Résumé :
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NMRI (Nuclear Magnetic Resonance Imaging) investigations have attempted to exploit the Blood-Oxygen-Level-Dependent (BOLD) contrast in the skeletal muscle as an estimator of physiological parameters. However, beside changes in hemoglobin saturation, which induce changes in apparent T2 and T2*, other mechanisms are involved in the BOLD contrast. For instance, it has been shown that BOLD contrast depends both on perfusion and on vascular filling during reactive hyperemia (RH) in human skeletal muscle [1]. To improve understanding of the origins of BOLD signal changes during RH, interleaved multi-parameter measurements have been used [2]. Such experiments have confirmed T2* and T2 changes, with no significant alteration in T1 and M0 [2]. In the present work we investigated the feasibility of simultaneous measurement of NMR parameters (T1, T2 and M0). Experimental data were acquired on a 3.0 T whole body scanner (Siemens TimTrio, Erlangen, Germany). Multiple parameters were obtained in human calf using the IR TrueFISP [3] sequence. Image resolution = 2.3 x 2.3 mm2, slice thickness = 8.0 mm. Dynamic multi-parameter measurements were performed in healthy volunteers who underwent a three phase protocol: rest (10 min), ischemia of the calf (5-10 min), reactive hyperemia (15 min). Time resolution for these multiparametric measurements was ~12 s. At rest, T1 and T2 maps showed excellent agreement with those obtained from standard methods. In all volunteers parameter time-courses showed a complex pattern, with concomitant changes in T1, T2 and M0, whose origins need to be disentangled. The ability to monitor dynamically and simultaneously the main parameters of the NMR signal might help to define indices that reflect muscle oxygenation more closely than does standard BOLD contrast. [1] Duteil et al, MRM 55, 450-4 (2006) [2] Klarhöfer et al, Proc. ISMRM 3800 (2007) [3] Schmitt et. al., MRM 51, 661-7 (2004)
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