Spatially localized proton NMR correlation spectroscopy
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Date
1997-03-01T00:00:00Z
Authors
Zhu, Jian-Ming
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Abstract
The work presented in this thesis involves the development, experimental implementation and optimization of a spatially localized, in vivo two-dimensional proton correlation experiment. This non-invasive technique, achieved by selective volume excitation with sequences of RF pulses and magnetic field gradients, provides chemical-shift-correlated 2D spectra from spatially localized volumes. This thesis started with the analysis of general spectral localization NMR pulse sequences through spin product operator formalism. The principle of coherence transfer pathway selection was applied to realize and guide the design of volume localized spectroscopic experiments. A localized signal from a different coherence pathway was realized in the localized STEAM experiment. This signal, which was not selected in conventional experiment, caused the loss of half of the localized signal intensity in the stimulated-echo localization experiments. A new pulse sequence was developed to select both coherence pathways simultaneously, thus enhancing the sens tivity of the localized volume signal. Experiments were demonstrated to verify the pulse sequence, performed on phantoms and in vivo rat brain using a 9.4 T system. The proper selection of the coherence transfer pathway also promoted the incorporation of 2D chemical shift correlation experiments into volume-localized spectroscopy. The newly developed z-COSY-STEAM experiment is based on the STEAM sequence for volume localization, and coherence transfer processes occur among spins of interest in the localized volume. Experiments demonstrated that the pulse sequence is remarkably free of slice interference, and the quality of the acquired localized COSY spectra, both from phantoms and in vivo rat brain, is superior to those previously published. The acquisition parameters and experimental conditions were carefully optimized and designed to improve the sensitivity and avoid excessive experimental duration. The proposed approach, along with adequately modified experimental parameters, should be easily adapted for human studies at lower field strengths on clinical magnetic resonance systems. As compared to the conventional localized 1D experiment, the COSY spectra obtained by the z-COSY-STEAM experiment revealed better-resolved peaks for the J-coupled protons of several metabolites. There are several important features associated with this z-COSY-STEAM method. First, a minimum of three slice-selective radio frequency pulses was used for coherence transfer and spatial localization. Thus, experiments can be easily controlled with the specific absorption rate (SAR) limitations, permitting the implementation of this method on clinical systems for human studies. Second, the coherence transfer processes in this pulse sequence are very efficient. Therefore, the signal intensities of cross peaks relative to those of diagonal peaks are expected to be improved in comparison to the results of other long sequences of pulses for localized 2D experiments. Finally, the spectral resolution in the COSY spectra, acquired with the z-COSY-STEAM experiment, is much better than in the J-resolved spectra, acquired by the J-PRESS method. The spectra are also easy to interpret, beneficial to both spectral assignments and peak quantitation. It is expected that many important applications, particularly with brain and tumor studies, will be possible in the near future.