Double-quantum filtered ²³Na NMR and MRI: Selective detection of ordered sodium in an inhomogeneous B₀ field

Double-quantum filtered ²³Na NMR experiments with one or two "magic angle" (54.7°) pulses in the filter step are widely used for selective observation of sodium ions that are interacting with ordered biological structures ("ordered sodium") and hence exhibit a distribution of quadrupolar splittings in their NMR spectrum. This approach has recently been extended to ²³Na MRI where the conventional experiment has been modified, omitting the 180° pulse to reduce the absorption of radiofrequency energy during human studies. Here, the "magic angle" double-quantum filtered ²³Na NMR experiment (without a 180° pulse) is analysed in terms of coherence pathways that lead to refocusing in an inhomogeneous B₀ field ("echoes") and those that do not ("antiechoes"). It is shown that the echo and antiecho pathways can be separated by phase cycling and that the antiecho pathway contributes very little to the overall signal in an inhomogeneous B₀ field. Hence, a double-quantum filtered ²³Na NMR experiment that utilises just the echo pathway and so achieves complete refocusing of the effects of B₀ inhomogeneity without making use of a 180° pulse is proposed. The new method is demonstrated both in ²³Na NMR spectroscopy in an inhomogeneous B₀ field and in ²³Na MRI of a three-component phantom.