Quantification of the Tissue Sodium Concentration in the Human Calf at 7T With Reduced Acquisition Time: Influence of the Nominal Spatial Resolution
To evaluate the repeatability and consistency of rapid quantitative 23Na MRI of the human calf, an acquisition‐weighted stack‐of‐stars (AW‐SOSt) sequence was implemented on a 7T MRI system and used for all measurements. Three variants, differing only in nominal in‐plane spatial resolution (2.5, 5.0, and 7.5 mm) and corresponding acquisition times (8:04, 4:02, and 2:41 min), while maintaining a constant slice thickness of 15 mm, were employed for imaging the calf skeletal muscle. Quantitative consistency across resolutions and repeatability was evaluated in simulations and consecutive measurements of 10 healthy volunteers. The apparent tissue sodium concentration (aTSC) was determined using a postprocessing pipeline consisting of a B0, B1, relaxation, and partial volume correction (PVC). Deviations of the determined aTSC from simulated ground truth were below 6.0%. The simulated coefficient of variation (CV) improved with decreasing spatial resolution from 1.1% to 0.9%. The in vivo findings matched the simulation results. The CV improved with increasing voxel size, decreasing from 2.7% at 2.5‐mm resolution to 2.0% at 7.5‐mm resolution. Measurements remained highly consistent across different in‐plane resolutions, with a small increase in aTSC (≈0.2 mM) observed as a difference between lower resolutions (7.5 mm) and the highest resolution (2.5 mm). All applied protocols showed good repeatability and high consistency between the protocols. aTSC quantification using low‐resolution 23Na MRI showed improved repeatability, whereas in vivo measurements showed no effect on quantitative accuracy. This study demonstrates that low‐resolution 23Na MRI with short acquisition times combined with PVC could be a practical alternative to commonly used “high‐resolution” techniques with long acquisition times for quantifying aTSC in calf muscle tissue.