Abstract
A hybrid two‐dimensional relaxometry (2DR) sequence was used to simultaneously measure both the spin‐spin (R~2~) and spin‐lattice relaxation rates (R~1~) 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 (R~2~ < 200 s^−1^), designated B, C, and D, with R~2~ values of 42.7, 26.5, and 8.1 s^−1^, and fractional volumes of 52, 35, and 7%, respectively. The R~1~ values of B and C were similar, ≅0.7 s^−1^, but different from that of D (P < 0.001), which had an R~1~ 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 R~2~ 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 R~1~ and R~2~. Magn Reson Med 46:1093–1098, 2001. © 2001 Wiley‐Liss, Inc.