| Compared with superconducting MR(magnetic resonance),movable PMR(Permanent Magnetic Resonance)sacrifices certain sensitivity and requires no coolant,which has the advantages of low acquisition cost,low detection cost and low maintenance cost.In addition,its movable nature extends the application of MR from specialized laboratories to food safety,drug testing,oil exploration and other application scenarios.For PMR image device and spectrometer,the homogeneity of the magnetic field directly affects the quality of the signal,and the study of high homogeneity magnets is a difficult and hot point for system development.For PMR relaxometry,a novel and efficient magnet design that meets the scenario application requirements is the focus of instrument development.In this paper,we will investigate the specific magnets applied to three different PMR application scenarios from the magnet perspective,including high homogeneity magnet,high depth magnet,and logging magnet.The details are as follows.1)Study of high homogeneity Halbach magnetsTo address the problem of homogeneity degradation due to processing errors in the practical implementation of high performance Halbach magnets,this paper proposes the PS(Passive Shimming)arrays that can generate each harmonic terms up to 3rd order based on the spherical harmonic function expansion method,and analyzes the homogeneity accuracy of PS arrays of different sizes and positions.The effectiveness and reproducibility of the proposed method for improving the magnetic field homhogeneity was verified by optimizing the number of each group of PS arrays in two sets of Halbach magnets,and the magnetic field homogeneity improved from 811 ppm and 1103 ppm to 4.7 ppm and 125 ppm,respectively.In addition,the PS mechanism of moving main magnet is explored and analyzed.This part is the difficult and innovative point of this paper.2)Single-sided magnet design for fatty liver detectionAccording to the requirements of permanent magnet used in the in vivo MR detection of human fatty liver,after analyzing the limitations of the U-shaped magnet,the structural improvement and dimensional optimization such as magnet grouping,introduction of PS blocks and optimization of the iron yoke were carried out to finally design and realize a high depth measurement magnet with a detection depth of 8 cm,gradient of 73.5 Gs/cm and field strength of 0.0725 T.By combining other hardware of MR system to build the prototype and the human fatty liver detection simulation platform,the relaxation signals of the phantom with different fat content are measured,and the effectiveness of the magnet for detecting samples with different fat content and the possibility of human body detection in vivo are analyzed and verified.This part is the innovation of this paper.3)Design of magnetic resonance logging magnetsBased on the improved Coulomb method,the influence of the inner diameter,length and spacing of the magnet in the Inside-out magnet on the magnetic field performance is analyzed.Combined with the finite element simulation verification,a model with comparable magnet performance with commercially available on-demand MR logging instruments was designed,and a magnet with a measurement depth of 21.5 mm and a field strength of 114 Gs was obtained according to the reduced realization and PS.A design method was developed from the optimization of the magnet model calculation to PS.The PS arrays proposed in this paper has been applied to several Halbach magnets in the laboratory,which has significantly improved the magnetic field homogeneity.The single-sided magnet for in vivo detection of human fatty liver has been installed and applied to human experiments in succession.For the logging magnet,the system will be built and applied accordingly in the next step,and these works are of great significance to promote the research of permanent magnets in PMR system in China. |
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