| Magnetic resonance imaging (MRI) is an important medical imaging technology, and now it is applied at clinic widely. MRI and related technology have great influence and scientific values because the scientists get the Nobel prizes five times totally in this field. The prominent advantages of MRI are the wonderful solution for soft issues and no risk of radioactivity. From these points of view, MRI is much better than other medical imaging technology such as X-ray CT, ultrasound etc. It is still at the beginning for the development of medical MRI technology and there should be great achievement for its application at clinic in prospect.Since the MRI system was developed several decades ago, it has been developed very rapidly. The strength of the main magnetic field covers from 0.2 Tesla to more than 10Tesla. The low field system is open, costs less money, has no restrict demand for the power supply. It is a good choice for small hospitals in rural areas. The high field system, such as the 1.5 Tesla system or the 3.0 Tesla system, is the first choice for big hospitals. The high field system could provide highly qualified medical imaging for doctors. It is very interesting that the different systems are present at the same time and shared by different hospitals due to their distinctive advantages.The MRI system is composed of main magnetic, gradient coil, radiofrequency (RF) coil, spectrometer system, power supply system and other affiliated parts. The RF coil is in charge of the emission and receive of the RF signals. It is one of the core parts in MRI system. Research of RF coil is very active and important in MRI field. There are many scholars who do great deal of research abroad. However the domestic research groups are much less, only several groups could do some research work in this field. RF coil could be divided into two kinds as the emission RF coil and the receive RF coil according to the different functions in the MRI system. The signals of RF coil are from tens of mega hertz to hundreds of mega hertz. The research of RF coil follows the ordinary laws in the electromagnetic field and also its own specialties.The research method is different for RF coil working in different frequency. Generally speaking, quasi-static method, also named equivalent circuit method, is used to analyze the RF coil at low field. This method is simple and could provide accurate results of resonance frequency, decoupling, matching in RF coil. With the increase of the resonance frequency, the errors will increase obviously. While the sized of RF coil or human body are similar with the wave length, this method is not adaptive any more. For high field RF coil, full wave analysis method is developed to analyze the RF coil. The full wave method, including the time domain finite difference method (FDTD), the method of moment (MoM), the finite element method (FEM), could provide complete analysis of RF coil at high field without the restriction of the frequency. The FDTD, MoM and FEM are all taken as tools for the analysis of high field RF coil. The advantage of FDTD is that it is convenient to analyze the inhomogeneous media in the electromagnetic field. The disadvantage of FDTD is that the need of calculation resources is much. The advantage of MoM is that it is a good choice to get the current density distribution of RF coil with complex structures. The disadvantage of MoM is that it is not appropriate for the analysis of inhomogeneous media. The FEM method is usually used to solve problems in frequency field. The grid method of FEM has much relation to the accuracy. The latest development includes the hybrid method, such as the hybrid MoM/FDTD or the hybrid FEM/MoM. The hybrid method could provide both the advantages of the two methods and has promising application in RF coil field. The author has done some research work using the above methods and published some papers.For the different demands of medical imaging, the geometrical shapes of RF coils are very different. There are solenoidal coil, saddle coil, birdcage coil, orthogonal coil, surface coil, array coils, implanted coil, etc. According to the different imaging parts of human body, the coils are divided as head coil, neck coil, head-neck coil, body coil, extremity coil, small organ coil, intra-cavity coil, etc. So the complicated structures of different RF coils have versatile medical applications. Because the RF coil is based on MRI system always, the development of RF coil is also related to the development of MRI system closely.For the development of MRI system, it is always the theme trying to reach the new clinical demand. Recently it is a new development of MRI system for intra-operative application, especial for the MRI guided focused ultrasound surgery, which is developing very rapidly and becoming very attractive in this field. The thermal surgery of tumors is a main method to kill tumors. During the process of surgery, it is necessary to monitor the temperature of the targeted organ to control and evaluate the surgery. There are two ways to monitor the temperature, one is the invasive method and the other is the non-invasive method. Obviously the non-invasive method is better than the invasive one. The MR thermometry is a wonderful non-invasive method. Many parameters in MRI, including the proton resonance frequency, relaxation time, are temperature sensitive. The accuracy of MR thermometry is very encouraging and it is within±1℃at high field. The research to combine the MRI system and the focused ultrasound devices is the hot topic recently. There are great potential values of MRgFUS system in the surgery of fibroid, breast cancer, prostate cancer, liver cancer, brain cancer, etc. MRgFUS technology is even called "disruptive technology innovation"RF coil is one of the core parts of MRI system, the research of RF coil in MRgFUS system is certainly an important topic. Comparing to the ordinary RF coil, the demand of intra-operative MRI system is different. It includes:(1) the physical path should be left for the pass of ultrasound beam, or for the intra-operative purposes; (2) the targeted organ is decided at first. At the same time, the general demands for RF coil, such as the homogeneous magnetic field for emission coil and high signal to noise ratio for receive coil, are still there. There is no specialized RF coil for intra-operative MRI system. Under the guidance of the supervisor professor Chen Wufan, with the aid of the 973 programs, a theoretical model, which is organ-oriented and specialized for intra-operative MRI, is proposed. The theoretical model includes the inverse method for the design of the organ-oriented RF coil, and the hybrid MoM/FDTD method for the optimization of RF coil. The prototype coils are built to verify the theoretical model.In the inverse method, the current density distribution of the surface of RF coil is calculated from the magnetic field distribution of targeted organ. The coil design is deduced from the current density distribution. The latest development of inverse method includes the inverse design of breast coil proposed by Li,Y and the inverse design of phased array coils proposed by Muftuler, L.T. Li, Y designed a cone-shape breast coil and made MRI scan with the designed coil. Muftuler, L.T designed the phased array coils to shorten the time spent in MRI. By now, there are no reports of the application of inverse method in the design of the intra-operative RF coil. The reasons maybe are:(1) the technologies in the intra-operative MRI field are not well researched because of the youth of these technologies; (2) the need of intra-operative RF coil could be satisfied partially by the modification of traditional RF coil; (3) it is difficult to design the specialized intra-operative RF coil because of the complex structures. The physical path in the RF coil will make the design and manufacture more difficult. In order to solve the problems mentioned above, the author gives a new method to design the intra-operative RF coil, it is the organ-oriented inverse method. The new method is based on the current research, especial the research by Li,Y. The object of the new method is to improve the homogeneity of the B1 field. Because of the physical aperture in the designed RF coil, the mathematical model should be specialized. It is necessary to solve the integration equations to get the current density distribution on the surface of the RF coil. Usually the integration equations are ill-conditioned, they should be regularized to get the solutions. The final design of the inverse method is contoured from the solutions of the current density. So there must be errors between the final design and the original targeted distribution of magnetic field. The final design of the inverse method should be optimized further. Li,Y etc set several control nodes, and adjusted the positions of the different nodes according to the calculation results by quasi-static method. It is simple but there are two disadvantages. One is the causal setting of the control nodes and the other is that the quasi-static method could not meet the demand at high field.In order to optimize the design result by the inverse method, the author proposed the hybrid MoM/FDTD method combining the human-body tissue model as load. The new hybrid MoM/FDTD method is based on the proposals of Huygens equivalent surfaces from Feng,Liu. Furthermore the complex coil-tissue interactions are considered in the new method. The current density distribution of the intra-operative RF coil is calculated by MoM method. It is necessary to set appropriate parameters of resonance capacitors, decoupling capacitors, detuning circuits and matching circuits. Then the Huygens equivalent surfaces are set according to the Huygens equivalence law. The Huygens equivalent surfaces are the interconnections between MoM and FDTD. The current density distribution of the Huygens equivalent surfaces is decided through mapping from the MoM results. The inner space surrounding by the Huygens equivalent surfaces is the FDTD domain, where the targeted organ is located. The electromagnetic model of the targeted organ is established and taken as load in FDTD domain. The author takes the advantage of FDTD to calculate the electromagnetic field with the inhomogeneous targeted organ model. The appropriate time and space steps are set in the FDTD solver. The perfect matched layers are set also. The accuracy of the solution is related to the resources of the computer also. After the steady solution is achieved in the FDTD domain, the current density distribution on the Huygens equivalent surfaces is changed accordingly. Then the new current density distribution is mapped back to change the current density distribution on the surfaces of the RF coil. After that a new round of optimization would restart again till the current density distribution on the Huygens equivalent surfaces reach the final stabilization and would not change any more. Thus the optimization is finished and the final design of the RF coil is decided. Some research work using the hybrid MoM/FDTD is accepted for publication in the IEEE ISBI meeting in 2011.Finally the prototype RF coil is built to verify the theoretical model. It is proved that the proposed theoretical model is practical and could meet demand of the MR thermometry. The related research results were published in "Concepts in Magnetic Resonance Part B-Magnetic Resonance Engineering" which is SCI indexed.
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