Study of chemical-shift imaging in nuclear magnetic resonance tomography

Abstract

The nuclear magnetic resonance (NMR) phenomenon has been applied to the area of chemistry since the 1940``s. Its application has mainly been in NMR spectroscopy for the analysis of chemical compounds. The NMR spectrum which is measured with an NMR spectrometer is a result of the chemical shift which is an intrinsic characteristic of chemical compounds. Recently, however, NMR has evolved into the area of imaging. This development of NMR tomographic imaging has made three dimensional imaging of an object possible. NMR imaging means the spatial mapping of the distribution of a certain nucleus together with its relaxation time constants, i.e., $T_1$ and $T_2$. In the imaging process, homogeneity of the main magnetic field is linearly distorted for the spatial mapping in both two ro three dimensions. In conjunction with the spatial mapping of the nuclear distribution, NMR spectral information can be resolved at each spatial position; this is called Nmr spectroscopic imaging or chemical-shift imaging. This chemical-shift imaging is a new area believed to have significant impacts on physics, chemistry, mechanics, and medicine. To obtain thsi 4-dimensional information, i.e., 3-dimensional spatial information together with 1-dimensional spectral information, "NMR chemical-shift imaging" has been developed. This generic term of NMR chemical-shift imaging may be subdivided into chemical-shift selective imaging, 4-dimensional Fourier imaging, echo-time encoded spectroscopic imaging, and 4-dimensional projection reconstruction. In this thesis, both advantages and limitations of these chemical-shift imaging methods will be discussed from both theoretical and experimental view points. Finally, various perturbations such as static field inhomogeneity, chemical shift, and magnetic susceptibility will be discussed and their characteristic behaviors and effects on imaging will be detailed.