3D 23Na MRI of human skeletal muscle at 7 Tesla: initial experience

To evaluate healthy skeletal muscle pre- and post-exercise via 7 T Na-23 MRI and muscle proton T-2 mapping, and to evaluate diabetic muscle pre- and post-exercise via 7 T Na-23 MRI. The calves of seven healthy subjects underwent imaging pre- and post-exercise via 7 T Na-23 MRI (3D fast low angle shot, TR/TE = 80 ms/0.160 ms, 4 mm x 4 mm x 4 mm) and 1 week later by H-1 MRI (multiple spin-echo sequence, TR/TE = 3,000 ms/15-90 ms). Four type 2 diabetics also participated in the Na-23 MRI protocol. Pre- and post-exercise sodium signal intensity (SI) and proton T-2 relaxation values were measured/calculated for soleus (S), gastrocnemius (G), and a control, tibialis anterior (TA). Two-tailed t tests were performed. In S/G in healthy subjects post-exercise, sodium SI increased 8-13% (p < 0.03), then decreased (t (1/2) = 22 min), and H-1 T-2 values increased 12-17% (p < 0.03), then decreased (t (1/2) = 12-15 min). In TA, no significant changes in sodium SI or H-1 T-2 values were seen (-2.4 to 1%, p > 0.17). In S/G in diabetics, sodium SI increased 10-11% (p < 0.04), then decreased (t (1/2) = 27-37 min) without significant change in the TA SI (-3.6%, p = 0.066). It is feasible to evaluate skeletal muscle via 3D Na-23 MRI at 7 T. Post-exercise muscle H-1 T-2 values return to baseline more rapidly than sodium SI. Diabetics may demonstrate delayed muscle sodium SI recovery compared with healthy subjects.