Time-Efficient Interleaved Spin Density-Weighted and Inversion Recovery 23Na MRI of the Human Calf Muscle at 7 T.
PURPOSE Spin density-weighted (SDW) and inversion recovery (IR) 23Na MRI provide different sodium contrasts with complementary information. Therefore, the aim was to develop a time-efficient sequence scheme capable of providing both contrasts by acquiring SDW and IR 23Na MRI data within a single sequence without additional measurement time. METHODS In the developed pulse sequence, the conventional 180° inversion pulse was divided into two successive 90° pulses, with an additional readout inserted between them. The sequence was initially evaluated in simulations and phantom measurements. Afterwards, it was applied in calf muscle measurements of six healthy volunteers to assess its practical feasibility and signal characteristics. All 23Na measurements were acquired at 7 T and compared to standard SDW and IR sequences. RESULTS The interleaved SDW/IR sequence yielded image quality and contrast comparable to standard SDW and IR sequences. Phantom experiments showed slightly higher remaining T1-weighting in the interleaved sequence compared with standard SDW, resulting in only minor changes in image contrast. Fluid suppression was effective across all IR approaches, though efficiency decreased modestly with longer effective inversion pulse lengths. Spin dynamic simulations for IR 23Na MRI indicated slightly increased sensitivity to fluid suppression artifacts arising from B0 inhomogeneities in the interleaved sequence. In vivo calf muscle measurements did not reveal significant differences in the mean muscle signal between the interleaved SDW/IR sequence and the corresponding standard sequences. CONCLUSION The proposed interleaved sequence enables simultaneous acquisition of 23Na SDW and IR contrasts, thereby reducing overall scan time and eliminating the need for co-registration.