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1 Department of Exercise Science, University of Massachusetts, Amherst, MA, USA
2 Department of Radiology & Radiological Science, Vanderbilt University, Nashville, TN, USA
* To whom correspondence should be addressed. E-mail: janekb{at}excsci.umass.edu.
Although skeletal muscle perfusion is fundamental to proper muscle function, in vivo measurements are typically limited to those of limb or arterial blood flow, rather than flow within the muscle bed itself. We present a non-invasive functional MRI technique for measuring perfusion-related signal intensity (SI) changes in human skeletal muscle during and after contractions, and demonstrate its application to the question of occlusion during a range of contraction intensities. Eight healthy men (aged 20-31 years) performed a series of isometric ankle dorsiflexor contractions from 10-100% maximal voluntary contraction (MVC). Axial gradient-echo echo-planar images (repetition time = 500 ms, echo time = 18.6 ms) were acquired continuously before, during and following each 10 s contraction, with 4.5 min rest between contractions. Average SI in the dorsiflexor muscles was calculated for all 240 images in each contraction series. Post-contraction hyperemia for each force level was determined as peak change in SI after contraction, which was then scaled to that obtained following a 5-min cuff occlusion of the thigh (i.e., maximal hyperemia). A subset of subjects (n = 4) performed parallel studies using venous occlusion plethysmography to measure limb blood flow. Hyperemia measured by fMRI and plethysmography demonstrated good agreement. Post-contraction hyperemia measured by fMRI scaled with contraction intensity up to ~ 60% MVC. Functional MRI provides a non-invasive means of quantifying perfusion-related changes during and following skeletal muscle contractions in humans. Temporal changes in perfusion can be observed, as can the heterogeneity of perfusion across the muscle bed.
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