic resonance elastography (MRE) and ultrasound shear wave elastography (SWE), to quantify the mechanical properties of skeletal muscle. We incorporated diffusion tensor imaging to determine the fascicle orientation and integrated this information into the direct inversion of the wave equation in MRE. This approach allowed for the quantification of anisotropic mechanical properties under the assumption that skeletal muscle behaves as an incompressible transversely isotropic material. This approach was first validated through comparison with ex vivo rheometry measurements, demonstrating a good agreement between the two techniques, and then applied in vivo to the medial gastrocnemius (MG), demonstrating muscle anisotropy as well. We also compared this technique with a commercial ultrasound SWE system, which assumes skeletal muscle to be isotropic, by measuring both ex vivo muscle samples and the MG in vivo. By quantifying shear wave velocity using both elastography techniques, we observed a moderate to strong correlation between SWE and MRE in ex vivo muscle samples and a strong correlation in the MG in vivo. These findings suggested that the isotropy assumption in commercial ultrasound SWE systems does not substantially affect the quantification of muscle mechanical properties. 

In the fourth study, we used MRI to evaluate changes in calf muscle morphology and intramuscular fat content 12 months after the first botulinum neurotoxin type A (BoNT-A) injection in children with cerebral palsy (CP), who were naive to muscle tone reduction therapy. Our findings showed that the calf muscle growth was not impaired 12 months after BoNT-A injection, as indicated by increased absolute muscle volume and unchanged normalized muscle volume. However, the calf muscle growth was compromised by concurrent intramuscular fat infiltration, evidenced by increased intramuscular fat content. 

The ultrasonography and MRI techniques presented in this thesis provide the biomechanics field with different options for quantifying skeletal muscle morphology and mechanical properties. These techniques not only contribute to the medical imaging methodological development but also offer practical implications for clinical assessments and rehabilitation strategies.

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