Résumé :
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Background. There is a strong need of non-invasive outcome measures for monitoring the natural progression of muscle disorders, particularly at the early stages of involvement. In this context, quantitative NMR imaging might provide objective markers of muscle damages and degenerative changes, and an alternative to serial muscle biopsies. Among other possible NMR muscle markers, variations in T2 relaxation time have been correlated to cell damages and inflammatory response. Nonetheless, lack of easily implementable, fast, accurate and reproducible methods have hampered the adoption of T2 measurement as a non-invasive tool for skeletal muscle characterization.Purpose. Introduce a fast and robust methodology for in vivo T2 mapping of skeletal muscle.Materials and methods. The approach is based on the partially Spoiled Steady State Free Precession (pSSFP) technique proposed recently by Bieri et al1. Accurate T2 estimation can be obtained from two pSSFP NMR imaging data sets with different linear phase increments.Results. The accuracy of this method was demonstrated in various simulations and phantom experiments. Clinical applicability was assessed by examples in selected patients. While the T2 map obtained in a healthy volunteer with this sequence presents a distribution of T2 values centred in 37 ms (panel A), the T2 map of a patient with a necrotizing myopathy, obtained using the same sequence and parameters, provided a much spreader T2 distribution with its median value shifted to 44 ms (panels C and D). Regions of very long T2 (up to 180 ms) were visible in the patient T2 map, reflecting a massive tissue oedema (panel B). These same regions were observed as hyper signal in a fat suppressed T2 weighted (T2w) image. In addition, all apparently normal heads of patient quadriceps (i.e., excluding oedematous tissues) on T2w images were abnormal with a mean T2 value > 40 ms, a fact that was totally overlooked by visual inspection. Conclusion. The pSSFP method allows water-selective, 3D, fast and robust mapping of skeletal muscle T2. As it can reasonably easily be installed on commercial scanners, it offers an alternative to standard T2w or T2 mapping schemes, whose utilisation is marred by severe limitations. 1. Bieri O, Scheffler K, Ganter C., Fast T2 Mapping Using Partially Spoiled Steady State Free Precession. In Proceedings of the Annual Meeting of ISMRM-ESMRMB, Stockholm, 2010, 2634.
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