Symmetric wideband five port reflectometer for microwave-imaging based brain injury diagnosis

Abstract

Brain injury is considered as one of the vital reasons for death worldwide with more than 15 million people suffer from brain stroke attack each year, according to World Health Organization (WHO). The limitations of conventional head imaging techniques such as MRI and CT-scan have been pointed out in the thesis where a portable and prompt diagnosis features are not made possible. Radar-based imaging (RBI) is addressed as a potential solution due to its effectiveness and aptness for a primary diagnosis of brain injury. However, the bulky structure and high-cost of vector network analyzer (VNA) limit the RBI potential. Five port reflectometer (FPR) has potential to substitute VNA. Two prototypes of FPR have been proposed in this thesis. First prototype involves a single negative (SNG) metamaterial array located at the ground of single ring FPR, whereas the second one involves double tier compensating network in additional to the first central ring. In the first prototype, the single ring FPR is designed based on the theoretical parameters integrated with SNG metamaterial array at the ground plane which has been optimized to obtain a larger bandwidth. It is observed that the effective permittivity of the substrate is changed due to the influence of SNG metamaterial which eventually changed the characteristic impedance of the transmission lines of the FPR at the front side of the substrate. The metamaterial array enhances the overall performance of single ring FPR with an increment of 65.62% fractional bandwidth (BW-10 dB) in the first band and 76.23% in the second band as compared to the design without metamaterial array. The first prototype has a dual-band operating zone extending from 0.93 GHz to 2.19 GHz and from 3.27 GHz to 4.49 GHz. The second prototype consists of double tier networks with inter-tier transmission lines and multi-section matching at each of arms. In the evolution of the second prototype, inter-tier transmission lines are shifted by 36˚ (which is half factorized value of inter-port angular distance of 72˚) in several optimizing steps, namely, a) non-shifted b) partially shifted and c) fully shifted design. Fully shifted design which has 36˚ shifted inter-tier and another 36˚ shifted arms has created additional electrical length traversed by inter-port transmission signals to enhance the bandwidth up to 88.04% (from 1.004 GHz to 2.583 GHz). In addition of bandwidth achievement, such compactness of proposed FPR is contributed by the curved lines at the outer matching sections which enable a reduction of 43.09% in length and 43.12% in width compared to the non-compact design. Both prototypes have been fabricated and measured. Discrepancies between simulated and measured results are assessed using mean absolute deviation. The 88.04% bandwidth of the proposed fully shifted FPR is the highest bandwidth among literatures which potentially leads to a higher accuracy of microwave imaging-based brain injury diagnosis.