Low-field nuclear magnetic resonance magnet（2 MHz） is required for rock core analysis. However, due to its low field strength, it is hard to achieve a high uniform B0 field only by using the passive shimming. Theref...Low-field nuclear magnetic resonance magnet（2 MHz） is required for rock core analysis. However, due to its low field strength, it is hard to achieve a high uniform B0 field only by using the passive shimming. Therefore, active shimming is necessarily used to further improve uniformity for Halbach magnet. In this work, an equivalent magnetic dipole method is presented for designing shim coils. The minimization of the coil power dissipation is considered as an optimal object to minimize coil heating effect, and the deviation from the target field is selected as a penalty function term. The lsqnonlin optimization toolbox of MATLAB is used to solve the optimization problem. Eight shim coils are obtained in accordance with the contour of the stream function. We simulate each shim coil by ANSYS Maxwell software to verify the validity of the designed coils. Measurement results of the field distribution of these coils are consistent with those of the target fields.The uniformity of the B0 field is improved from 114.2 ppm to 26.9 ppm after using these shim coils.展开更多
For a superconducting magnet of magnetic resonance imaging （MRI）, the novel approach presented in this paper allows the design of cylindrical gradient and shim coils of finite length. The method is based on identifi...For a superconducting magnet of magnetic resonance imaging （MRI）, the novel approach presented in this paper allows the design of cylindrical gradient and shim coils of finite length. The method is based on identification of the weighting of harmonic components in the current distribution that will generate a magnetic field whose z-component follows a chosen spherical harmonic function. Mathematical expressions which relate the harmonic terms in the cylin- drical current distribution to spherical harmonic terms in the field expansion are established. Thus a simple matrix inversion approach can be used to design a shim coil of any order pure harmonic. The expressions providing a spherical harmonic decomposition of the field components produced by a particular cylindrical current distribution are novel. A stream function was applied to obtain the discrete wire distribution on the cylindrical-surface. This method does not require the setting of the target-field points. The discussion referring to matrix equations in terms of condition numbers proves that this novel approach has no ill-conditioned problems. The results also indicate that it can be used to design cylindrical-surface shim coils of finite length that will generate a field variation which follows a particular spherical harmonic over a reasonably large-sized volume.展开更多
A novel kind of SC（superconducting） solenoid coil is designed to satisfy the homogencity requirement of the magnetic field.In this paper,we first calculate the current density distribution of the solenoid coil secti...A novel kind of SC（superconducting） solenoid coil is designed to satisfy the homogencity requirement of the magnetic field.In this paper,we first calculate the current density distribution of the solenoid coil section through the linear programming method.Then a traditional solenoid and a nonrectangular section solenoid are designed to produce a central field up to 7 T with a homogencity to the greatest extent.After comparision of the two solenoid coils designed in magnet field quality,fabrication cost and other aspects,the new design of the nonrectangular section of a solenoid coil can be realized through improving the techniques of framework fabrication and winding.Finally,the outlook and error analysis of this kind of SC magnet coil are also discussed briefly.展开更多
基金Project supported by the State Key Development Program for Basic Research of China(Grant No.2014CB541602)the National Natural Science Foundation of China(Grant Nos.51677008 and 51707028)the Fundamental Research Funds of Central Universities,China(Grant No.106112015CDJXY150003)
文摘Low-field nuclear magnetic resonance magnet（2 MHz） is required for rock core analysis. However, due to its low field strength, it is hard to achieve a high uniform B0 field only by using the passive shimming. Therefore, active shimming is necessarily used to further improve uniformity for Halbach magnet. In this work, an equivalent magnetic dipole method is presented for designing shim coils. The minimization of the coil power dissipation is considered as an optimal object to minimize coil heating effect, and the deviation from the target field is selected as a penalty function term. The lsqnonlin optimization toolbox of MATLAB is used to solve the optimization problem. Eight shim coils are obtained in accordance with the contour of the stream function. We simulate each shim coil by ANSYS Maxwell software to verify the validity of the designed coils. Measurement results of the field distribution of these coils are consistent with those of the target fields.The uniformity of the B0 field is improved from 114.2 ppm to 26.9 ppm after using these shim coils.
基金Project supported by the National Natural Science Foundation of China (Grant No. 60871001)
文摘For a superconducting magnet of magnetic resonance imaging （MRI）, the novel approach presented in this paper allows the design of cylindrical gradient and shim coils of finite length. The method is based on identification of the weighting of harmonic components in the current distribution that will generate a magnetic field whose z-component follows a chosen spherical harmonic function. Mathematical expressions which relate the harmonic terms in the cylin- drical current distribution to spherical harmonic terms in the field expansion are established. Thus a simple matrix inversion approach can be used to design a shim coil of any order pure harmonic. The expressions providing a spherical harmonic decomposition of the field components produced by a particular cylindrical current distribution are novel. A stream function was applied to obtain the discrete wire distribution on the cylindrical-surface. This method does not require the setting of the target-field points. The discussion referring to matrix equations in terms of condition numbers proves that this novel approach has no ill-conditioned problems. The results also indicate that it can be used to design cylindrical-surface shim coils of finite length that will generate a field variation which follows a particular spherical harmonic over a reasonably large-sized volume.
基金Supported hy National Natural Science Foundation of China(91026001)High-tech Projects
文摘A novel kind of SC（superconducting） solenoid coil is designed to satisfy the homogencity requirement of the magnetic field.In this paper,we first calculate the current density distribution of the solenoid coil section through the linear programming method.Then a traditional solenoid and a nonrectangular section solenoid are designed to produce a central field up to 7 T with a homogencity to the greatest extent.After comparision of the two solenoid coils designed in magnet field quality,fabrication cost and other aspects,the new design of the nonrectangular section of a solenoid coil can be realized through improving the techniques of framework fabrication and winding.Finally,the outlook and error analysis of this kind of SC magnet coil are also discussed briefly.
