Parallel Computing Method Research Of The Open NMR Magnet Design

Common intracranial diseases such as encephalitis，intracranial tumors and braininjuries, with features of sudden strong, fast development of disease course, high fatalityrate, and also immovable of patient, will cause intracranial edema or hematoma,therefore, on clinical a small, portable, real–time monitoring devices is needed fordoctors in real time observing intracranial condition in order to determine or change thetreatment of programmes, or saving lives.The current common diagnostic equipment such as CT, MRI and Meg machine,with a imaging of high resolution, but can not provide bedside real–time monitoring forpatients, Electrical Impedance Tomograpy（EIT）, Magnetic Induction Tomograpy（MIT） technology which were small in size, portable and with open structure, can beused for real–time monitoring, but with a low imaging resolution and poor repeatability.Considering the shortcomings of existing monitoring equipment, main purpose ofthis thesis is aimed at bedside real-time monitoring of intracranial disease, and to designan open NMR main magnet structure. Within the thesis, the key of the research wasfocus on the theory and method of permanent magnet field calculation, structure designmethod of open NMR’s main magnet, parallel computing of the permanent magneticfield, and preliminary study of NMR experiment. Specific content includes thefollowing several aspects:1）Theoretical research on permanent magnet magnetic field calculation. As thebasis for open NMR magnet designing, this thesis focus on two aspects of thepermanent magnet’s magnetic field calculation based on the magnetic charge equivalentmodel: First, first order–finite element method of magnetic field calculation of thearbitrary permanent magnet; Second, second order–finite element method of magneticfield calculation of permanent magnet. Single rectangular permanent magnet’s magneticfield was calculated by the analytical method, first–order FEM method andsecond–order FEM method, the calculation result indicated that the error of themagnetic field can be reduced about one times by the second–order FEM methodcompared with the first–order FEM method, which can be controlled about2%.2）Magnet structure designing of the open NMR. In order to design the open mainmagnet structure, the closed Halbach magnet structure was opened to generate asemi–ring magnet structure, a magnet structure adjustment and optimization methodwas proposed to obtasin a static main magnetic field in object region of 10mm×50mm×50mm in which the field is flat distribution in horizontal direction andgraded distribution in depth direction, with range of0.0630~0.1635T, average of0.1071T, gradient of2.01mT/mm.3）Parallel computing of the magnetic field. In this thesis, five commercialcomputers and gigabit internet were hired to build the parallel cluster, the Jacobi matrixpreconditioned conjugate gradient method was carried out to parallelism solve largesparse symmetric positive definite linear equations which was generated by FEMmethod, the follow four aspects was detailed study: First，derived data type was definedto complete the communicating between the computing nodes, based on of which theparallel sparse matrix vector multiplication was carried out; Second, a static loadbalanced method was proposed by evenly distributed whole the nonzero elements ofFEM stiffness matrix to all computing nods, in order to balance the main load andreduce the barrier time; Third, mesh grid was renumbered by the proposed modified ADalgorithm in order to reduce the communication time also the barrier time, and speed upof4was obtained when parallel computing the equations; Fourth, the parallelcomputing software of permanent magnetic field was constituted by using theJacobi–CG as the solver, added with the geometry creation with code, model meshingalgorithm, and post–processing module. At last, only1925seconds was consumed whendesigning the magnet structure by parallel computing software. Speed up of3.14wasobtained compared with the6042seconds of sequential computing time, and also theefficiency of30times improvement was achieved compared with matlab program.4）Mearment of the designed magnet and experiment study of the open NMR. First,the real magnet structure was constructed, the magnetic field distribution of the ROIwith dimension of10mm×50mm×50mm was measured on the three dimensionalmagnetic field measurement platform, as a result, the magnetic field range was0.0630~0.1642T, the average was0.1077T, the gradient strength was2.03mT/mm, theactual measured magnetic field distribution agreed with calculated magnetic fielddistribution; Second, the NMR system was set up with NMR spectrometer, radiofrequency amplifier, main magnet and radio frequency coil, for sample with rubber,three NMR experiment were conducted to demonstrate the feasibility of the magnetdesigning methods and the available of the design magnets.