Wideband Microwave Imaging Systems for the Diagnosis of Fluid Accumulation in the Human Torso

According to the World Health Organization (WHO), cardiovascular diseases (CVDs) are the leading causes of death worldwide, with one third of deaths attributed to CVDs in 2012. Pulmonary oedema and pleural effusion are the most apparent symptoms of many diseases categorized under CVDs such as heart failure and lung cancer, at which fluid (mainly with high water content) is accumulated in or around the lungs. Therefore, constant monitoring of fluid levels inside the lungs is one of the most efficient ways of early detection of CVDs. Chest X-Rays and computational tomography (CT)-scans are the most widely used devices for fluid detection; however, they suffer from lack of sensitivity and ionizing radiation, respectively, that makes them unsuitable for long term monitoring purposes. Currently, magnetic resonance imaging (MRI) is the most reliable device that can be utilized for fluid accumulation detection. However, considering the fact that more than 75% of the CVDs occur in countries with low or middle income, it is not widely available. Moreover, due to their bulky structures, the abovementioned devices lack the capability of being used in mobile emergency units such as ambulances or clinics at rural areas. To that end, this thesis is dedicated to design and fabrication of a low cost, portable and non-invasive device that can be used as an initial decision making tool for medical staff to pursue further investigations to define the exact cause of the oedema. First chapter of the thesis is allocated to introduction of the cardiovascular diseases and their effects on the dielectric properties of the tissues inside the lungs. A complete literature review on various alternative methods for replacing the conventional devices is performed. The obtained results by these systems and their advantages as well as their limitations are discussed. Microwave imaging technique is then presented in chapter two as a robust method which can both provide information about the presence and location of the accumulated fluid. This is specifically of great importance for cases where biopsy is required to remove or take sample of the accumulated fluid for saving the life of the patient. Chapter two is also allocated to the introduction of microwave-based medical diagnostic and monitoring systems for different applications such as breast cancer detection and brain imaging. A prospect of the possible realizable systems is investigated and existing scanning approaches are discussed. The main contributions of the thesis that are the design of several complete platforms, design of novel and unidirectional microwave sensors (antennas), promotion of novel scanning and detection methods are clarified in these chapters. In chapter three, firstly the optimum operating frequency for torso imaging is defined. By applying a circuit model that models different layers of torso as circuit elements, it is shown that a wide operating bandwidth at lower ultra-high frequency (UHF) band provides a reasonable compensation between the resolution of the obtained images and signal penetration inside the body. It is explained that due to the limited allowed microwave power for safety considerations unidirectional antennas are required. Then, it is explained that due to the large wavelengths at lower UHF band the sizes of the prospective antennas are expected to be large. To that end, novel miniaturization techniques are proposed to reduce the sizes of the conventional antennas in chapters three and four. These antennas are categorized under three dimensional (3-D) and planar structure. A folding technique is introduced and used in the proposed 3-D structures and it is shown that by using this technique both size and directivity/back radiation suppression is improved. 3-D slot-antenna and cubic monopole-fed antennas are also proposed that wide operating bandwidth is achieved using slot impedance transformer, and multiple resonance-merging techniques, respectively. Regarding the planar structures that are presented in chapter four, it is shown that by combining the loop-dipole modes, both wide-operating bandwidth and directivity enhancement is achievable. Capacitive-loading of a loop antenna is the other proposed technique in which a loop antenna is partially and/or non-uniformly loaded with capacitors in the forms of simple slots and mu-negative (MNG) metamaterial-unitcells that help miniaturizing the size of the antenna by lowering its first resonance frequency. In chapters five and six, several platforms using single and multiple antennas with linear and circular configurations are presented and the utilized imaging technique for data processing is explained. The platforms are presented in a systematic progressive manner in which each system is covering the limitations of its previous prototype. Two final clinical platforms in the shape of clinical bed and doughnut-shaped chamber are proposed and the obtained test results on artificial phantom, animal lungs and human tests are presented. Based on the obtained results on healthy human beings it is shown that the scattered-field from torso of people with different body sizes vary in a reasonably limited range that is a welcoming result for building a global-database to define a threshold for healthy range. Chapter seven concludes the discussions made throughout the thesis and explains future works that can be carried out to further improve the reported systems.