(A) study on generalized inhomogeneity correction for NMR imaging

Abstract

In NMR Imaging, since time varying gradient fields which are usually used for spatial localization or spatial encoding are superimposed on the main magnetic field, some dynamic inhomogeneities arising from the uncompensated residual gradients are inevitably added to the main field inhomogeneity. These dynamic inhomogeneities sometimes cause artifacts similar to those arising from the main magnetic field inhomogeneity. However, due to their dynamic nature, they have seldom been studied and few successful results of correcting the effects have been reported. In this thesis, an eddy current model is established for the NMR imaging system. Based on this model, the effect of the eddy current (or the dynamic inhomogeneity) is studied in spectroscopic imaging methods and some correction methods are also presented together with experimental results. As a related topic, a new field inhomogeneity mapping method is developed and its usefulness is also demonstrated by experiments under the presence of the dynamic inhomogeneity. Finally, a new eddy current measurement method is developed. By employing the phase of the NMR signal, the accuracy of the eddy current measurement could be much improved compared to the conventional method using a sampling coil. With the aid of this measurent method, eddy current compensation was performed on the KAIS 2.0 Tesla whole body NMR imaging system. In the appendix, two methods of correcting the stationary inhomogeneity effect in NMR imaging are presented. These are a water-fat chemical shift imaging method which forms the bases of this thesis and a new chunk-3D imaging sequence by which chemical shift as well as the inhomogeneity effect can be eliminated. Both methods are based on echo-time-encoding technique. And the usefulness of these are demonstrated by accompanying experiments.