| Magnetic Resonance Imaging (Nuclear Magnetic Resonance imaging, NMRI) is a kind of temography technology, The external RF field produces different RF pulses acting on the organization in biology body and detecting the signal of Nuclear Magnetic Resonance from the biological body to imaging. Compared to other imaging technologies, such as the X-ray and CT (Computed Tomography), Magnetic Resonance Imaging technology has many advantages:first of all, MRI (magnetic resonance imaging) doesn’t need to human body exposure in the ionizing radiation environment, so the Magnetic Resonance Imaging technology is more security. Second, Magnetic Resonance Imaging could get many parameters, it could provide many diagnosis information. Third, Magnetic Resonance Imaging has the high contrast, it can get detailed anatomy imaging. Fourth, Magnetic Resonance Imaging could get the function of tomography at arbitrary level, we can observe on the human body from any direction. Fifth, MRI (Magnetic Resonance Imaging) will not appear gas or bone artifact.The actual engineering problem of electromagnetic field is very complicated, such as boundary irregular shape, complex material structure, and the material properties and the distribution of the electromagnetic field of the nonlinear sex, and so on. Therefore, before the computer invented, people can only take some simplified measures, get the approximation, or using the simulation method to satisfy the requirements of the engineering result. When the computers were invented, it provides us a powerful tool for the solution of complex electromagnetic problems. As digital electronic computer has the advantages, such as fast calculation speed, storage capacity, strong and accuracy calculating function, it made computation field produced startling development. Since the1960s, with the introduction of the development of computer technology, some of the electromagnetic field numerical calculation method developed, and widely used. Relative to the classical electromagnetic theory, the numerical method by the shape of the boundary constraint is greatly reduced, can solve all types of complex problems. Numerical solution method of electromagnetism problem are divided into two categories:time domain and frequency domain. Frequency domain technologies are MOM and finite difference, the frequency domain technologies were developed early, and more mature. The time domain method is mainly a finite difference time domain technology. But, all kinds of numerical calculation method has advantages and disadvantages. The finite difference time domain method (FDTD) is suitable for non-uniform complex dielectric electromagnetic parameters (such as the human body), but it need to make the three dimensional space grids, and set absorption edge conditions in the space boundary, the time of calculating is relatively longer, and also it need more physical memory. Instead, MOM does not require for the whole space grids in the calculation process, also needn’t to set the absorbing boundary, it only to have the current distribution area of discrete, the calculation is relative fast, so MOM is very suitable for similar RF coil, which is complex structure. But at the same time, MOM can’t deal with complex medium load such as the human body. Inverse method is the reverse consideration question, it calculate the coil structure by the ideal goal field. A complex problem is often difficult to rely on a single method to solve, it often need to combine a variety of methods, mutual complement each other, so hybrid method has be attention by people increasingly.In order to study the complex mechanism of coil-tissue electromagnetic interactions, the method of establishing human electromagnetic model was intended for designing the RF coil in MR. Based on CT scanning images, the3d model was reconstructed via a precise manual segmentation and volume rendering method. And insert the corresponding electromagnetic parameters to the different organizations, then we establishe the real human3d electromagnetic model, as the load, To optimize the distribution of RF field.The purpose of this paper is to improve the simulation accuracy of MR RF coil, then to improve the quality of MR image. We take advantages of method of moment (MOM), finite difference time domain (FDTD) method and inverse method. That is, MOM is suitable for the calculation of current distribution on the RF coil structure; FDTD is suitable for the electromagnetic field analysis in the case of human body; Inverse methods can simulate ideal target field. We used the Huygens’equivalent surface to combine the MOM/FDTD and Inverse method/FDTD to form a new hybrid method. With the proposed real human body3D electromagnetic model, we considered the complex coil tissue interactions effectively. For the MOM/FDTD, the signal-to-noise ratio (SNR) of the image, which is designed by the method, is193.4dB,38.7dB more than which does not consider the interactions. Consequently, the results verified the accuracy of the design. |
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