Assessments of Skeletal Muscle Architecture and Energetics by Magnetic Resonance Diffusion Tensor Imaging and 31P Spectroscopy

Skeletal muscle is responsible for the voluntary movement of materials through the body and locomotion; however, the performance and efficiency of the muscle depend on many factors including alterations in oxygen concentrations and myofiber dimensions. The primary objective of this dissertation was to utilize magnetic resonance techniques, namely diffusion tensor imaging and 31P spectroscopy, to evaluate muscle performance under different pathological and environmental conditions such as sarcopenia (loss of muscle mass because of aging), catabolic stress (caused by 30% caloric restriction and six-wks intensive aerobic exercise) and hypoxia (breathing 10% O2 gas mixture). To achieve these goals, a 1H/31P retunable surface coil along with an animal apparatus were designed and fabricated for in vivo experiments conducted in a high field, vertical magnet. Diffusion tensor imaging was utilized to study changes in skeletal muscle architecture caused by sarcopenia, as well as potential treatments including exercise and/or dietary supplementation. Catabolic stress and acute hypoxic tolerance also were evaluated with diffusion tensor imaging. Potential muscle architecture changes were determined by measuring parameters such as the apparent diffusion coefficient, fractional anisotropy and eigenvalues of water diffusion. To assess muscle function and physiology, 31P NMR spectroscopy was utilized to investigate dynamic concentrations of high energy phosphates and recovery rates of these substrates under the impacts of chronic catabolic stress and acute hypoxia as a function of dietary constraints and genetic background. Results showed that the surface coil was highly sensitive to both proton and phosphorus nuclei indicated by the high quality factor of the coil for both operating modes. Results also suggested that the third eigenvalue was the most sensitive parameter for changes in myofiber dimensions, which were reduced with age. As a treatment for sacropenia, a 0.46-g/kg/day supplement of and #946;-hydroxy and #946;-methylbutylate (HMB) was found to partially recover some of the age-related losses in the muscle and reduce fat mass; however, it did not have an impact on the lean body mass. Further, the effects of the dietary supplement were overwhelmed by the resistive exercise when combining the two treatments. In contrast, catabolic stress caused large reductions in fat muscle mass but less dropoff in lean muscle mass. As a result, DTI parameters were less sensitive to these alterations, and HMB supplementation's impact may have been limited to a blunting of fat mass losses. However, reductions in resting phosphocreatine concentrations as assessed by NMR spectroscopy did track with potential losses in muscle function and/or increased inflammation in skeletal muscle under catabolic stress. Finally, DTI data showed that the C57BL and the BALBc mouse strains had similar myofiber dimensions; however, the recovery rates of phosphocreatine in the C57BL mice appeared to be slower under acute hypoxia, which seemed to have a reduced effect on the BALBc strain, likely because of different genetic backgrounds that are linked to improved cardiovascular tolerances. In conclusion, overall results demonstrated the sensitivity of magnetic resonance techniques utilized in this dissertation to alterations in skeletal muscle structure and metabolism caused by different genetic and environmental experimental conditions.