Résumé :
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Background. Except for fatty infiltration mapping, skeletal muscle T1-weighted (T1w) NMR imaging is reputed to be non-informative, in particular with regard to myocyte inflammation, oedema, necrosis or more broadly speaking cell damage. These pathological features are typically looked for on T2-weighted images or better with T2 mapping sequences, the latter being unfortunately prone to errors owing to instrumental imperfections. Basic molecular dynamics predicts that increased water mobility responsible for longer T2 in soft tissues ought to be most often paralleled by T1 lengthening, not detectable by T1w but probably measurable by T1 mapping.Purpose. To determine whether skeletal muscle T1 was abnormally long in dystrophic muscle of GRMD dogs and correlated with concurrent T2 modifications.Materials and Methods. GRMD (n=5) and control (n=5) dogs were scanned at 3T and 3 fore-limb and 4 hind-limb muscles were studied longitudinally at the ages of 2, 4, 6 and 9 months. At each time-point, a variety of NMR indices were measured, T2w/T1w, T2w, T2, T2 and T1 heterogeneities, Gd enhancement&. to which a T1 determination was added. Limb T1 was measured using a selective inversion Turbo-Flash sequence, with inversion times from 250 to 8000ms. Results. Mean T1 of fore-limb and hind-limb muscles were longer in GRMDs than in controls: 1.50s vs 1.59s, p<.03 and 1.46s vs 1.61s, p<.0002, respectively. They were statistically significant T1 differences across muscles (p<.00001), tibialis cranialis having the shortest T1 (1.47s) and flexor digitorum superficialis the longest (1.60s) but no GRMD muscle was particularly affected and T1 increase was identical in all muscles investigated. There was an overall decrease in muscle T1 with age, which did not concern preferentially either GRMD or control dogs, or specific muscles.Skeletal muscle T1 was correlated with T2, maximum Gd relative enhancement and T2w heterogeneity, with Spearman coefficients of .73, .65 and .42, respectively, all with p<0.0001. There was no relation between muscle T1 and fat infiltration, which is almost absent in GRMDs within this age range.Conclusion. While this possibility had been so far overlooked, we demonstrated here that T1 mapping is an additional robust NMR imaging marker of muscle dystrophy. As compared to T2 mapping, its simplicity and reliability compensates for the low dynamic range of changes while limitations owing to poor specificity are the same.
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