Study On Superconducting Magnet For 1.5T Dedicated Neonatal Self Shielded Magnetic Resonance

Magnetic Resonance Imaging equipment is a medical Imaging equipment designed according to the principle of nuclear Magnetic Resonance(NMR).It has the advantages of being suitable for soft tissue Imaging,no ionizing radiation and high imaging resolution.MRI equipment has greatly promoted the development and progress of medical diagnosis technology.With the continuous development of China's medical level,specialized MRI equipment has become a key development direction in the field of medical equipment.Therefore,1.5T neonatal self-shielding MRI superconducting magnet was studied in this thesis.Strong magnetic field with high uniformity can improve the image signal-to-noise ratio of MRI equipment and shorten the signal acquisition time,so magnet is the key component of MRI equipment.Superconducting magnets have natural advantages in generating uniform and strong magnetic fields.Superconducting MRI equipment uses superconducting magnets to generate the required magnetic field,which has the advantages of high magnetic field intensity,small size,and self-shielding technology to reduce the range of stray field.In 1.5T neonatal MRI equipment,in order to ensure the imaging quality,the magnetic peak-peak uniformity in the target imaging space should be less than 8 PPM.In this paper,the passive uniform field method was used to simulate the magnetic field correction of the processed and installed magnets.The main research contents of this paper include the multi-objective optimization design of neonatal MRI superconducting magnets,the design of magnetic field passive uniform field algorithm in the imaging region of magnets and the simulation verification of structural stability of magnets with multi-physical field coupling.For the design of neonatal self-shielded superconducting magnets,an optimal design method based on the preformed finite element model combined with the second generation of multi-objective genetic algorithm(NSGA-?)is proposed in this paper based on the traditional MRI optimization design algorithm.First,linear programming algorithm is used to obtain the initial position of the magnet coil current cluster,obtain the layout of the magnet structure,then according to the initial structure parameters in business more than physical field finite element analysis software COMSOL main coil,establish magnet coil shield and ferromagnetic shield of initial model,build COMSOL links with the MATLAB platform,form the two-way data communication,Using the multi-physical field simulation function of COMSOL,the bidirectional coupled finite element analysis was carried out,and the optimal design parameters were selected from the Pareto optimal solution iteratively calculated in the NSGA-? algorithm,so as to realize the optimal design of 1.5T neonatal self-shielded superconducting MRI magnet.The designed and installed MRI superconducting magnet needs to passively homogenize the magnetic field in the target imaging area before actual operation.For the passive leveling algorithm,this paper proposes a nonlinear weighted multi-imaging space passive leveling algorithm based on the traditional passive leveling algorithm,which takes the weighted sum of the uniformity and the amount of leveling slice as the optimization objective.This leveling method can achieve the requirements of the system leveling field while controlling the total amount of leveling slice consumed to a minimum.And the magnetic field uniformity of the imaging space with smaller diameter is higher.The less number of uniform field films can avoid the influence of eddy current effect on the imaging,and the improvement of magnetic field uniformity in the smaller diameter imaging space is more conducive to the diagnosis of neonatal brain disease.In this paper,the above passive shimming algorithm and the traditional passive shimming algorithm were packaged and the passive shimming software was written.The software was used to conduct shimming experiments on the 1.5T universal MRI equipment of the project partner company to verify the practicability and accuracy of the design of the passive shimming software.In order to verify the structural safety of the theoretical design,a multi-physical field coupling simulation model of neonatal MRI superconducting magnets was built,which included ferromagnetic shielding,superconducting coils,skeleton structure and passive uniform field bands.The influence of mechanical support structure of brass,aluminum alloy and stainless steel on the electromagnetic stress and strain of superconducting magnets is simulated and analyzed,and on this basis,the mechanical support structure is improved to optimize the distribution of electromagnetic stress and strain of superconducting magnets,and the safety margin of the magnet structure is improved.The established magnet model theoretically verifies the stability and safety of the designed 1.5T neonatal self-shielded MRI superconducting magnet.