Interleaved 31 P MRS/ 1 H ASL for analysis of metabolic and functional heterogeneity along human lower leg muscles at 7T.

Purpose MR offers the unique possibility to noninvasively investigate cellular energy metabolism via 31P MRS, while blood perfusion, which provides oxygen and substrates to the tissue, is accessible by arterial spin labeling (ASL) 1H MRI. Because metabolic and hemodynamic parameters are linked, it would be desirable to study them simultaneously. A 3D-resolved method is presented that allows such measurements with high spatiotemporal resolution and has the potential to discern differences along an exercising muscle. Methods Multi-voxel localized P-31 MRS was temporally interleaved with multi-slice pASL 1H MRI. Phosphorus spectra were collected from two adjacent positions in gastrocnemius medialis (GM) during rest, submaximal plantar flexion exercise and recovery, while perfusion and T2*-weighted axial images were acquired at the same time. Seventeen healthy volunteers (9 f / 8 m) were studied at 7 T. Results An increase of postexercise perfusion and T2*-weighted signal in GM positively correlated with end-exercise PCr depletion and pH drop. At proximal positions functional and metabolic activity was higher than distally, that is, perfusion increase and peak T2*-weighted signal, end-exercise PCr depletion, end-exercise pH, and PCr recovery time constant were significantly different. An NOE-induced SNR increase of approximately 20 % (P < .001), at rest, was found in interleaved P-31 spectra, when comparing to P-31-only acquisitions. Conclusions A technique for fast, simultaneous imaging of muscle functional heterogeneity in ASL, T2* and acquisition of time-resolved P-31 MRS data is presented. These single exercise recovery experiments can be used to investigate local variations during disease progression in patients suffering from vascular or muscular diseases.