| Magnetic resonance imaging(MRI)is another significant progress in the field of medical imaging after computed tomography(CT),which has experienced a development process from theory to practice,form to function,macro to micro.Highfield MRI has higher signal-to-noise ratio and faster resonance frequency,which is more conducive to high-resolution imaging of MR signals.Aimed at the activity of human cerebral cortex neural network,the structure and function of the neural system,and the human consciousness and emotional mechanism,high field MRI with non intrusion and fast response characteristics can be used tracking brain signal changes in real-time.For the current view,it is the most likely technology to decrypt intelligent information for human brain.However,the magnetic field deviation is larger for highfield MRI.How to ensure that the imaging area having a stable magnetic field with high uniformity,so as to obtain accurate phase space information in the image reconstruction stage,and realize the synchronization of time and space to achieve fast high-resolution scanning is always a difficult problem in MRI magnet design and engineering implementation.For this purpose,this paper carried out the following research:Firstly,starting from the electromagnetic structure design of large magnet,three design schemes of 14T MRI magnet are given in detail,and the heat transfer,mechanical stability,inductance,structure complexity,uniformity control and economical efficiency are compared between the schemes,and the best scheme is selected for this research problem.Compared with the wire wound magnet,it is more difficult to control the calculated deviation of the magnetic field for the large cross section composite conductor with multi-wire implanted in the copper groove.Therefore,an integral idea of current core is proposed to achieve high precision magnetic field calculation.Due to the inevitatble winding pattern of turn-to-turn transition area,which for most of electromagnetic design of MRI magnet,it is ignored,regarding all turn as independent ideal circle,however it should not be neglected for composite conductors with large current flow through the transition area.Therefore,a semicircle and double center electromagnetic integral strategy is devised to effectively solve the turn-to-turn transiton deviation of calculation for magnetic field.For the loss of homogeneity caused by the three dimensional complex structural deviation of the background magnet,high precision spherical harmonic calculation theory is established.The spherical harmonic characteristics and level is evaluated due to the different deviation.Different types of deviation corresponding harmonic field characteristics are obtained.Combined with the actual shimming ability,the actual manufacture tolerance can be determined.According to the non-uniform harmonic order calculated by the background magnet field,an active shimming coil with corresponding harmonic order is designed using the target field theory.The magnetic field to be solved is taken as the known condition,and the current density distribution of the coil is deduced inversely.Then the theory of flow function is used to discretize the region of current density so as to obtain the final structure of shimming coil winding.When the coil is designed using the target field method,it is inevitable to consider how to solve the first kind of Fredholm equation.In order to eliminate its ill-condition,a regularization idea is adopted to find the best solution by using the inflection point of the L-curve.In addition,when the source points of the two-dimensional Green’s function coincide with the field point,the singularity will appear in the integral,which will lead to the distortion of the shimming coil winding.Therefore,the multi-dimensional singularity integration problem is discussed,and an effective Monte Carlo integration idea is selected to effectively avoid the integral singularity.The passive ferromagnetic piece placed directly in the inside diameter of high field MRI will be saturated(according to its magnetization B-H curve,under the magnetic field of 0.5 T,ferromagnetic material would reach magnetization saturation state).Then the passive ferromagnetic piece will have the ability of self generating magnetic field.it can be considered to be composed by a series of small loops of current which can be called magnetic dipole from the micro view of level.The magnetic field expression of the magnetic dipole to the space field point is established,and then the exact magnetic field contribution of the final ferromagnetic piece is obtained by integrating every position of the magnetic dipole.Based on this,the magnetic field contribution and harmonic contribution of each passive ferromagnetic shimming piece to each field point in DSV are deduced and the sensitivity matrix is established.Then the linear programming is constructed based on the harmonic and magnetic field to determine the thickness and position of the passive ferromagnetic shimming pieces.Good optimization algorithm in electromagnetic design can improve the homogeneity fast and better.Therefore,three kinds of algorithm design frameworks for solving optimal problems are given:unconstrained definite optimization,constrained definite optimization and stochastic intelligent algorithm optimization respectively.Within the three kind of algorithm frameworks,the gradient descent method、Newton method、conjugate gradient method、BFGS、DFP、linear programming(simplex and interior-point method)、nonlinear programming(sequential quadratic programming and obstacle interior-point method),the particle swarm method are established.The implement processes and procedures for the specific algorithms are given.Comparing with existing software,the efficiency of the algorithm design has been proved. |
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