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.
핵자기 공명 영상에서 와 전류에 의해 생기는 잔여 경사자계가 균일 자장을 요구하는 분광학적 영상법에 미치는 영향을 고찰하였다. 잔여 경사자계는 이와같은 영상법에서 주 자장의 비 균일성과 유사한 효과를 나타내며 이에 대한 보상방법이 요구된다.
그 일환으로 본 논문에서는 물-지방 분리 및 분광학적 영상기법에 사용할 수 있는 효율적인 영상기법을 제시하였으며 관련된 분야로서 잔여 경사자계 하에서 주 자장의 균일도를 정확히 영상화 할 수 있는 방법을 제시하였고 실험을 통하여 입증하였다.
끝으로 와 전류 보상을 위한 새로운 잔여 경사자계 측정법을 개발하였다. 이를 이용하여 이제까지 문제시 되었던 커다란 시정수의 잔여 경사자계까지도 정확히 측정할 수 있었으며 와 전류 보상에 보다 정확성을 기할 수 있게 되었다.
본 측정법을 이용하여 20,000 가우스 초전도 핵자기 공명 영상시스템의 와 전류 보상을 실시하고 그 결과를 제시 하였다.
