Double-Quantum Filtered 23Na MRS and MRI: Targeted Detection of Ordered Sodium in Inhomogeneous Fields without 180° Refocusing.

Motivation: Visualizing sodium in ordered biological environments requires a 180° pulse, but this increases SAR levels, limiting in vivo applications, particularly at high magnetic fields. Goal(s): To enable 23Na MRS/I in ordered environments without 1800 pulse. Approach: The Double Quantum Filtered (DQF) 23Na MRS/I without a 180° pulse are analysed in terms of coherence pathways that lead to refocusing in an inhomogeneous B0("echoes") and those that do not ("antiechoes"). Results: It is demonstrated that echo and antiecho pathways can be separated through phase cycling. The proposed "echo" DQF 23Na MRS/I achieves complete refocusing of B0 inhomogeneity effects without the need for 1800 pulse. Impact: The new method is demonstrated both in 23Na MRS in an inhomogeneous B0 field and in 23Na MRI of a three-component phantom. This approach enables the minimization of SAR effects in sodium imaging at high magnetic fields.