BOLD MRI mapping of transient hyperemia in skeletal muscle after single contractions

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Abstract
Transient increases in signal intensity (DeltaSI, peak 2.6 +/- 0.6 %, mean +/- SE, n = 14) were observed in axial, gradient-echo, echo-planar magnetic resonance images acquired at 1.5 T from human anterior tibialis muscle following single, 1 s duration, isometric ankle dorsiflexion contractions. The magnitude of the MRI-measured DeltaSI was not significantly different using TR of 2000 vs 500 ms, or using spin-echo vs gradient-echo echo-planar pulse sequences. However, DeltaSI measured by gradient-echo sequences was significantly greater at 3 vs 1.5 T (3.8 +/- 0.8 vs 1.6 +/- 0.2 %, n = 5). The time course of the transient DeltaSI (peak at 7.9 +/- 0.4 s after each contraction, decay with half-time of 4.6 +/- 0.6 s) was comparable to the time course of the transient increase in relative heme saturation (13 +/- 2 %, n = 5) measured after single contractions in another group of subjects by near-infrared spectroscopy (peak at 9.3 +/- 0.5 s, decay with half-time 6.2 +/- 0.8 s, n = 8). Simulations of intravascular and extravascular blood-oxygenation level-dependent (BOLD) effects in muscle suggested that intravascular BOLD makes a major contribution to the transient changes, although other factors such as increased vascular volume or increased muscle cell T2 may also contribute. The transients can be exploited for muscle functional imaging analogous to BOLD-based brain functional imaging, and might provide an index of peripheral vascular function.