Research On MRI Magnet-system With CAD Method

With rapid development of modern medical imaging technology, it has been a necessary clinical diagnosis approach to get symptom information from human organic imaging. MRI (Magnetic Resonance Imaging) is a medical imaging method which has lots of advantages such as multi-parameter imaging, no ionizing radiation, arbitrary direction imaging and so on. MRI is based on NMR (Nuclear Magnetic Resonance) theory, so static magnetic field is required for imaging. In MRI system, magnet system is the main part that generates imaging magnetic field. Its performance is an important factor to affect image quality. Therefore, the research on methods for magnet system has important theoretical and applied values. Magnet system is mainly composed of magnet and shimming component. Magnet's responsibility is to generate homogeneous magnetic field in imaging work space. Shimming component is used to improve magnetic field homogeneity to satisfy MRI requirement.Because the research work on MRI theory is started late in our country, we have lagged behind in this technology field. At present, most MRI equipments need to be imported from foreign companies. To improve the capability of MRI system made in China, this thesis has a deep research on design methods for permanent magnet system. The investigation contents include four parts: magnetic circuit design of permanent magnet, magnet configuration optimization, passive shimming calculation and active shimming design. Primary innovation works are described as the following:(1) Magnetic circuit design of permanent magnetIn MRI system, permanent magnet's configuration is decided by imaging magnetic field performance parameters. In this thesis, we combined magnetic circuit design theory and FEA (Finite Element Analysis), and had a detailed research on permanent magnet design method. Using simulation analysis method, we studied the magnetic field distribution characteristic of permanent magnet. It made sure that magnetic circuit configuration and magnet performance achieved the MRI requirement. In addition, we designed shim-loop and trapezoid shim-board to improve the magnetic field homogeneity and magnetic induction intensity.(2) Permanent magnet configuration optimizationTo lighten the weight of permanent magnet, this thesis presented a simulation method to study and improve the optimization technique of magnet configuration. With finite element method, we analyzed the magnetic field distribution of permanent magnet. Based on the magnetic field distribution characteristic, the redundancy parts of magnetic circuit configuration were removed accurately. The magnet weight was lightened significantly. Moreover, we optimized the shim-loop configuration. This made the imaging magnetic field more homogeneous.(3) Passive shimming calculationGenerally, the original magnetic field homogeneity of permanent magnet can't satisfy MRI requirement. Passive shimming is usually used to correct the magnetic field. In this thesis, we presented a novel algorithm to calculate the position and number of shimming ferromagnetic elements. With magnetic induction intensity samples, we analyzed imaging magnetic field distribution characteristic. Based on electromagnetic theory, we established the correcting magnetic field mathematical model, and proposed a localizing method to program the shim radius. To reduce the quantities of the ferromagnetic material and avoid repeated correction, successive approximation was presented to calculate the number of shimming ferromagnetic elements. The experiment results show that this method can effectively guide passive shimming work and obtain highly homogeneous magnetic field.(4) Active shimming designThe magnetic field homogeneity will descend when temperature changes. Generally, active shimming is used to re-correct magnetic field. We proposed a set of tabulate shim coils to improve the imaging field homogeneity for permanent magnet. By studying the magnetic spherical harmonics and its Taylor expansion, the proposal of designing lower-order radial coils was derived including x, y, xz, xz~2, x~2-y~2 etc. In addition, a novel composite design was presented for the axial coils. By controlling the shim currents of axial coils, different axial magnetic fields can be obtained. This method simplified the design process of axial coils, and made the axial correcting magnetic field more precise.(5) Shim current calculationWhen shim coils are excited by appropriate current, they can generate correcting magnetic field to compensate the imaging magnetic field unhomogeneity. Based on the indirect calculation method, we proposed a novel iterative optimization algorithm to calculate shim current. Because there is a direct proportion relationship between FID (Free Induction Decay) signal and magnetic field homogeneity, we use the intensity of FID signal as feedback information to the calculation. Shim current can be worked out in less than 3 minutes. This satisfied the requirement that active shimming should be performed in real time.