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

Abstract

PurposeThe purpose of this study was to characterize the 1H downfield MR spectrum from 8.0 to 10.0 ppm of human skeletal muscle at 7 T and determine the T1 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° 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 T1 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 T1 = mean ± SD (ms): T1‐broad,8.0ppm = 2108.2 ± 664.5, T1‐sel,8.0ppm = 753.6 ± 141.0 (p = 0.003); T1‐broad,8.2ppm = 2033.5 ± 338.4, T1‐sel,8.2ppm = 135.3 ± 35.3 (p < 0.0001); and T1‐broad,8.5ppm = 1395.4 ± 75.4, T1‐sel,8.5ppm = 107.1 ± 40.0 (p < 0.0001). Using the Solomon model, we found T1 = 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 T1 between peaks. The cross‐relaxation rate σAB = mean ± SD (Hz) of each peak was σAB,8.0ppm = 0.76 ± 0.20, σAB,8.2ppm = 5.31 ± 2.27, and σAB,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 T1 and cross‐relaxation rates of 1H resonances between 8.0 and 8.5 ppm in the healthy human calf muscle at 7 T.