Fast In Vivo Na-23 Imaging And T-2* Mapping Using Accelerated 2d-Fid Ute Magnetic Resonance Spectroscopic Imaging At 3 T: Proof Of Concept And Reliability Study

Purpose: To implement an accelerated MR-acquisition method allowing to map T-2* relaxation and absolute concentration of sodium within skeletal muscles at 3T.Methods: A fast-UTE-2D density-weighted concentric-ring-trajectory Na-23-MRSI technique was used to acquire 64 time points of FID with a spectral bandwidth of 312.5 Hz with an in-plane resolution of 2.5 x 2.5 mm(2) in similar to 15 min. The fast-relaxing Na-23 signal was localized with a single-shot, inversion-recovery-based, non-echo (SIRENE) outer volume suppression (OVS) method. The sequence was verified using simulation and phantom studies before implementing it in human calf muscles. To evaluate the 2D-SIRENE-MRSI (UTE = 0.55 ms) imaging performance, it was compared to a 3D-MRI (UTE = 0.3 ms) sequence. Both data sets were acquired within 2 same-day sessions to assess repeatability. The T-2* values were fitted voxel-by-voxel using a biexponential model for the 2D-MRSI data. Finally, intra-subject coefficients of variation (CV) were estimated.Results: The MRSI-FID data allowed us to map the fast and slow components of T-2* in the calf muscles. The spatial distributions of Na-23 concentration for both MRSI and 3D-MRI acquisitions were significantly correlated (P < .001). The test-retest analysis rendered high repeatability for MRSI with a CV of 5%. The mean T-2Fast* in muscles was 0.7 +/- 0.1 ms (contribution fraction = 37%), whereas T2Slow* was 13.2 +/- 0.2 ms (63%). The mean absolute muscle Na-23 concentration calculated from the T-2*-corrected data was 28.6 +/- 3.3 mM.Conclusion: The proposed MRSI technique is a reliable technique to map sodium's absolute concentration and T-2* within a clinically acceptable scan time at 3T.