Basic Research On Key Technologies Of Ultra-low Filed Resistive-magnet MRI System For Brain Imaging

As an acute cerebrovascular disease,stroke has the characteristics of high disability rate and high mortality rate.At present,this disease has become the first cause of life injury and the third cause of death in China.With the gradual acceleration of the aging process of population in China,the incidence and mortality of stroke will be further increased,which will bring great challenges to the healthy development of our economy and society.Screening and timely diagnosis of patients by medical imaging technology can effectively reduce the incidence and mortality of stroke.Magnetic resonance imaging(MRI)technology plays a key role in the diagnosis of stroke;nevertheless,most of the MRI systems equipped in clinic are superconducting MRI systems,which has huge volume,weight and high technical complexity.Neither cost and efficiency are not competent for large-scale diagnosis and screening.The ultra-low field MRI system reduces the volume and weight of the imaging system by reducing the background magnetic field.The imaging system can be applied to various scenes such as ward,ambulances,which provides a new means for timely diagnosis and screening of stroke.The aim of this thesis is to reduce the weight and volume of MRI system and enhance the ease of use of MRI technology in stroke diagnosis and screening.This thesis mainly studied the optimum design method and theory of electromagnetic system of MRI system;to validate the optimization method and the function of the prototype of electromagnetic system,imaging under ultra-low magnetic filed is also studied.The work accomplished in this thesis includes:1 Study on design of an open resistive electromagnet and processing error sensitivity of different electromagnetic body structure.To ensure the convenience of patient transfer and the comfort of patient during scanning,the open and disc-shaped electromagnet structure was chosen.A hybrid method for electromagnet optimization is proposed,which consists of linear programming,coil bundle adjustment and nonlinear optimization.MRI has high requirements for magnetic field homogeneous,in order to control the negative effect of processing error on the magnetic field of electromagnet,the effect of processing error on the magnetic field homogeneous of different structures is analyzed which support that electromagnet with 3 coil bundles has the lowest processing error sensitivity.The prototype of electromagnetic body is also built,the designed maximum current is 50 A,the designed maximum magnetic field is 10 m T,the radius of spherical region of interest is 150mm,the designed magnetic field inhomogeneity is 15 ppm.With 50 A current excitation,the measured magnetic field is less than 80 ppm.2 Optimization of gradient coil based on equivalent magnetic dipole method and study of geometric parameters of gradient coil.In gradient design,two points should be considered:one is to design gradient coil according to the structural characteristics of electromagnet,the other is to increase the slew rate of gradient coil as much as possible and thus shorten the time of magnetic resonance scanning.In this thesis,the mathematical model of gradient coil optimization is established based on the equivalent magnetic dipole method.The design intention is included in the mathematical model by partitioning the feasible district winding gradient coil,constraining the magnetic field in ROI and the electromagnetic energy of gradient coil.The optimization method of gradient coil can only be carried out under the given geometric parameters(size,spacing,coil pattern,density).In order to ensure the rationality of the given geometric parameters,this work reveals the influence of geometric parameters on the performance of gradient magnetic field by finite-element-simulation(FEM)analysis.Finally,the optimum gradient coil is designed and corresponding prototype is also built,the magnetic field efficiency of the gradient coil in different directions is greater than 19?T/(m·A),and the measurement non-linearity is less than 3%.3 Optimization of the structure of a head RF coil and study on its performance parameters.In order to improve the signal-to-noise ratio(SNR)of RF coils as much as possible,a close-fitting head RF coil structure is proposed.The head surface is irregular,to optimize the wingding pattern of RF coil on an irregular surface,a linear programming method based on FEM simulation is proposed.The electromagnetic calculation of the fixed coil position is carried out by FEM simulation,and then the winding pattern is determined by linear programming.After the study on coil design method,the relationship between the magnetic field homogeneity and SNR of RF coil is analyzed by FEM simulation which revealed that the RF magnetic field homogeneity and SNR of the close-fitting head RF coil are a pair of competitive attributes.Finally,an optimal RF coil is designed and fabricated,and the inhomogeneity of the optimal RF magnetic field in ROI is less than 25%.4 Function verification of imaging system and study on ultra-low field quantitative imaging.After completing the construction of the electromagnetic system of ultra-low field MRI system,this thesis verifies the design method of electromagnetic system and the function of electromagnetic system prototype through head-shaped phantom and human brain imaging.On the basis of the verification of imaging system,the method of ultra-low field T₁quantitative imaging is further studied,the T₁parameter measurement and T₁quantitative imaging for copper sulfate phantom verify the feasibility of magnetic resonance quantitative imaging and imaging acceleration at ultra-low field.In summary,the design method and theory of electromagnetic system of ultra-low field MRI system is systematically studied;with which the prototype of ultra-low field MRI system is also built.The function of the prototype is verified by imaging experiment;quantitative MRI under ultra-low field is explored and realized in a filed of less than 10m T eventually.