Quantification of cross-relaxation in downfield H-1 MRS at 7 T in human calf muscle

PurposeThe purpose of this study was to characterize the H-1 downfield MR spectrum from 8.0 to 10.0 ppm of human skeletal muscle at 7 T and determine the T-1 and cross-relaxation rates of observed resonances. MethodsWe 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 degrees 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-1 recovery and (2) a Solomon model explicitly including cross-relaxation effects. ResultsThree 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-1 = mean +/- SD (ms): T-1-broad,T-8.0ppm = 2108.2 +/- 664.5, T-1-sel,T-8.0ppm = 753.6 +/- 141.0 (p = 0.003); T-1-broad,T-8.2ppm = 2033.5 +/- 338.4, T-1-sel,T-8.2ppm = 135.3 +/- 35.3 (p < 0.0001); and T-1-broad,T-8.5ppm = 1395.4 +/- 75.4, T-1-sel,T-8.5ppm = 107.1 +/- 40.0 (p < 0.0001). Using the Solomon model, we found T-1 = mean +/- SD (ms): T1-8.0ppm = 1595.6 +/- 491.1, T1-8.2ppm = 1737.2 +/- 963.7, and T1-8.5ppm = 849.8 +/- 282.0 (p = 0.04). Post hoc tests corrected for multiple comparisons showed no significant difference in T-1 between peaks. The cross-relaxation rate sigma(AB) = mean +/- SD (Hz) of each peak was sigma(AB,8.0ppm) = 0.76 +/- 0.20, sigma(AB,8.2ppm) = 5.31 +/- 2.27, and sigma(AB,8.5ppm) = 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). ConclusionWe found significant differences in effective T-1 and cross-relaxation rates of H-1 resonances between 8.0 and 8.5 ppm in the healthy human calf muscle at 7 T.