| As a worldwide disease with a mortality and disability rate much higher than the average,the effective stroke treatment is to adopt the method of "early diagnosis and early treatment";therefore,rapid screening,pre-diagnosis,bedside monitoring,and monitoring are strokes There is an urgent need for patient treatment and control of the disease.At present,small-scale mobile CT is mainly used clinically to meet the purpose of rapid screening and pre-diagnosis;for the monitoring and monitoring of diagnosed patients,pressure sensors are implanted to monitor intracranial pressure to monitor the development of stroke in real-time.However,these two methods have certain limitations.The ionizing radiation of the small mobile CT has a more significant impact on the human body.It cannot clearly distinguish between hemorrhage and ischemic stroke,so it can only be diagnosed by the "exclusive" method;pressure sensor.The intracranial implantation method caused secondary injury to the patient.It not only has the risk of infection but also has low sensitivity.It cannot show the patient’s stroke development status and cannot provide sufficient diagnostic reference information for further stroke treatment.In summary,there is an urgent need for a non-radiation,non-invasive stroke diagnosis and treatment equipment to solve the current diagnosis and treatment difficulties.It can provide medical staff with accurate and effective stroke disease information and reduce stroke mortality and disability and provide safer and more reliable disease detection methods for stroke patients.Magnetic resonance imaging technology is widely used in the medical field due to its non-invasive characteristics(MRI),and for stroke,magnetic resonance has better imaging effects and more evident image contrast than CT.With its clear imaging quality and fast imaging speed,the current medical magnetic resonance system mainly uses superconducting high-field magnetic resonance imaging systems(1.5T,3.0T)and permanent magnetic medium and low-field magnetic resonance imaging(0.7T,0.35 T,etc.).However,whether it is a superconducting system or a permanent magnet system,which weighs tens of tons and covers a large area,it also needs to build electromagnetic shielding and magnetostatic shielding areas.These restrictions make it difficult to miniaturize and lighten and cannot be used as a mobile device.This thesis aims to design an ultra-low-field magnetic resonance imaging system for stroke imaging and realize the theoretical design of a set of dual-radius ultra-low-field magnetic resonance electromagnetic coils,gradient coils,and shim coils.It carries out engineering practice on the design results,evaluation,and performance testing.The research content is as follows:(1)Dual-radius ultra-low field MRI magnet for brain imaging: Compared with the current research status of lightweight magnetic resonance for brain imaging at home and abroad,the dual-radius electromagnet structure was finally selected.In this paper,an electromagnetic head magnetic resonance imaging magnet is studied,and a double-radius electromagnet for head imaging is designed.First,the current status of electromagnets used for ultra-low-field magnetic resonance imaging is compared,and the mechanical structure of dual-radius electromagnets used for head imaging is described;Then dual-radius electromagnet coil structure is mathematically modeled,and design requirements parameters are set through the inverse problem method;Next,morphological methods are used to deal with the irregular and overlapping current clusters calculated in the quadratic programming and convert the irregular-shaped current clusters into regular shapes;Finally,considering the homogeneity degradation caused by artificial treatment and the winding methods,the genetic algorithm is used to optimize the structural parameters.After the optimal algorithm is accomplished,a dual-radius MRI electromagnet with 7ppm over 260 mm region of interest is obtained.The electromagnet background magnetic field is up to 20 m T,and the weight of the whole structure is about 132.88 kg.Sensitivity analysis is carried out according to the designed main magnet structure to evaluate the effect of engineering errors on the magnetic field uniformity during actual manufacturing.In the repeated evaluation of sensitivity,the maximum value of magnetic field uniformity is 206.88 ppm,and the average value of magnetic field uniformity is 112.81 ppm.(2)Research on the electromagnetic inverse problem of the dual-radius magnet gradient coil.After the dual-radius main magnet coil is designed according to actual requirements,its matching gradient field system needs to be designed according to the main magnet geometry.First,design the gradient coil wiring space according to the magnetic resonance main magnet structure,and perform three-dimensional modeling of the wiring area;Then use the electromagnetic field inverse problem method to construct the corresponding magnetic field objective function of the gradient coil;Next,customize the constraint equation according to the coil dimension parameters and material requirements;Finally,select the corresponding solution method to calculate the optimization problem.Simultaneously,the influence of actual machining and assembly errors on the linearity of gradient coils is discussed.(3)Research on the electromagnetic inverse problem of dual-radius magnet shimming coil.Considering the influence of actual engineering errors on the main magnetic field’s uniformity,the shimming coil group needs to be designed accordingly.The shimming coil is designed according to the worst case of the main magnetic field’s sensitivity analysis.The second-order shimming coil includes five sets of coils,XY,XZ,YZ,X2-Y2(named as X2 coil),and 2Z2-X2-Y2(named as Z2 coil).Through the spherical harmonic function expansion of the magnetic field in the main magnet’s target area,each active shimming coil group is used to cancel the high-order coefficients of the spherical harmonic function to achieve the final shimming goal.In this paper,a dual-radius ultra-low field MRI system was all-around researched,based on the works of electromagnetic components design theory,performance testing,and engineering evaluation.It will provide a new possible to earlier prehospital diagnosis and bedside monitoring for stroke. |
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