| Uniform static magnetic field is an essential basis to obtain the high-resolution spectra for nuclear magnetic resonance (NMR) experiments. Since an important development of modern NMR-an increase on the strength of the magnetic field, shimming becomes a more difficult and time-consuming matter. Thus, how to homogenize a magnetic field faster and more efficiently has become an important significance for the technical development of high-field NMR spectrometers.In this thesis, the study on the shim system of our home-built high-field NMR spectrometer can be summarized as two aspects:(1) we achieve the completely independent design and manufacture of a system of the room temperature (RT) shim coils, which is a key component for the high NMR spectrometer; (2) the automatic shimming methods including the automatic search shimming method and the gradient shimming method are improved in both experimental technique and data processing.Firstly, we realize a RT shim coil system for the high-field NMR spectrometer, for which coils are designed with discrete windings and based on a global search approach called OQNLP. The related design standards are shown as follows:(1) the shim coils have been implemented with 54 mm outside diameter (OD) and 45.0 mm inside diameter (ID); (2) The system is consisting of 28 types of shim functions, including on-axis coils up to the seventh order and off-axis coils up to the fifth order; (3) the field error after optimization can be less than 1% in the effective sample space. Shimming is performed at 500 MHz on a non-spinning standard 1H line shape test and given in terms of the full width at 50%,0.55% and 0.11% of the peak. The final line shape of 0.48/5.42/7.99 Hz has achieved the high-resolution standard of the international commercial high-field NMR spectrometers (<0.5/6.0/12.0 Hz), which demonstrates that the design and manufacture of shim coils meet the standards of the industrialization. Furthermore, the shim coil system combined with our new automatic shimming methods can provide a more effective shimming for installation.Then, in order to correct the shortcomings about parameter settings and shimming efficiency, we propose an improved automatic search shimming method. Firstly, the initial step and the convergence of the search algorithm can be determined by fitting a theoretical quadratic function relation between the second moment of the absorbed spectra and the current offset of the shim coil, for which the tedious parameter settings on manual operation can be replaced by the automatic and accurate calculations. Secondly, shimming based on the line shape of spectrum can adequately utilize the properties including the full width at half maximum (FWHM) and symmetry of peaks during the optimization process. The shimming results of the proposed method show the improvements in both the quality and efficiency of shimming relative to the conventional automatic search shimming based on deuterium lock level or FID area.Also, we proposed a new gradient shimming method based on the regularized estimation for Bo-field and shim functions. Compared to the conventional phase difference imaging technique ignoring noise and T2* effects, this improved gradient shimming method based on a statistical analysis utilizes the Gaussian noise information and a priori properties about Bo-field or shim functions to establish a Penalized Maximum Likelihood estimation:the noise in the MR measurements is Gaussian, the Bo-field maps tend to be smooth, and each shim coil is generally built to generate a spherical harmonic field. The estimation of shim functions in both simulations and real experiments have shown that the proposed regularized estimator is more reliable, especially when the noise is obvious or Bo homogeneity is poor. The final shimming results also demonstrate that this new shimming method can significantly improve the quality and efficiency of shimming at "cold shim" or low signal-to-noise ratio (SNR). |
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