Quantification of the 31P metabolite concentration in human skeletal muscle from RARE image intensity

A method is described for quantifying the cellular phosphorus-31 (31P) concentration in human skeletal muscle based on RARE (rapid acquisition with relaxation enhancement) image intensities. The 31P concentrations were calculated using relaxation rates, RF coil spatial characteristics, and RARE signal intensities from foot muscle and an external 31P standard. 31P RARE and 1H T2-weighted images of the foot muscles in 11 normal subjects were acquired at 3.0 T using a double-tuned (31P/1H) birdcage coil. 31P PRESS (point-resolved spectroscopy) spectra were acquired to verify the measurable 31P concentrations in a multiecho acquisition. The mean measured concentration was 26.4 +/- 3.1 mM (mean +/- SD) from RARE signal intensities averaged over the entire imaged foot anatomy and 27.6 +/- 4.1 mM for a 3 x 3 pixel region-of-interest measurement. The 31P RARE image acquisition time was 4 min with a 0.55 cm3 voxel size. These results demonstrate that the 31P concentration can be accurately measured noninvasively in human muscle from RARE images acquired in short scan times with relatively high spatial resolution.