Résumé :
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Muscle imaging, and in particular NMR imaging, will play an increasingly important role, together with dynamometry and actimetry, for the evaluation of therapeutic interventions in patients with neuro-muscular disorders. To fulfil this mission, NMR imaging must be able to detect early changes with high sensitivity, precision and reproducibility. Qualitative evaluation by visual inspection will not achieve this goal and one needs to turn to truly quantitative imaging protocols.Many possibilities have emerged recently and a range of optimized protocols are now ready to be introduced in clinical trials. Muscle trophicity can be very accurately evaluated, particularly using 3D anatomical protocols.In chronic diseases, fatty degeneration and infiltration can be precisely measured using optimized Dixon sequences. Fine compensations for RF field inhomogeneities have opened perspectives for voxel by voxel signal quantitation that may identify signal loss caused by interstitial fibrosis. Muscle inflammation, necrosis or sarcolemma integrity can be assessed by time-course studies of tissue signal enhancement after injection of Gd contrast agents, possibly coupled to albumin. Different patterns of enhancement are observed and may orient towards a diagnosis.Skeletal muscle T2 mapping can also provide valuable information on the possible existence of oedema, inflammation or loss of membrane integrity. As compared to the relatively long Gd kinetic studies, T2 measurement is short, fully non-invasive but extremely prone to errors caused by RF inhomogeneities as well as by fatty infiltration. Robust processing schemes are now being proposed to handle these problems and provide exact measures of muscle T2. In parallel, new acquisition schemes have been developed, that are immune to such confounding factors.In addition, P31- NMR spectroscopy identifies a number of metabolic alterations in dystrophic muscle, that seem to parallel disease severity and may constitute functional biomarkers of membrane integrity.The tools that had long been identified are now reaching maturity and quantitative NMR imaging and spectroscopy are available for widespread use as non-invasive outcome measures in therapeutic trials.
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