Magnetic Shielding And Magnet Design For Magnetic Resonance Guidance Accelerator System

Cancer is the number one disease that threatens people's health.Radiotherapy is an important way to treat cancer.Comparing with traditional computed tomography,magnetic resonance imaging has the advantages of high resolution,no ionizing radiation and so on.The magnetic resonance guidance accelerator system can provide accurate located image,excellent soft tissue resolution,tumor range / location / contour information and motion information to achieve better therapeutic effect.Therefore,the magnetic resonance guidance accelerator system has great clinical significance and prospect.One of the main technical challenges when integrating magnetic resonance imaging(MRI)system with medical linear accelerator is the strong interference of fringe magnetic fields from the MRI system with the electron beams of linear accelerator,making the linear accelerator not to work properly.In order to minimize the interference of magnetic fields,a magnetic shielding cylinder with an open structure made of high permeability materials is designed.ANSYS Maxwell is used to simulate Helmholtz coil which generate uniform magnetic field instead of the fringe magnetic fields which affect accelerator gun.The parameters of shielding tube,such as permeability,radius,length,side thickness,bottom thickness and fringe magnetic fields strength are simulated,and data is processed by MATLAB to compare the shielding performance.The design of magnetic resonance superconducting magnets for magnetic resonance accelerator system,which is not only necessary to satisfy the requirements of general magnetic resonance superconducting magnets,including the uniformity and magnetic leakage range,but also the problem that the beam passes through the magnet will reduce the dose and the position where the magnetic field of the electron gun and the acceleration tube will affect the normal operation of the accelerator need to be considered.Firstly,we used the 0-1 integer linear programming algorithm and made use of the least length of the superconductor as the objective function.Then,considering the requirement of magnetic field uniformity and leakage field,we opened a window at the center of the magnet which along the axis and increased the constraint to make the magnetic field of electron gun and the accelerating tube's position small enough.After these steps,we got the initial current density distribution of the superconducting magnet which satisfy our requirements.Then we changed the initial current density distribution of superconducting magnet into rectangular shape to get the final distribution parameter of the coil.Using a vectorization technique to optimize coil magnetic field calculations,which reduce calculation time.In order to make the superconducting magnet uniform,we used genetic algorithm,multigroup genetic algorithm and designed genetic-powell algorithm to optimize the magnet parameters.The magnetic shielding cylinder designed in this paper shows the shielding effectiveness table with different side thickness and bottom thickness at the optimal radius and length,which can make the accelerator output without loss.The designed magnet meets the engineering requirements of the magnetic flux leakage limit,the opening meets the requirement of the accelerator beam pass,and the magnetic field of the accelerator is limited to 20 gauss.Within the 500 mm diameter area,the uniformity has reached 10 ppm.The above design meets the needs of magnetic resonance guided accelerator systems.