Research On The Design Of Shimming Coils For MRI System Of Permanent Magnet Type

In our country, due to low cost and convenient maintenance, the magnetic resonance imaging system of permanent magnet type have been most widely used. Magnetic field uniformity is the crucial technical parameters that affect the equipment's quality.However, with the influence of raw ferromagnetic material and processing technology, magnetic fields is often inhomogeneous.The equipment defects greatly reduce image quality and finally hinder illness diagnoses. In order to solve this problem, the researchers have done lots of research. Among of all works, using external equipment to correct the main field is considered the most simple and effective way.In this paper, we will pay attention to designing shim coils in the 20cm diameter spherical volume field for magnetic resonance imaging system.We can use a set of auxiliary equipment and design a certain shape shimming coils in the magnetic field.Then adjust the current of the coil to generate the magnetic field components in different directions. Finally the main magnetic field in the region becomes more uniform. This paper introduces the theory about magnetic resonance imaging.Three conditions must be needed for magnetic resonance imaging.Then the paper gives a brief introduction to the general structure of magnetic resonance imaging system.Learning from the electromagnetic theory, we can get the series expression of magnetic field intensity of Z axis.With discussing Taylor series expansion of magnetic field generated through the arc, the methods of the design of radial and axial shimming coils can be found. Designing shimming coils not only need pay attention to analyzing the characteristics of the non-uniform magnetic field produced by permanent magnet, but also require a combination of shim coils for engineering application. In order to optimize the design, we use the Matlab simulation software, through the design of specific parameters, find the relationship among the radius of shim coils,the plate spacing and the magnet field linear. Finally, on the basis of the coils in the low-level,the paper analyzes the design direction of the high-level coils.