Quantification of cross-relaxation in downfield H MRS at 7 T in human calf muscle.

PURPOSE

The purpose of this study was to characterize the H downfield MR spectrum from 8.0 to 10.0 ppm of human skeletal muscle at 7 T and determine the T and cross-relaxation rates of observed resonances.

METHODS

We performed downfield MRS in the calf muscle of 7 healthy volunteers. Single-voxel downfield MRS was collected using alternately selective or broadband inversion-recovery sequences and spectrally selective 90° E-BURP RF pulse excitation centered at 9.0 ppm with bandwidth = 600 Hz (2.0 ppm). MRS was collected using TIs of 50-2500 ms. We modeled recovery of the longitudinal magnetization of three observable resonances using two models: (1) a three-parameter model accounting for the apparent T recovery and (2) a Solomon model explicitly including cross-relaxation effects.

RESULTS

Three resonances were observed in human calf muscle at 7 T at 8.0, 8.2, and 8.5 ppm. We found broadband (broad) and selective (sel) inversion recovery T  = mean ± SD (ms): T  = 2108.2 ± 664.5, T  = 753.6 ± 141.0 (p = 0.003); T  = 2033.5 ± 338.4, T  = 135.3 ± 35.3 (p < 0.0001); and T  = 1395.4 ± 75.4, T  = 107.1 ± 40.0 (p < 0.0001). Using the Solomon model, we found T  = mean ± SD (ms): T  = 1595.6 ± 491.1, T  = 1737.2 ± 963.7, and T  = 849.8 ± 282.0 (p = 0.04). Post hoc tests corrected for multiple comparisons showed no significant difference in T between peaks. The cross-relaxation rate σ  = mean ± SD (Hz) of each peak was σ  = 0.76 ± 0.20, σ  = 5.31 ± 2.27, and σ  = 7.90 ± 2.74 (p < 0.0001); post hoc t-tests revealed the cross-relaxation rate of the 8.0 ppm peak was significantly slower than the peaks at 8.2 ppm (p = 0.0018) and 8.5 ppm (p = 0.0005).

CONCLUSION

We found significant differences in effective T and cross-relaxation rates of H resonances between 8.0 and 8.5 ppm in the healthy human calf muscle at 7 T.