Towards an improved microwave tomography system
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This dissertation outlines work taken towards the understanding, implementation, and improvements to the process of creating of quantitative images of the bulk-electrical parameters of the interior of unknown objects via the use of electromagnetic scattering data. Improvements are considered to both theory and experiments using low-power radiation in the microwave frequency range, known as Microwave Tomography (MWT). A detailed derivation of the Multiplicative-Regularized Contrast-Source Inversion (MR-CSI) method is given, and we compare the performance of MR-CSI with the other leading inversion technique used in MWT: the Gauss Newton/Distorted Born Iterative Method. The inversion results of the two algorithms are very similar, and thus most of the differences between them are in the relative ease of implementation and computational resource use. We further introduce a new version of the CSI algorithm, based on the Finite-Difference method. Using this algorithm, we show that when accurate information about a scatterer is known before the inversion process, this information is best utilized as an artificial computational background, as opposed to an initial guess of the scatterer. The MWT problem is also formulated inside of a conductive enclosure, which significantly changes the physics, and resultant Green's function, of the MWT problem. The implications and possible advantages of this type of MWT are discussed, and synthetic inversion results for a circular enclosed system are presented. These results show that the enclosure is capable of improving the inversion in some regions, although more research is required to realize the full potential of conductive-enclosure MWT. In the final section, experimental results from both open-region and conductor-enclosed type MWT systems developed at the University of Manitoba are shown. For the open-region system, we show that antenna coupling is a major factor affecting the data collection, and provide a simple method for avoiding the frequencies where this coupling is too strong to prevent effective imaging. For the conductor-enclosed type system, we have found the system to be extremely sensitive to presence of antennas in the chamber, and show that effective MWT imaging is possible in this type of system by taking the antenna elements into account in the inverse solver.