The Application of Electromagnetic Theory in Microwave Therapy and Magnetic Resonance Imaging

Abstract

This thesis studies the application of electromagnetic theory in two aspects: characterizing the microwave thermal effect in cancer therapy and solving the low signal-to-noise ratio (SNR) issue in magnetic resonance imaging (MRI). In the first part of the thesis (Chapter 2), an invasive microwave breast cancer therapy which uses the needle insertion to guide microwave power into the tumor region is investigated through the calculation of specific absorption rate (SAR) in a simulated breast model in FEKO. It is shown by the simulation results that the heating effect can be adjusted by the direction of incident wave and the needle insertion direction, and the best heating and focusing effect in tumor region is obtained. Then a shielding method which consists of radially distributed needles is discussed, and the shielding effect is shown by the smaller Poynting vector values in the protected region. The second part of the thesis (Chapter 3 to Chapter 6) is to deal with the problem of low SNR in an MRI system. A vertical phased coil array which consists of a number of vertically stacked surface coils is proposed. The SNR increase is firstly explained in theory with the conclusion that SNR can be increased by increasing the number of coils in the array provided that the mutual coupling can be removed. Then the decoupling method is introduced through a simulated MRI system in a laboratory experiment, and good decoupling results are obtained, thus validating the feasibility of the proposed vertical phased coil array. The SNR variation with the number of coils in the array is shown through a series of rigorous numerical experiments, and it is found that in the situation of decoupling, the SNR of the system is significantly increased by the vertical phased coil array. Subsequently a multi-layered surface coil array which consists of multiple surface coils in both the vertical and horizontal directions is developed to increase the SNR of MRI with large field of view (FOV) for scanning large samples. The SNR performance of the multi-layered surface coil array is investigated through numerical experiments, and improved SNR performance is obtained.