Fast in vivo 23Na imaging and 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 T2∗ relaxation and absolute concentration of sodium within skeletal muscles at 3T. METHODS A fast-UTE-2D density-weighted concentric-ring-trajectory 23 Na-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 × 2.5 mm2 in ~15 min. The fast-relaxing 23 Na 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 T2∗ 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 T2∗ in the calf muscles. The spatial distributions of 23 Na 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 T2Fast∗ in muscles was 0.7 ± 0.1 ms (contribution fraction = 37%), whereas T2Slow∗ was 13.2 ± 0.2 ms (63%). The mean absolute muscle 23 Na concentration calculated from the T2∗ -corrected data was 28.6 ± 3.3 mM. CONCLUSION The proposed MRSI technique is a reliable technique to map sodium's absolute concentration and T2∗ within a clinically acceptable scan time at 3T.