| Magnetic Resonance Imaging, MRI is a non-invasive approach of mapping the internal structure and function of the human body. Compared with other modalities of medical imaging, MRI has many advantages, such as without ionizing radiation, imaging with multiple parameters in arbitrary plane, and providing excellent soft-tissue contrast. All those advantages make MR a very important tool for both research and clinical applications.RF coil is an important component in magnetic resonance imaging system, which is located in the most front-end imaging system, receiving signals in volume of interest. In clinical application, the signal to noise ratio, SNR and the uniformity are important for medical imaging, and both the magnetic field intensity and the uniformity of the magnetic field are directly related to SNR and uniformity. Thus the designers of RF coils must take account of simulation of electromagnetic to inspect field distribution of the coil.In engineering, finite difference time domain method and method of moments are the most commonly used two kinds of electromagnetic methods. FDTD method is first proposed by Yee, and the FDTD method belongs in the general class of grid-based differential time-domain numerical modeling methods. The time-dependent Maxwell's equations (in partial differential form) are discretized using central-difference approximations to the space and time partial derivatives. For it is a time-domain method, solutions can cover a wide frequency range with a single simulation run. Therefore, the FDTD method is not restricted by the complex dielectric parameters, such as the human body, so it is widely used in the simulation of RF coils. MoM is another method for electromagnetic analysis, which bases on a rigorous integral equation numerical method. The precision of MoM depends on the target geometry model accuracy and the correct choice of the base function and weight function. The idea of MoM is to mainly discrete the geometry target, and then create integral equation, and, with the weight function, test this integral equation, resulting in a matrix equation. The current distribution on the target geometry is obtained by solving the matrix equation. Compared to FDTD, MoM requires less computing resources, and is very suitable for more complex geometry of RF coils, so it is often used for the design of RF coils.When the human body is loaded into a coil, the coupling from body into the coil is generated; thereby the distribution of the electromagnetic field in the coil is changed, so we need to reconsider design of RF coils by electromagnetic simulation for coil. However, when the coil is loaded by human body, the electromagnetic simulation of coil is greatly difficult, because the FDTD method which needs to compute three regions as the computational domain, including the human body, free space between the body and coil, and the RF coil would take significant computing resources. In addition, when the FDTD method is applied, the PML dielectric layer need to be set, the thickness of each grid in the dielectric layer, which is the same thickness of each grid in free space, is important factor to absorption of electromagnetic wave. If the span of grid of the PML is too great, the PML would not work; if too small, the calculation would greatly increase. Therefore the direct use of the FDTD method to simulate the RF coil with load is not desirable. MoM needs to construct the sophisticated Green's function to solve complex dielectric electromagnetic problem, but in this way the large matrix is raised. So MoM is not expert in simulating the RF coil with load, such as human body.In this paper, we introduce the hybrid method of FDTD/MoM for simulation of RF coils with human body. At first, the current distribution on the RF coil is obtained by MoM, then the Huygence equivalent surface which envelopes human model is set in the free space between the human and coil. And with the current distribution mapped onto the equivalent surface, the FDTD method is taken to salve the problem of simulation of RF coils with human.Firstly, the principle of magnetic resonance imaging was introduced, followed by a discussion of the theory of the FDTD method and MoM. Then, the technology of RF circuit related to RF coils also was discussed in this paper; At last, the hybrid method was taken to design the RF coil. Using the hybrid method, the specific multi-channel receiving RF coil was developed, and phantom imaging test has proved the developed coil prototype's significantly improved performance. |
PDF Full Text Request