Reconfigurable antennas for electromagnetic torso imaging

Abstract

Pleural effusion and pulmonary edema are conditions characterized by excess fluid accumulation in pleural cavity and lung air sacs, respectively. These conditions stem from several diseases such as different types of cancer and heart failure. Besides the significance of monitoring these conditions in managing the primary source of exudates, the diagnosis and hence, circumventing these manifestations are crucial since the excess fluid inside the chest wall is followed by serious breathing complications. Therefore, early-stage detection of the fluid accumulation inside the chest area, and more importantly monitoring its progression are valuable in managing many life-threatening situations. Chest X-ray, magnetic resonance imaging (MRI), and computed tomography (CT) scan are the conventional techniques used for the detection of the excess fluid inside the torso. Despite their notable performance, these devices are not suitable for frequent monitoring due to exposing patients to ionizing radiation, lack of sensitivity for detecting a small amount of fluid, and shielding requirements. Therefore, researchers have proposed alternative approaches such as Electromagnetic imaging (EMI) systems. The accumulation of fluid inside the torso leads to a variation in the dielectric properties of the lung tissue, which can be detected by EMI techniques. Despite the conventional tools, EMI systems are low-cost, low profile, portable, and safe due to their low-power non-ionizing radiation, which makes them appropriate alternatives for diagnosis and monitoring purposes.In EMI systems, antennas have the essential role of transmitting/receiving the electromagnetic signals into/from the targeted area. Different types of wideband antennas with fixed unidirectional radiation have been proposed for EMI systems. Despite their notable performance, their main limitation is the fixed radiation pattern. Therefore, to scan different parts of the torso area, an array of antennas along the torso or a mechanically movable structure is needed. However, this results in a complex and bulky system that is prone to mechanical movement errors. To overcome this limitation, pattern reconfigurable antennas can be used as the scanning platform.This thesis presents different types of pattern reconfigurable antennas used in EMI systems to electronically scan the whole human chest area for detecting the fluid accumulation inside the lungs. Different methodologies and theoretical analyses are proposed to develop new generations of pattern reconfigurable antennas, that resulted in six different types of antennas. The proposed antennas were implemented and successfully tested in an EMI system using realistic phantoms to detect the excess fluid inside the chest. The first contribution of this thesis is allocated to the design of planar loop-dipole beam switched antennas, capable of scanning the whole torso area. The antenna consists of a one-wavelength loop, a modified half-wavelength bow-tie dipole, and two parasitic directors. The beam steering is achieved using parasitic directors to alter the current distribution on the loop and thus, enables beam switching.The second contribution is the development of high impedance surfaces to increase the gain and reduce the back-radiation. First, a wideband pattern reconfigurable bow-tie antenna on an inductive reflector is proposed. The antenna can steer the radiation pattern in one plane. Then, another beam steerable mechanism based on corrugated cross-slot radiators with parasitic slots on an inductive reflector is developed. The antenna is capable of two-dimensionally (2-D) switching the beam both on azimuth and elevation planes.To achieve low profile structures and reduce the total height of the antennas, reflecting surfaces can be replaced by transmitting metasurfaces. As a result, metasurfaces are thoroughly investigated as the third contribution. An offsetting technique is first presented to design a beam steering metasurface. The mechanism of steering the beam is based on the excitation of each metasurface unit cell with different phase delays by changing the location of the radiating slot. To overcome the limitation of the offsetting technique for 2-D beam steering, another beam steerable mechanism based on activating/deactivating unit cells is proposed.The fourth contribution of the thesis is the development of body-matched lens antennas. Metasurfaces are effective in creating focused plane wave radiation inside the imaging domain. However, they still suffer from skin reflections, which significantly reduce wave penetration inside the human torso. Therefore, there is a need for structures that can act as a matching medium while forming a focused field at near-field inside the imaging domain. To that end, a near-field focused beam body matched gradient-index lens (GRIN) antenna is proposed.The last but not least contribution is the verification of the capability of the fabricated designs to build EMI platforms. The test setup includes the proposed antennas as radiating elements, an artificial torso phantom, a microwave transceiver, and a processing unit to detect fluid accumulation inside the chest. A modified radar-based microwave imaging algorithm is used that can successfully detect the accumulated fluid inside the human torso phantom.