【作者】 李会香；

【导师】 谢德馨；

【作者基本信息】 沈阳工业大学 ， 生物医学工程， 2011， 硕士

【摘要】 由于医用磁共振成像(Magnetic Resonance Imaging, MRI)系统在检测人体时,对病人无损害,所以这种成像设备越来越广泛地应用于医疗诊断中,成为最重要的成像设备之一。随着磁共振设备需求的增大,其成像质量也相应地备受关注。根据磁共振的成像原理,主磁场的磁场强度和磁场均匀度与成像质量密切相关。本文对两种开放式磁共振主磁体进行了提高磁场均匀度方法的研究。对于开放式永磁型MRI系统,磁体结构不改变的情况下,在磁体表面放置小磁片对成像空间的磁场进行校正,达到提高磁场均匀度的目的,即无源匀场。基于运筹学中整数线性规划思想和从实际匀场过程得到的中心匀场思想,本文提出了一种无源匀场数学模型。采用FORTRAN语言编制单纯形法和分枝定界法程序对模型进行求解,得到极面上特定位置处垫片的数目。数学模型的正确与否需要验证,为了节省资源，本文利用商用软件ANSOFT对匀场方案进行验证。根据匀场方案,建立垫片阵的ANSOFT模型,计算垫片阵成像空间产生的磁场。通过比较匀场前后成像空间的磁场分布,可以得知数学模型的正确性。本文对两台不同的双柱型磁体进行了仿真研究,磁场不均匀度均有显著降低,证明了模型的正确性及有效性。对于开放式超导型MRI,在保证成像空间磁场强度的同时,还可通过优化磁体结构来提高磁场均匀度。本文对1.5T开放式超导型MRI系统主磁体进行了优化设计的前期研究。在不采用专门的优化算法的前提下,基于电磁场基本理论和磁路理论,利用商用软件ANSOFT对磁体部分进行了多方案设计,为提高成像空间的磁场均匀度寻找优化方向。设计方案的可变参数主要有：铁心长度、铁轭位置、极面形状和线圈电流分布。本文分别对单独的设计方案及混合的设计方案进行仿真分析,为开放式超导型MRI磁体设计提供了研究基础与方向。更多还原

【Abstract】 As medical Magnetic Resonance Imaging (MRI) are harmless to patients when detecting the human body, it is more and more widely used in medical diagnosis, and has become one of the most important imaging devices. With the increasing demand for MRI devices, its imaging quality receives much concern. According to imaging principles of MRI, the imaging quality is closely related to magnetic field strength and magnetic field homogeneity of the main magnetic field.In this paper, the method of improving magnetic field homogeneity are studied for two type main magnets of open-type MRI. For open permanent MRI system, in the condition of no changes to the magnet structure, small shims are placed on the magnet planes to correct the magnetic field in imaging space, to achieve the purpose of improving magnetic field homogeneity, that is passive shimming. Based on integer linear programming of operations research and the central shimming idea obtained from actual shimming process, a mathematical model of passive shimming is proposed in this paper. Simplex method and branch and bound program is compiled in FORTRAN to solve the model, and the number of shims at special position on the pole faces is obtained. The correctness of mathematical model needs to be verified. In order to save resources, commercial software ANSOFT is used to verify the shimming scheme. According to shimming scheme, the ANSOFT model of shims array is established, and then the magnetic field of imaging space produced by the shims is calculated. By comparing the magnetic field distribution in imaging space before and after shimming, we can know the correctness of the mathematical model. Simulation research is done on two different double-column type magnets. The magnetic field inhomogeneity of the two magnets reduce significantly, which proves the correctness and validity of the model.For open superconducting MRI, the magnetic field homogeneity in imaging space is improved by optimizing the magnet structure, meanwhile, the magnetic field strength is ensured. Preliminary research of optimization and design for a 1.5T open superconducting MRI main magnet is carried out in this paper. Without considering special optimization algorithm, based on the basic theory of electromagnetic and magnetic circuit theory, the magnet is designed with several schemes using commercial software ANSOFT to find optimization direction for improving the magnetic field homogeneity in imaging space. The variable parameters for design schemes are as following:core length, yoke position, pole face shape and coil current distribution. In this paper, several single and mixed design schemes are simulated and analyzed, and the results provide some research foudation and direction for open superconducting MRI magnet design.更多还原