【作者】 武海澄；

【导师】 周荷琴；

【作者基本信息】 中国科学技术大学 ， 模式识别与智能系统， 2007， 博士

【摘要】 随着现代医学影像技术的快速发展，利用人体组织器官的影像进行临床诊断已经成为一种必不可少的诊疗途径，极大地促进了医疗水平的提高。磁共振成像就是其中一种新兴的医学影像方法，主要利用核磁共振原理对处于均匀静磁场中的人体器官进行成像，具有成像参数多、无电离辐射、可任意方向断层等诸多优点，在临床诊断中被广泛应用。在磁共振成像设备中磁体系统是一个重要的组成部分，负责产生均匀静磁场，为磁共振成像提供基础物理环境，其性能的优劣是决定磁共振图像质量的重要因素，也是衡量整个成像系统性能的主要参考指标，因此具有重要的研究意义。磁体系统主要由磁体和匀场部件两部分组成，其中磁体负责在成像空间产生一定强度的静磁场，匀场部件负责对磁场进行补偿校正，使磁场均匀度满足磁共振成像要求。由于我国的磁共振成像研究工作起步较晚，在这一领域的相关技术比较落后，目前成像系统仍然以设备引进为主。为了提高我国磁共振成像设备的国产化能力，本文结合我国磁共振成像设备的实际发展需要，针对性价比较高的永磁型磁体系统设计方法进行了深入研究，内容涉及到永磁体磁路设计、磁体结构优化以及成像磁场的无源匀场计算和有源匀场设计等，主要在以下几个方面进行了有特色和创新性的研究工作：(1)磁体磁路设计磁共振成像永磁体的结构需要根据系统对成像磁场的性能要求进行设计，本文将磁路计算理论与有限元分析方法相结合，利用磁体仿真模型对磁路结构及成像磁场进行分析研究，确保了磁场性能参数满足设计要求。另外针对永磁体成像磁场均匀性差的缺点，设计了匀场环部件用以减弱磁场的边缘效应，增强了成像磁场均匀性，同时改进了磁体匀场板结构，提高了磁场的磁感应强度。(2)磁体结构优化针对永磁体重量大的缺点，本文利用计算机仿真分析方法对磁体中的磁场分布特征进行了详细研究，根据精确的磁场分布数据对磁体结构进行优化，去除了磁体支架材料中的冗余部分，大幅减轻了磁体重量，同时对磁体匀场环的尺寸结构进行了优化设计，进一步提高了成像磁场的均匀性。(3)无源匀场计算由于永磁体的初始磁场均匀性通常无法满足磁共振成像要求，一般采用无源匀场方法对磁场进行校正。本文提出了一种利用磁场分布特征对匀场磁片进行综合规划的方法，根据成像磁场样本数据分析磁场分布特征，利用电磁理论建立磁片校正磁场的数学模型，以此为基础提出了极值定位方法对磁片匀场轨道半径进行规划，同时提出了逐次逼近算法进行匀场计算，减少了磁场的重复校正。该算法能够准确算出匀场磁片的校正位置及使用数量，有效地指导无源匀场工作。(4)匀场线圈设计当外界温度发生变化时成像磁场的均匀性会随之改变，通常采用有源匀场方法对磁场进行再校正。本文针对永磁体的组成结构提出了一种平板式匀场线圈设计方法，通过研究成像磁场的球谐函数以及通电圆弧在自由空间产生磁场的泰勒展开级数，导出了x、y、xz、xz~2、x~2-y~2等低阶径向线圈的设计方法，同时针对轴向线圈提出了一种新的混合式设计方案，能够通过控制线圈中的电流组合实现对不同轴向磁场分量的获取，简化了轴向线圈的设计过程，提高了校正磁场的准确性。(5)匀场电流计算对匀场线圈施加适当的电流后才能产生校正磁场消除成像磁场的非均匀分量，本文在匀场电流间接计算方法的基础上，利用磁共振成像原理中磁场均匀性与系统FID信号强度成正比的关系提出了一种电流迭代优化算法，将系统FID信号强度的变化作为反馈信息，能够快速计算出匀场电流对成像磁场进行补偿校正，平均耗时不到三分钟，满足磁共振成像系统对有源匀场的实时性要求。更多还原

【Abstract】 With rapid development of modern medical imaging technology, it has been a necessary clinical diagnosis approach to get symptom information from human organic imaging. MRI (Magnetic Resonance Imaging) is a medical imaging method which has lots of advantages such as multi-parameter imaging, no ionizing radiation, arbitrary direction imaging and so on. MRI is based on NMR (Nuclear Magnetic Resonance) theory, so static magnetic field is required for imaging. In MRI system, magnet system is the main part that generates imaging magnetic field. Its performance is an important factor to affect image quality. Therefore, the research on methods for magnet system has important theoretical and applied values. Magnet system is mainly composed of magnet and shimming component. Magnet’s responsibility is to generate homogeneous magnetic field in imaging work space. Shimming component is used to improve magnetic field homogeneity to satisfy MRI requirement.Because the research work on MRI theory is started late in our country, we have lagged behind in this technology field. At present, most MRI equipments need to be imported from foreign companies. To improve the capability of MRI system made in China, this thesis has a deep research on design methods for permanent magnet system. The investigation contents include four parts: magnetic circuit design of permanent magnet, magnet configuration optimization, passive shimming calculation and active shimming design. Primary innovation works are described as the following:(1) Magnetic circuit design of permanent magnetIn MRI system, permanent magnet’s configuration is decided by imaging magnetic field performance parameters. In this thesis, we combined magnetic circuit design theory and FEA (Finite Element Analysis), and had a detailed research on permanent magnet design method. Using simulation analysis method, we studied the magnetic field distribution characteristic of permanent magnet. It made sure that magnetic circuit configuration and magnet performance achieved the MRI requirement. In addition, we designed shim-loop and trapezoid shim-board to improve the magnetic field homogeneity and magnetic induction intensity.(2) Permanent magnet configuration optimizationTo lighten the weight of permanent magnet, this thesis presented a simulation method to study and improve the optimization technique of magnet configuration. With finite element method, we analyzed the magnetic field distribution of permanent magnet. Based on the magnetic field distribution characteristic, the redundancy parts of magnetic circuit configuration were removed accurately. The magnet weight was lightened significantly. Moreover, we optimized the shim-loop configuration. This made the imaging magnetic field more homogeneous.(3) Passive shimming calculationGenerally, the original magnetic field homogeneity of permanent magnet can’t satisfy MRI requirement. Passive shimming is usually used to correct the magnetic field. In this thesis, we presented a novel algorithm to calculate the position and number of shimming ferromagnetic elements. With magnetic induction intensity samples, we analyzed imaging magnetic field distribution characteristic. Based on electromagnetic theory, we established the correcting magnetic field mathematical model, and proposed a localizing method to program the shim radius. To reduce the quantities of the ferromagnetic material and avoid repeated correction, successive approximation was presented to calculate the number of shimming ferromagnetic elements. The experiment results show that this method can effectively guide passive shimming work and obtain highly homogeneous magnetic field.(4) Active shimming designThe magnetic field homogeneity will descend when temperature changes. Generally, active shimming is used to re-correct magnetic field. We proposed a set of tabulate shim coils to improve the imaging field homogeneity for permanent magnet. By studying the magnetic spherical harmonics and its Taylor expansion, the proposal of designing lower-order radial coils was derived including x, y, xz, xz~2, x~2-y~2 etc. In addition, a novel composite design was presented for the axial coils. By controlling the shim currents of axial coils, different axial magnetic fields can be obtained. This method simplified the design process of axial coils, and made the axial correcting magnetic field more precise.(5) Shim current calculationWhen shim coils are excited by appropriate current, they can generate correcting magnetic field to compensate the imaging magnetic field unhomogeneity. Based on the indirect calculation method, we proposed a novel iterative optimization algorithm to calculate shim current. Because there is a direct proportion relationship between FID (Free Induction Decay) signal and magnetic field homogeneity, we use the intensity of FID signal as feedback information to the calculation. Shim current can be worked out in less than 3 minutes. This satisfied the requirement that active shimming should be performed in real time.更多还原