三维开放式磁共振电阻抗成像的研究

发布时间：2018-01-29 19:18

本文关键词： 磁共振电阻抗成像电导率电流密度三维重建　出处：《天津大学》2013年博士论文　论文类型：学位论文

【摘要】：生物组织的电特性是现代生物医学领域的研究热点之一。由于生物体的各组织之间，以及正常组织与病变组织之间的电导率均有较大差异，所以对生物组织电导率的研究有助于临床上的病理诊断和对人类自身奥秘的探索。电阻抗断层成像技术通过测量低频电流激励下成像体表面的电势分布来重建成像体内部的电导率分布图像。由于测量信息有限，传统的电阻抗成像方式在静态成像中面临空间分辨率低的瓶颈，，限制了其临床应用。磁共振成像是利用人体组织中的氢原子核在磁场中受到射频脉冲激发而产生的磁共振信号，通过计算机重建，获得人体不同层面图像的成像技术。若向置于磁共振系统中的人体注入特定的电流，则该电流产生的周围磁场将影响磁共振信号。磁共振电阻抗成像技术即借助磁共振扫描系统通过测量该电流在成像体内部产生的磁感应强度分布来实现电导率分布的重建，它不仅提供了精确的成像体形状信息，而且改善了传统的电阻抗断层成像方式单纯利用边界电压的情况，从而降低了电阻抗成像的病态性，能够获得较高分辨率的电导率图像，为电阻抗成像的临床应用提供了条件。本文将磁共振电阻抗成像发展到三维空间，并特别针对竖直静磁场方向的开放式磁共振系统，提出了若干提高成像速度和改善成像效果的图像处理方法以及电流注入模式，为磁共振电阻抗成像技术的发展奠定了基础。 1．以乳腺癌为例，对三维磁共振电阻抗成像的临床应用进行了仿真分析。在由二维到三维的磁共振电阻抗成像的推广中发现，由于涉及庞大的灵敏度矩阵计算和超大方程组的求解，需要较大的内存资源和较长的重建时间；通过对灵敏度矩阵的分析，提出了改进的分层灵敏度重建算法，降低了三维重建所需的计算机硬件需求和计算时间，从而在保证重建精度的前提下显著提高了三维磁共振电阻抗成像的重建效率。 2．针对MREIT的快速成像问题，在分析磁共振电阻抗成像的数学模型和正逆问题的基础上，提出利用单次电流的磁感应强度分量即可进行电导率图像重建的方法，缩短了磁感应强度的测量时间和电导率分布图像的重建时间；单次电流的注入亦将减少电极之间的电磁干扰，降低激励和测量系统的复杂性；并通过仿真与仿体实验验证了该方法的有效性。 3．针对主磁场为竖直方向的开放式磁共振系统，提出一种环形电极模式应用于开放式磁共振电阻抗成像。首先通过对开放式磁共振系统中不同电流注入模式的分析，提出了基于环形电极的电流注入模式；然后用仿真实验证明了该电极模式下，开放式磁共振电阻抗成像的可行性，并且发现该电极模式下电流密度在主磁场方向分量的分布与电导率的分布基本一致，从而使磁共振电流密度成像具有更加广泛的临床意义；基于真实人体小腿模型的仿真实验和环形电极的仿体实验都对环形电极对于开放式磁共振电阻抗成像的可行性和有效性进行了验证。 4．在磁共振电阻抗成像中，图像质量与磁共振系统的测量信噪比密切相关，而测量信噪比在一定程度上又由静磁场强度决定。为了提高中低场强下的磁共振电阻抗成像的图像质量，提出了一种图像后处理方法。其具体步骤包括：背景区域填充；图像灰度归一化；直方图均衡化；和灰度的线性拉伸。应用所开发的磁共振电阻抗成像软件系统，在0.36T磁共振系统下进行的仿体实验证明经过该方法处理后所获得的电阻抗图像灰度分布更加均匀，图像对比度得到显著提高。
[Abstract]:The electrical characteristic of biological tissue is one of the hot topics in modern biomedical field . Because of the large difference between the tissues of the organism and the electrical conductivity between normal tissue and diseased tissue , the study of biological tissue conductivity contributes to the clinical pathological diagnosis and the exploration of the human ' s own mystery . The electrical impedance tomography technique is used to reconstruct the electric conductivity distribution image inside the imaging body by measuring the potential distribution of the surface of the imaging body under the excitation of low frequency current . The invention provides a magnetic resonance imaging technique for reconstructing the conductivity distribution by measuring the magnetic induction intensity distribution generated in the imaging body by measuring the current in the magnetic resonance system . 1 . Based on breast cancer as an example , the clinical application of three - dimensional magnetic resonance imaging is analyzed . It is found that large memory resources and longer reconstruction time are needed because of the large sensitivity matrix calculation and the solution of the super - large equations . By analyzing the sensitivity matrix , the improved layered sensitivity reconstruction algorithm is proposed , which reduces the computer hardware requirement and the calculation time required for the three - dimensional reconstruction , and improves the reconstruction efficiency of the three - dimensional magnetic resonance electrical impedance imaging remarkably . 2 . On the basis of analyzing the mathematical model and positive inverse problem of MREIT , it is proposed to use the magnetic induction intensity component of single current to reconstruct the conductivity image . The measurement time of the magnetic induction intensity and the reconstruction time of the electric conductivity distribution image are shortened ; the injection of single current will also reduce the electromagnetic interference between the electrodes , reduce the complexity of the excitation and measurement system , and verify the validity of the method through simulation and simulation experiments . 3 . An open type magnetic resonance imaging system is proposed for the main magnetic field . First , the current injection mode based on the ring electrode is presented by the analysis of different current injection modes in the open magnetic resonance system . Then , the feasibility of the open magnetic resonance imaging is demonstrated by simulation experiments . The results show that the distribution of the current density in the main magnetic field direction is basically consistent with the distribution of the conductivity , so that the magnetic resonance current density imaging has more extensive clinical significance . 4 . In MR imaging , the image quality is closely related to the measured signal - to - noise ratio of the magnetic resonance system , and the signal - to - noise ratio is determined to some extent by the static magnetic field intensity . In order to improve the image quality of the magnetic resonance electrical impedance imaging in the middle and low field intensity , an image post - processing method is proposed .

【学位授予单位】：天津大学
【学位级别】：博士
【学位授予年份】：2013
【分类号】：R310

【参考文献】