| In this paper,based on the harmonic analysis method,by introducing the power consumption equation,gradient coils and shim coils with low inductance,high linearity and high efficiency are designed to meet the needs of portable MRI based on Halbach array magnet.The specific results are as follows:(1)Development of Halbach type gradient coils and high order shim coils with high efficiency and low power consumption:Based on the classical target field method,the harmonic analysis method is improved,and the Green's function is introduced to establish the corresponding model of harmonic coefficient of magnetic field and current density.In order to reduce the heating of gradient coil,the power consumption equation is introduced.Through regularization and nonlinear programming method,Halbach type gradient coils with high efficiency and low power consumption are designed.COMSOL software is used to simulate the target magnetic field distribution,heat transfer,stress and strain of gradient coil.The difference of positive and negative wiring and the advantage of introducing power consumption are obtained by thermal imager.The resistance of three directional gradient coils fabricated with flexible PCB is less than 0.5?,the inductance is lower than 5 u H,the linearity is more than 95%,and the coil efficiency is more than 108)8)-1-1.Secondly,in order to improve the space utilization,the flexible PCB is used for forward wiring,and each shim coil is simulated and tested by Gauss meter.Relying on the gradient skeleton of 3D printing,a set of high-order active shim device for Halbach magnet is developed,which can meet the requirements of later active shimming.(2)Construction of gradient coils and shim coils NMR experimental platform:in order to test the gradient coils and shim coils,a set of high stability Halbach MRI platform is built.It includes the design of widening coil probe,the development of preamplifier and the integration of instrument circuit.Taking vegetable oil as the sample,the active shimming test was carried out.The magnetic field uniformity is improved from 39.5 ppm to 1.98 ppm in the range of?3mm×3mm;In the range of?20mm×20mm,the magnetic field uniformity increases from 358.57 ppm to 21.1ppm.(3)Imaging experiments of gradient coils and shim coils:by making a standard sample of 2 mm water film and using one-dimensional imaging pulse to obtain the spin echo signal of NMR,the efficiency of X and Y gradient coil is calculated to be 13.158)8)-1-1 and 13.348)8)-1-1,respectively.To further verify the applicability of the formula,the z-gradient coil is tested with vegetable oil as a sample in a test tube and its efficiency is 10.508)8)-1-1.The test results show that the gradient efficiency is basically consistent with that measured by Gauss meter.Finally,the gradient coil is used for two-dimensional imaging of standard samples.The imaging results show that the size of the sample is basically consistent with the real object,and the water film can also be observed.Therefore,the reliability of the gradient coil and shim coil designed in this paper is proved. |
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