Fast in vivo 23Na imaging and T2* mapping using accelerated 2D-FID magnetic resonance spectroscopic imaging at 3 T: Proof of concept and reliability study
Purpose: To implement an accelerated MR-acquisition method allowing to map sodium T2* relaxation and absolute concentration within skeletal muscles at 3T. Methods: A fast-2D density-weighted concentric-ring-trajectory 23Na-MRSI technique was used to acquire 64 time-points of FID with a spectral bandwidth of 312.5 Hz from a 2.5 x 2.5 mm2Na in-plane resolution within about 15 minutes. The fast relaxing 23Na signal was localized with a single-shot, inversion-recovery based, non-echo (SIRENE) OVS
... echo (SIRENE) OVS method. The sequence was verified using simulation and phantom studies before implementing it in human calf muscles. Within two same-day sessions, 2D-SIRENE-MRSI (UTE = 0.55 ms) and 3D-MRI (UTE = 0.3 ms) data were acquired. The T2* values were fitted voxel-by-voxel using a bi-exponential model for the 2D-MRSI data. Within-subject coefficients of variation were estimated for both acquisition methods. Results: The MRSI-FID data allowed for fast and slow T2* mapping of the calf muscles in vivo with minimal sensitivity reduction. The spatial-distributions of 23Na concentration for both in vivo MRSI and 3D-MRI acquisitions were significantly correlated (r = 0.7, P<0.001). The test-retest results rendered high reliability for both MRSI (CV = 5%) and 3D MRI (CV = 6%). The mean T2*fast in calf muscles was 0.7 ± 0.1 (contribution fraction = 37%), while T2*slow was 13.2 ± 0.2 ms (63%). The mean absolute muscle 23Na 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.