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

A noninvasive magnetic resonance imaging (MRI) method to assess the distrib ution of perfusion and metabolic demand ((Q) over dot/(V) over dotO(2)) in exercising human skeletal muscle is described. This method combines two MRI techniques that can provide accurate multiple localized measurements of (Q ) over dot/(V) over dotO(2) during steady-state plantar flexion exercise. T he first technique, P-31 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 ((V) over dotO(2)). The second MRI technique allows the measurement of both spat ially and temporally resolved muscle perfusion in vivo by using arterial sp in 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) over dot/(V) over dotO(2), revealing areas of a pparent under- and overperfusion for a given metabolic turnover. Analysis o f these data in a similar fashion to that employed in the assessment of ven tilation-to-perfusion matching in the lungs revealed a similar second momen t of the perfusion distribution and PCr distribution on a log scale (log SD (Q) over dot and log SDPCr) (0.47). Modeling the effect of variations in lo g SD(Q) over dot and log SDPCr in terms of attainable (V) over dotO(2), ass uming no diffusion limits, indicates that the log SD(Q) over dot and log SD PCr would allow only 92% of the target (V) over dotO(2) to be achieved. Thi s communication documents this novel, noninvasive method for assessing (Q) over dot/(V) over dotO(2), and initial data suggest that the mismatch in (Q ) over dot/(V) over dotO(2) may play a significant role in determining O-2 transport and utilization during exercise.