| In this thesis, a 3D microwave imaging method is developed for a microwave imaging system with an arbitrary background medium. In the previous study on the breast cancer detection of our research group, a full wave inverse method, the Diagonal Tensor approximation combined with Born Iterative Method (DTA-BIM), was proposed to reconstruct the electrical profile of the inversion domain in a homogenous background medium and a layered background medium. In order to evaluate the performance of the DTA-BIM method in a realistic microwave imaging system, an experimental prototype of an active 3D microwave imaging system with scanning antennas is constructed. For the objects immersed in a homogenous background medium or a layered background medium, the inversion results based on the experimental data show that the resolution of the DTA-BIM method can reach finely to a quarter of wavelength of the background medium, and the system's signal-noise-ratio (SNR) requirement is 10 dB. However, the defects of this system make it difficult to be implemented in a realistic application. Thus, another active 3D microwave imaging system is proposed to overcome the problems of the previous system. The new system employs a fixed patch antenna array with electric switch to record the data. The antenna array introduces a non-canonical inhomogeneous background in the inversion system. The analytical Greens' functions employed in the original DTA-BIM method become unavailable. Thus, a modified DTA-BIM method, which use the numerical Green's functions combined with measured voltage, is proposed. This modified DTA-BIM method can be used to the inversion in a non-canonical inhomogeneous background with the measured voltages (or S 21 parameters). In order to verify the performance of this proposed inversion method, we investigate a prototype 3D microwave imaging system with a fixed antenna array. The inversion results from the synthetic data show that this method works well with a fixed antenna array, and the resolution of reconstructed images can reach to a quarter wavelength even in the presence of a strongly inhomogeneous background medium and antenna couplings. In addition, a time-reversal method is introduced as a pre-processing step to reduce the region of interest (ROI) in our inversion. Furthermore, a Multi-Domain DTA-BIM method is proposed to fit the discontinued inversion regions. With these improvements, the size of the inversion domain and the computational cost can be significantly reduced, making the DTA-BIM method more feasible for rapid response applications. |
