Gradient reversal technique and its applications to chemical-shift-related NMR imaging.

The chemical-shift effect often degrades slice selectivity thereby degrading the contrast and spatial resolutions in a high-field NMR imaging. We propose a new pulse sequence which can compensate for this chemical-shift effect during slice selection. This technique provides correct slice definition as well as clear separation between water and lipid protons in high-field proton NMR imaging. Particular applications of this technique are 2D imaging with a thin-slice selection and chunk 3D imaging, where correction of the chemical-shift effect in slice selection is important. Another interesting application of the technique is chemical-shift-selective (CHESS) imaging. This technique does not require spectral-selective rf pulses and it takes advantage of the chemical-shift effect during slice selection. The latter is found to be useful for human in vivo chemical-shift imaging in high-field NMR. In this paper, the theoretical derivation of the proposed gradient reversal technique, its applications, and related experimental results are presented.