数值混合方法设计与优化磁共振射频线圈

发布时间：2018-06-07 17:50

本文选题：磁共振成像 + 逆方法　；参考：《南方医科大学》2012年硕士论文

【摘要】：磁共振成像(Nuclear Magnetic Resonance imaging, NMRI)是一种断层成像技术,它通过外部射频场发射不同的射频脉冲序列对生物体内各个组织进行激发,通过接收探测生物体内产生的核磁共振信号来对体内组织器官的内部生理与化学特征成像。相比其他的一些成像技术,例如X-ray和CT (Computed Tomography),磁共振成像技术具有很多优势：首先,磁共振成像不需要将人体曝露在电离辐射的环境下,因此磁共振成像技术更具安全性。其次,磁共振为多参数成像,能够提供许多诊断信息。第三,磁共振成像对比度高,可得到详细的解剖图谱。第四,磁共振具有对任意层面断层成像的功能,可以从三维空间上直接观察人体。第五,磁共振成像不会出现气体及骨伪影等现象。工程上实际的电磁场问题是很复杂的,如边界形状不规则,复杂的物质结构,材料性能的非线性以及电磁场的分布性等等。因此,在计算机出现以前人们在实际的设计计算工作过程中,只能采取一些简化措施,得出近似的解析解,或者用模拟实验的方法来满足工程要求的近似结果。电子计算机的出现,给解算复杂的电磁工程技术问题提供了强有力的工具。由于数字电子计算机具有运算速度快,储存容量大,计算功能强和准确度高等优点,使得计算领域产生了惊人的发展。20世纪60年代以来,随着电子计算机技术的发展,一些电磁场的数值计算方法发展起来,并得到广泛地应用,相对于经典电磁理论而言,数值方法受边界形状的约束大为减少,可以解决各种类型的复杂问题。电磁学问题的数值求解方法主要分为时域和频域两大类。频域技术主要有矩量法、有限差分方法等,频域技术发展得比较早,也比较成熟。时域法主要有时域差分技术。时域法的引入是基于计算效率的考虑,某些问题在时域中讨论起来计算量要小。但是,各种数值计算方法都有优缺点：时域有限差分方法(FDTD)适合对非均匀复杂电磁参数电介质(例如人体)进行有效地分析,但是需要对整个三维空间进行网格化,还要在空间边缘设定边界吸收条件,计算时间相对比较久,需要的物理内存也相对比较大。相反,矩量法(MOM)在计算过程中不需要对整个空间网格化,也不许要设定吸收边界,只需要对有电流分布的区域进行离散化,计算相对快速,非常适用于对类似射频线圈这样的复杂结构进行电磁仿真分析。但同时,MOM不能处理类似人体这样的复杂介质的负载。而逆方法则是反向考虑问题,由预设理想目标场出发反演计算得出线圈结构。一个复杂的问题往往难以依靠一种单一方法解决,常需要将多种方法结合起来,互相取长补短,因此混和方法日益受到人们的重视。为了提高磁共振射频线圈射频场的性能,解决射频线圈与人体组织复杂电磁作用问题,我们提出了以真实人体为基础建立三维电磁模型的方法。以人体CT断层扫描图像为基础,采用精确的人工分割方式和体绘制三维重建方法,并赋上不同组织相应的电磁参数,建立真实人体三维电磁模型,作为射频场FDTD域负载。将建立起来的真实人体三维电磁模型作为线圈负载,优化磁共振射频线圈的射频场分布。本本研究的目的在于增强磁共振射频线圈仿真的真实性,提高射频线圈设计水甲,从而提高线圈的成像质量。充分利用三种方法的长处：矩量法(MOM)在计算复杂结构线圈上电流分布的优势；时域有限差分(FDTD)方法在仿真人体模型等复杂的不均匀电磁介质的优势；逆方法可以模拟理想目标场。通过惠更斯等效面将MOM和FDTD以及逆方法和FDTD有机结合到一起,形成混合射频线圈设计方法,并提出了一种新的真实人体电磁模型建立方法,融入到混合方法中,充分考虑线圈与真实人体组织之间复杂的电磁相互作用。其中,由MOM和FDTD混合方法设计的原型线圈扫描图像的信噪比达到193.4dB,相比先前未考虑人体与线圈之间作用的方法得到的图像信噪比提高了38.7dB,从而验证了仿真设计的正确性。
[Abstract]:Nuclear Magnetic Resonance imaging (NMRI) is a technique of tomography. It excuses the tissues of the organism by transmitting different radiofrequency pulses from the external radiofrequency field, and imaging the internal physiological and chemical characteristics of the organs and organs by receiving the MRI signals produced in the organism. Compared to some other imaging techniques, such as X-ray and CT (Computed Tomography), magnetic resonance imaging technology has many advantages: first, magnetic resonance imaging does not need to expose the human body to ionizing radiation, so magnetic resonance imaging technology is more secure. Secondly, MRI is a multi parameter imaging, which can provide a lot of diagnostic information. Third, the contrast of magnetic resonance imaging is high, and a detailed anatomical map can be obtained. Fourth, magnetic resonance has the function of tomography at any level, and can directly observe the human body from the three-dimensional space. Fifth, the magnetic resonance imaging will not appear gas and bone artifacts and so on.
The actual electromagnetic problem in engineering is very complicated, such as irregular boundary shape, complex material structure, nonlinear material property and distribution of electromagnetic field. So, in the process of actual design and calculation before the computer appears, some simplified measures can be taken to obtain approximate analytical solutions, or use models. The quasi experimental method meets the approximate results of the engineering requirements. The emergence of the electronic computer provides a powerful tool for solving the complex electromagnetic engineering technology problems. Because the digital electronic computer has the advantages of fast operation speed, large storage capacity, high computing function and high accuracy, the computing field has produced an amazing development of.2. Since the 60s zeroth Century, with the development of electronic computer technology, the numerical calculation method of some electromagnetic fields has been developed and widely used. Compared with the classical electromagnetic theory, the numerical method is greatly reduced by the boundary shape constraints, and can solve various types of complex problems. The numerical solution method of electromagnetics is the main method. It is divided into two categories in time domain and frequency domain. Frequency domain technology mainly includes moment method, finite difference method and so on. Frequency domain technology is developed early and mature. Time domain method is the main time difference technique. The introduction of time domain method is based on calculation efficiency, and some problems are discussed in time domain. The method has the advantages and disadvantages: the time domain finite difference method (FDTD) is suitable for the effective analysis of the inhomogeneous and complex electromagnetic parameter dielectric (such as the human body), but it needs to mesh the whole three-dimensional space and set the boundary absorption conditions at the edge of the space. The calculation time is relatively long, and the required physical memory is relatively large. In the calculation process, the method of moment (MOM) does not need to mesh the whole space, and does not have to set the absorption boundary. It only needs to discretize the area with current distribution, and is relatively fast. It is very suitable for electromagnetic simulation analysis of complex structures like radio frequency coils. But at the same time, MOM can not handle the similar human body. The load of complex medium is the inverse problem, and the coil structure is obtained from the presupposition ideal target field. A complex problem is often difficult to rely on a single method. It often needs to combine a variety of methods to complement each other, because this mixing method has been paid more and more attention.
In order to improve the performance of RF field of magnetic resonance radio frequency coil and solve the problem of complex electromagnetic action of radio frequency coil and body tissue, we put forward a method to establish a three-dimensional electromagnetic model based on real human body. Based on the CT tomography image of human body, we use accurate artificial segmentation method and body to draw three-dimensional reconstruction method. With the corresponding electromagnetic parameters of the organization, a three-dimensional electromagnetic model of the real human body is set up as the load of the FDTD field in the radio frequency field. The three-dimensional electromagnetic model of the real human body is set up as the coil load to optimize the RF field distribution of the magnetic resonance radio frequency coil.
The purpose of this study is to enhance the authenticity of the simulation of magnetic resonance radiofrequency coils, improve the design of water nail by the RF coil, and improve the imaging quality of the coils. The advantages of the method of moment (MOM) are fully utilized to calculate the current distribution on the coils of complex structures, and the time domain finite difference (FDTD) method is used to simulate the model of the human body. The advantage of heterogeneous electromagnetic medium, the inverse method can simulate the ideal target field. Through the combination of MOM and FDTD and the inverse method and FDTD, the design method of mixed radio frequency coil is formed through the Huygens equivalent surface. A new method for establishing the real human body electromagnetic model is put forward, and the coils are fully considered in the mixing method. The complex electromagnetic interaction with the real human tissue, in which the signal to noise ratio of the prototype coil scanned by the MOM and FDTD hybrid method is 193.4dB, and the image signal-to-noise ratio (38.7dB) is improved compared to the previously unconsidered method of the interaction between the human body and the coil, thus verifying the correctness of the simulation design.
【学位授予单位】：南方医科大学
【学位级别】：硕士
【学位授予年份】：2012
【分类号】：R310

【参考文献】