Sodium relaxometry using 23 Na MR fingerprinting: A proof of concept

Abstract

Purpose: To evaluate the feasibility of ²³ Na MR fingerprinting (MRF) for simultaneous quantification of T₁ , $T_{2l}^{\ast }$ , $T_{2s}^{\ast }$ , $T_2^{\ast }$ in addition to ΔB₀ .

Methods: A framework for sodium relaxometry using MRF at 7T was developed, allowing simultaneous measurement of relaxation times and inhomogeneities in the static field. The technique distinguishes between bi- and monoexponential transverse relaxation and was validated in simulations with respect to the ground truth. In phantom measurements, a resolution of 2 × 2 × 12 mm³ was achieved within 1 h acquisition time, and the resulting parameter maps were compared to results from reference methods. Relaxation times in five healthy volunteers were measured with a resolution of 4 × 4 × 12 mm³ .

Results: Phantom experiments revealed an agreement between the relaxation times obtained via ²³ Na-MRF and the reference methods. In white matter, a longitudinal relaxation constant of T₁ = 38.9 ± 4.8 ms was found, while values of $T_{2l}^{\ast }$ = 29.2 ± 4.9 ms and $T_{2s}^{\ast }$ = 4.7 ± 1.2 ms were found for the long and short component of the transverse relaxation. In cerebrospinal fluid, T₁ was 67.7 ± 6.3 ms and $T_2^{\ast }$ = 41.5 ± 3.4 ms.

Conclusion: This work demonstrates the feasibility of ²³ Na-MRF for relaxometry in sodium MRI in both phantom and in vivo studies. Simultaneous quantification of T₁ , $T_{2l}^{\ast }$ , $T_{2s}^{\ast }$ , $T_2^{\ast }$ and ΔB₀ was possible within a 1 h measurement time.

Keywords: MRI; X-Nuclei; magnetic resonance fingerprinting; relaxometry; sodium.