On the use of two-dimensional-J NMR measurements for in vivo proton MRS: measurement of homonuclear decoupled spectra without the need for short echo times.

Abstract

The potential of two-dimensional (2D)-J NMR for in vivo proton MRS is examined. Single voxel measurements on the rat brain were performed at 4.7 T using point-resolved spectrocopy localization with a voxel size of 64 microliter and total measuring times of 10-15 min. It is shown that a series of measurements with only 16 or fewer different echo times (TE) enables good signal localization in the f1 axis corresponding to the coupling patterns. For data evaluation, the 2D-J NMR spectrum as well as cross-sections at given f1 values and projections onto the f2 axis are used. A comparison between cross-section spectra taken at different f1 values may help to solve problems of peak assignment. The projection of the 2D magnitude spectrum onto the f2 axis corresponds to a homonuclear decoupled 1D proton spectrum. Because the T2 relaxation times of several coupled resonances (e.g., myo-inositol and glutamate) are rather long, only minor losses in the quality of the projection spectra occur if the measurements with short TE (< or = 50 ms) are not used for data processing. Thus, homonuclear decoupled proton spectra detecting uncoupled and several coupled resonances can be measured with high quality in vivo, even on MR systems that are not equipped with actively shielded gradients, prohibiting data acquisition with TEs of 50 ms or less.