Two-dimensional time correlation relaxometry of skeletal muscle in vivo at 3 Tesla

A hybrid two-dimensional relaxometry (2DR) sequence was used to simultaneously measure both the spin-spin (R2) and spin-lattice relaxation rates (R1) of skeletal muscle in vivo. The 2DR sequence involved a 180° inversion pulse followed by a variable delay time (30 values from 40 to 7000 ms); a projection presaturation (PP) scheme to localize a 16-ml cylindrical voxel; and a CPMG sequence (950 even echoes, effective echo spacing = 1.2 ms, equilibrium time = 12 s). The 2DR data were collected at 3.0 Tesla from the flexor digitorum profundus of eight healthy males, 26 ± 2 years old. Analysis was performed with a 2D version of the non-negative least-squares algorithm and a one-way ANOVA. All subjects exhibited at least three spin-groups (R2 < 200 s−1), designated B, C, and D, with R2 values of 42.7, 26.5, and 8.1 s−1, and fractional volumes of 52, 35, and 7%, respectively. The R1 values of B and C were similar, ≅0.7 s−1, but different from that of D (P < 0.001), which had an R1 of 1.0 s−1. The results suggest that exchange between B and C ranges from 0.7–16.2 s−1, while exchange between either of these spin-groups with D is slower. If the data are interpreted with a compartment model, in which spin-groups with short and long R2 values are attributed to extra- and intracellular fluid, respectively, the exchange of water across the cell membrane in living skeletal muscle is slow or intermediate relative to both R1 and R2. Magn Reson Med 46:1093–1098, 2001. © 2001 Wiley-Liss, Inc.