| The magnetic field distortion caused by the difference in magnetic susceptibility between sample and surrounding materials has been a hot research topic in the field of magnetic resonance.Especially in the combination of miniature magnetic resonance instruments and high-field microfluidic magnetic resonance,which have emerged in recent years,the influence of magnetic susceptibility on the quality of NMR signals is prominent.The high-degree harmonic components of the distorted magnetic field caused by magnetic susceptibility account for a large proportion,so multiple sets of high-degree active shim coils are needed to shim,which is in contradiction with the requirements of small assembly space,low thermal effect,and convenience in smallsized NMR instruments.The low cost requirement and the harsh material selection make the zero magnetic material solution of high-field spectrometer unsuitable for small-sized NMR instruments.Therefore,the inhomogeneous magnetic field caused by the interference of surrounding medium magnetic susceptibility near sample area is one of the problems that need solving urgently in the study of small-sized NMR spectrometer.Compensation-type passive shimming methods that introduce specific substances to reduce changes in susceptibility cannot be applied to complex susceptibility distortions,and the compensation efficiency is low.In view of this deficiency,this dissertation proposes a method for designing shim coils to compensate for the influence of susceptibility in permanent magnets.After the susceptibility field was modeled,the influence of sample susceptibility changes,main magnetic field instability and assembly deviation on susceptibility field was analyzed.Using the concept of magnetic dipole,a shim coil was designed for the interference field caused by bulk magnetic susceptibility effect(BMS),and the designed shim coil was categorized and wired.Finally,a 0.5T permanent magnetic NMR susceptibility compensation shim coil was developed.The specific work can be expounded as follows:Firstly,according to a target field method that relates the coil current expansion coefficient to the magnetic field spherical harmonic coefficient,the corresponding shim coil design process was formulated.A total of 15 sets of biplanar shim coils from first to third degree were designed and developed on a 1.2T permanent magnet NMR spectrometer,and a total of 9 sets of biplanar shim coils including first to second degree and Z4 were designed and developed on a 0.5T permanent magnetic NMR spectrometer.Then the overall shimming effects of these coils were experimentally verified on the independently developed 1.2T and 0.5T permanent magnetic NMR spectrometers respectively.On the 1.2T NMR spectrometer,after shimming,the half-height width of the water peak decreased from about 11.74ppm to 0.12ppm.On the 0.5T NMR spectrometer,after shimming,the half-height width of the water peak decreased from 24.07ppm to 0.46ppm.Secondly,for the problem of BMS in the shim area of a small-sized NMR spectrometer,we first analyzed the main magnetic field disturbance in sample area of the 0.5T permanent magnetic NMR spectrometer through modeling and simulation.Then based on the idea of active shim coil compensating nonuniform field,a permanent magnet shim coil to compensate the influence of magnetic susceptibility was proposed to solve this problem.At the same time,a detailed design scheme was provided,and in combination with specific parameters,we designed a shim coil with good compensated effect.Next,a method for winding the irregular coil was proposed.The result shows that on the 0.5T permanent magnetic NMR spectrometer,the compensated coil can reduce the magnitude of the nonuniform disturbance field in sample area caused by BMS from 10-7 to 10-8. |
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