Localized Double Spin Echo Spectroscopy of Weakly Homonuclear Coupled Spin Systems: Influence of Chemical Shift Artifacts

In localized double spin echo proton NMR spectroscopy of weakly homonuclear coupled spin systems unusual echo time dependent signal intensity modulations can be observed. Recently a model for a theoretical description assuming rectangular slice profiles and exact pulse angles was published. Here we use the product operator formalism to calculate the intensity modulations caused by the chemical shift between weakly coupled spins. We point out that the effects of this chemical shift artifact should be dependent on the size of the selected volume. A quantification of the effect is given, and experiments with different voxel sizes are performed with a solution of calcium lactate using a 1.5 T SIEMENS whole-body imager. The experimental results do not show the theoretically expected effects. It can be stated that the proposed model very well enables understanding of some dynamical properties of coupled spin systems in volume selective spectroscopy, but it does not solve the problem of sufficient theoretical description of the double spin echo spectroscopy.