Design Of Gradient Coils For Halbach Magnetic Resonance Imaging System

Halbach magnets are used widely,and they also have specific advantages for nuclear magnetic resonance,for example,the structures don't require an iron yoke and realize magnets with strong magnetic field,low weight,and small volume.In addition,the magnetic field at the center is much homogeneous,and the production is low-cost.Therefor,the Halbach magnet is the ideal structure for portable field MRI analysis and imaging devices,and also is a popular research direction for online NMR spectroscopy and imaging.Due to the fact that the direction of the magnetic field of the main magnetic field B₀ of the Halbach magnet is radial,which is different with superconducting and C-type permanent magnets MRI devices,and its'gradient coils are distributed on the cylinder.So coils of Halbach have special design requirements.In this dissertation,the equivalent magnetic dipole method is studied to design gradient coils and shim coils for a 2MHz Halbach magnetic.The details include the following aspects:?1?Study on equivalent magnetic dipole method to design gradient coils for Halbach magnetic.Based on the relationship between the stream function and the equivalent magnetization,the magnetic field distribution in the target region of the Halbch magnet was conbined to the stream function.Thus,the distribution of stream function on the cylindrical surface can be inversely solved according to the desired magnetic field,then the coil structures could be got.During the solution process,energy consumption is taken as the optimization goal,and the variance between expected magnetic field and calculated magnetic field is used as penalty function to obtain smooth structures of gradient coils.Afterwards,the magnetic field distributions of the coils were verified by Maxwell simulation software.Finally,the X,Y,and Z gradient coils were fabricated using double-layer flexible boards,and the magnetic field in the region of interest of each coil was actually measured by a Gauss meter.It was found that the magnetic field gradients generated by the X,Y and Z gradient coils were 3.022,3.013,and 1.7 mTm^-1A^-1,respectively,and gradient uniformity is less than 5%of the commercial requirements of gradient coil.?2?Study on active shim coils for Halbach magnet.The magnetic field uniformity of the 2MHz Halbach magnet is 108ppm after passive shimming,in order to further improve the uniformity,second-order shim coils are also designed through the equivalent magnetic dipole method,including XY,XZ,YZ,X²-Y² and 2Z²-X²-Y² coils.For the designed second-order shim coils,the magnetic field distributions were verified by Maxwell software,then eight coils were fabricated?including X,Y and Z gradient coils?.After that,active shimming experiments were finished,uniformity of the magnet was increased to 28 ppm,which achieved the requirement of uniformity in imaging experiments.?3?Study on imaging experiment of Halbach magnet.The above designed gradient coils were finally used to a preliminary study of Halbach magnet imaging experiments.In this process,two-dimensional spin echo sequence was used to perform imaging experiments.The acquired spin echo signals were filled with k-space and transformed by inverse Fourier.Cross-sectional vertical and sagittal images of cylindrical water sample was obtained eventaully.The successful completion of the imaging experiment and the shimming experiment proved the effectivty of the gradient coils and the second-order shim coils designed by the equivalent magnetic dipole method.The 2MHz low-field Halbach magnet is successfully used for imaging experiments,but the resulting image is not clear enough,and is slightly deformed.It is necessary to improve the structure of gradient coils and imaging sequence in the future research,and strength the research in imaging experiments.