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Design of RF Coils for Magnetic Resonance Imaging System Using Hybrid of FDTD/MoM Method

Author: HanJiJun
Tutor: ChenWuFan；XinXueGang
School: Southern Medical University,
Course: Biomedical Engineering
Keywords: Magnetic resonance imaging FDTD Method of Moments RF coil
CLC: R197.39
Type: Master's thesis
Year: 2011
Downloads: 68
Quote: 1
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Abstract

Magnetic resonance imaging (Magnetic Resonance Imaging, MRI) under conditions of non-invasive imaging of human or other living body tissue structure and function, it has a high image resolution, the imaging parameters, faults can be any direction, the human body without ionization radiation damage and other advantages. Therefore, the magnetic resonance imaging was able to be widely used in clinical and clinical and scientific research increasingly important imaging method. The RF coil (RF coil) is an important part of the magnetic resonance imaging system which is located at the forefront of the imaging system, receiving magnetic resonance signals from within the imaging area. In clinical applications, the signal-to-noise ratio (SNR) of the magnetic resonance image and the imaging uniformity is an important indicator of a measure of the RF coil and the RF coil of the magnetic field strength and magnetic field uniformity of the distribution and the image signal-to-noise ratio and imaging uniformity directly related. Thus for designers in the design of the RF coil, the coil electromagnetic simulation analysis, and thus the distribution of the coil of the magnetic field strength and magnetic field uniformity inspect. FDTD (finite difference time domain, FDTD) method and the moments method (method of moments MoM) in the project, the most commonly used two methods for electromagnetic simulation. FDTD method first proposed by Yee. Alternate sampling field components in time and space, and use the sampling Maxwell differential equations directly translate into a difference equation, combined with the initial and boundary conditions are gradually solving the electromagnetic field distribution in time and space propulsion. FDTD method is insensitive to electromagnetic frequency limit, the dielectric complex electromagnetic parameters (such as the human body) and is suitable for analysis, so widely used in the RF coil simulation. The method of moments is a the strict numerical method based on the integral equation, its accuracy depends on the target geometry modeling accuracy and correct choice of the base weight function and the calculation of the impedance elements. Thought the geometric target split discrete to build integral equations on a suitable definition of basis functions, and then, with the weight function test resulting in a matrix equation to solve the matrix equation, the current distribution can be obtained by geometric goal, so as to obtain the other near the far-field information. Relative to the FDTD, method of moments less computing resource requirements, and is very suitable for the analysis of the more complex geometry of the RF coil design RF coil process also frequently used method of moments to do its coil electromagnetic simulation analysis . When the human body into the coil, coupled with the coil, thus changing the electromagnetic field distribution in the coil, and therefore we need to consider the impact of the body coil in the coil design. Joining the human body, however, such that the RF coil of the electromagnetic simulation analysis greatly increases the difficulty of this is because of the FDTD method, the need for the calculation area to be subdivided, and join the body of the RF coil, the calculation region including the human body, the human body, and between the coils free space, the RF coil of three parts, these three calculation regions are using the FDTD method, it takes a great deal of computing resources. In addition, in the FDTD method is used, it is necessary to set the PML dielectric layer, the dielectric layer, the thickness of each grid i.e. in free space, the thickness of each mesh with PML dielectric layer absorbs electromagnetic waves performance has a direct relationship, if the free space in each network cell thickness is too general PML dielectric layer not absorb the electromagnetic wave; grid thickness is too small, the simulation calculated strength is greatly increased. For more complicated geometry of the RF coil, will cause errors in the segmentation process. Therefore, the direct use of the FDTD method simulation analysis and undesirable to join the human body RF coil. To simulation analysis of the RF coil, only discrete segmentation of the RF coil, and then based on free-space Green's function, small pieces or facets of the current distribution can be obtained using the method of moments. Here, the method of moments discrete general sub-one-dimensional or two-dimensional discrete triangular facets, the moment method is very suitable for the analysis of more complex geometry of the RF coil. However, the method of moments to solve complex electromagnetic parameters of dielectric electromagnetic analysis of the problem, you need to construct a complex Green's function, and in the realization of the process, you need to solve the matrix equations, method of moments is not like the FDTD method for complex electromagnetic parameters dielectric (such as the human body) electromagnetic analysis of advantages. In this paper, we introduce the FDTD / MoM hybrid method of adding the human body RF coil simulation analysis. In the hybrid approach, first using the Moment Method to obtain the RF coil current distribution on Huygens equivalent surface (Huygence equivalent surface) is provided in the free space between the body and the coil, so that the envelope of the human body model in the equivalent plane then mapped to the equivalent surface current distribution can FDTD method for equivalent surface (including the human body) area of ??electromagnetic analysis. This article introduced the first magnetic resonance principle FDTD method and the method of moments theory;, RF circuit technology applications in the RF coil to achieve process; FDTD / MoM hybrid method designed RF coil in the last to do a more detailed exposition. FDTD / MoM hybrid method designed RF coils elaborate FDTD method and the method of moments in MRI RF coil design, coil design in the body of the coupling of the RF coil is then considered into the introduction of FDTD / MoM hybrid method. Hybrid approach, we improve a specific coil that surgery open multi-channel RF receiver coil, and simulation analysis, improved coil and the primary coil, and finally achieve the improved prototype coil after coil, and through the prototype coil the water mold scan imaging, confirmed the significantly improve its image quality than the original coil.

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