Local perfusion and metabolic demand during exercise: a noninvasive MRI method of assessment.

A noninvasive magnetic resonance imaging (MRI) method to assess the distribution of perfusion and metabolic demand (Q/VO(2)) in exercising human skeletal muscle is described. This method combines two MRI techniques that can provide accurate multiple localized measurements of Q/VO(2) during steady-state plantar flexion exercise. The first technique, (31)P chemical shift imaging, permits the acquisition of comparable phosphorus spectra from multiple voxels simultaneously. Because phosphocreatine (PCr) depletion is directly proportional to ATP hydrolysis, its relative depletion can be used as an index of muscle O(2) uptake (VO(2)). The second MRI technique allows the measurement of both spatially and temporally resolved muscle perfusion in vivo by using arterial spin labeling. Promising validity and reliability data are presented for both MRI techniques. Initial results from the combined method provide evidence of a large variation in Q/VO(2), revealing areas of apparent under- and overperfusion for a given metabolic turnover. Analysis of these data in a similar fashion to that employed in the assessment of ventilation-to-perfusion matching in the lungs revealed a similar second moment of the perfusion distribution and PCr distribution on a log scale (log SD(Q) and log SD(PCr)) (0.47). Modeling the effect of variations in log SD(Q) and log SD(PCr) in terms of attainable VO(2), assuming no diffusion limits, indicates that the log SD(Q) and log SD(PCr) would allow only 92% of the target VO(2) to be achieved. This communication documents this novel, noninvasive method for assessing Q/VO(2), and initial data suggest that the mismatch in Q/VO(2) may play a significant role in determining O(2) transport and utilization during exercise.