target Audience

Physicists and clinicians involved in the field of muscle physiology, sports sciences, neuromuscular disorders

Abstract

Contractile activity of skeletal muscle is tightly linked to a number of variables including, among others, energy production, mitochondrial function, blood flow, oxygen consumprion... Fourty years ago, 31-phosphorus (31P) MR spectroscopy (MRS) was introduced as a non invasive investigative technique of muscle energetics. For a period of more than 30 years, it has been used in order to document muscle energy production in a variety of contexts including sports physiology and muscle physiopathology. This approach has been clearly recognized as a tool which can be used for improving our knowledge about energy production and the corresponding regulatory mechanisms and a large amount of papers have been published on that topic [1]. Over the past decade, methodological progress has been made in the field of MRI/MRS regarding magnetic field, gradients and coils. Some of them have been translated as valuable tools allowing to address directly or indirectly biological issues mainly regarding muscle energy production in various physiological and pathological conditions. Mitochondrial function, oxygen consumption, blood flow and muscle contraction will be addressed in this talk with a particular emphasis on new technological developments and applications. Among these methodological progresses, ultra high-field (UHF) MRS has played a major role for the investigation of mitochondrial function. Using 31P MRS measurements performed at 7T, it has been possible to detect in human skeletal muscle at rest a new inorganic phosphate (Pi) pool with a higher T1 relaxation time. While it has been assigned to the intra-mitochondrial pool [2], it has been suggested as an interesting index of mitochondrial function [3]. Taking advantage of the higher spectral resolution and signal-to-noise at 7T, a 3D mapping of the PCr resynthesis rate after a standardized exercise has been reported with a 12s temporal resolution [4]. As an extension of this work, a 3D gradient-echo sequence for frequency-selective excitations of the PCr and Pi signals in an interleaved sampling has been developed in order to assay PCr and pH changes in exercising and recovering skeletal muscle [5]. Besides 31P MRS measurements, original imaging techniques such as DTI and strain tensor measurements have been combined to assess tri-dimensionally strain during muscle contraction [6] while ultra-high field DTI has been used in order to investigate the intramuscular variability and sex differences in young adults [7]. Oxygenation and perfusion are key parameters of muscle function which have been documented in skeletal muscle using non-contrast MRI techniques combining ASL and magnetic susceptibility-based techniques [8, 9]. Overall, this whole set of newly-developed technique might offer the opportunities to assay muscle function non invasively in the coming future

Acknowledgements

No acknowledgement found.

References

1. Kemp, G.J., et al., Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review. Acta Physiol (Oxf), 2015. 213(1): p. 107-44. 2. Kan, H.E., et al., In vivo 31P MRS detection of an alkaline inorganic phosphate pool with short T1 in human resting skeletal muscle. NMR Biomed, 2010. 23(8): p. 995-1000. 3. Valkovic, L., et al., Skeletal muscle alkaline Pi pool is decreased in overweight-to-obese sedentary subjects and relates to mitochondrial capacity and phosphodiester content. Sci Rep, 2016. 6: p. 20087. 4. Parasoglou, P., et al., Rapid 3D-imaging of phosphocreatine recovery kinetics in the human lower leg muscles with compressed sensing. Magn Reson Med, 2012. 68(6): p. 1738-46. 5. Schmid, A.I., et al., Dynamic PCr and pH imaging of human calf muscles during exercise and recovery using (31) P gradient-Echo MRI at 7 Tesla. Magn Reson Med, 2016. 75(6): p. 2324-31. 6. Englund, E.K., et al., Combined diffusion and strain tensor MRI reveals a heterogeneous, planar pattern of strain development during isometric muscle contraction. Am J Physiol Regul Integr Comp Physiol, 2011. 300(5): p. R1079-90. 7. Foure, A., et al., Diffusion Properties and 3D Architecture of Human Lower Leg Muscles Assessed with Ultra-High-Field-Strength Diffusion-Tensor MR Imaging and Tractography: Reproducibility and Sensitivity to Sex Difference and Intramuscular Variability. Radiology, 2018: p. 171330. 8. Englund, E.K., et al., Combined measurement of perfusion, venous oxygen saturation, and skeletal muscle T2* during reactive hyperemia in the leg. J Cardiovasc Magn Reson, 2013. 15: p. 70. 9. Englund, E.K., et al., Simultaneous measurement of macro- and microvascular blood flow and oxygen saturation for quantification of muscle oxygen consumption. Magn Reson Med, 2018. 79(2): p. 846-855.