Résumé :
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Communication n° 502. Introduction : The NMR signal intensity depends, among many factors, on blood oxygenation, giving rise to blood oxygen level dependent (BOLD) contrast, the mechanisms of which are complex and depend on parameters prone to large variations, particularly in skeletal muscle. Yet, studies have used BOLD as a surrogate marker for muscle perfusion, despite the absence of experimental validation. Objective : To test the strength of the relation between BOLD contrast and perfusion determined by arterial spin labeling (ASL) in muscle - both available from the same acquired images- and whether this relation changed for varying degrees of leg vascular replenishment. Methods : We simultaneously measured ASL and BOLD responses in the calf muscle of 6 healthy subjects during post-ischemic reactive hyperemia after positioning the leg at different heights relative to the heart to produce varying degrees of leg vascular replenishment. Results : Whatever the degree of vascular repletion, calf muscle reperfusion profiles, quantified by ASL, were statistically indistinguishable over the first 240s after cuff release (p>0.2), whereas skeletal muscle BOLD response at reperfusion was much greater after vascular filling than in standard conditions or after vascular draining (p<0.001). Positive linear relationships were identified between BOLD and perfusion data for each condition during the first 30s of reperfusion, but the slope after vascular repletion was twice as steep (0.0014; p<0.01) as those measured in the reference condition and after draining (0.0007 and 0.0008, respectively). Conclusion : BOLD contrast is inadequate to probe arteriolar vasodilation of skeletal muscle, especially when changes in tissue vascular fractional volume are expected. Though combined analysis of ASL and BOLD signals can prove judicious to estimate muscle oxygen extraction, only the ASL technique combined to NMR imaging can offer non-invasive, quantitative and dynamic investigation of muscle perfusion in vivo.
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