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 degrees) 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 degrees pulse to reduce the absorption of radiofrequency energy during human studies. Here, the "magic angle" double-quantum filtered 23Na NMR experiment (without a 180 degrees 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 degrees 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.