Design and implementation of microwave imaging systems for medical applications

Abstract

A microwave medical imaging system processes scattered electromagnetic fields in the microwave region to create images. It is an alternative or complementary imaging tool that can be used in medical applications to assist the diagnosis of disease inside the human body. For example, microwave imaging offers many desirable characteristics as a cancer evaluation tool. It is a non-ionising radiation and during measurement, compression of the scanned body part is avoided. These benefits potentially lead to safer and more comfortable examinations. It also has the potential to be both sensitive and specific to detect small tumors, whilst being much lower cost than current methods, such as magnetic resonant imaging, positron emission tomography and ultrasound. However, it should be noted that all current imaging modalities have a relatively high incidence of false positive and false negative results. Moreover, microwave medical imaging designs can be made portable, so real time results for applications such as brain stroke detection is feasible. This thesis proposes several microwave medical imaging systems based on ultra-wideband technology and in doing so makes five contributions to the field of microwave imaging systems. The first contribution is the development of a UWB slotted Vivaldi antenna that operates as the transducer for imaging systems. The antenna is compact structure to overcome the shortcoming of conventional Vivaldi tapered-slot antennas. The antennas have a stable main-beam direction and high dynamic range. They are designed to work efficiently in a coupling medium. The designed antennas provide high sensitivity and excellent time-domain characteristics to enhance the capability of the proposed imaging systems. The second contribution of the thesis is the development of coupling medium and artificial phantoms that emulate the electrical properties of realistic tissue in order to test and assess the proposed microwave imaging systems before application to human subject. Two types of coupling medium are described. A breast phantom and a head phantom are designed and fabricated. The materials of the fabricated phantoms mimic the electromagnetic features of realistic human tissues. The electrical characteristics of all the fabricated materials show excellent matching when compared with the available data on real human tissue. The third contribution of the thesis is the design of a microwave medical imaging system for breast cancer detection. A planar scanning system is developed. The laboratory assessment of a two-dimensional and three-dimensional imaged breast phantom is explained. An experimental study of the breast cancer detection employing a fabricated semi-rectangular shaped phantom is conducted. An image reconstruction algorithm is developed to generate the images of the breast phantom using the measurement data. The results show the ability of the designed microwave imaging system to detect and localise tumors at different locations of the breast phantom. The fourth contribution of the thesis is the design of a microwave medical imaging system for brain stroke detection. In this case, a circular scanning system is developed. The laboratory assessment in a two-dimensional case of the imaged head phantom is explained. An experimental study is conducted using the head phantom. An imaging algorithm is developed for brain stroke detection and localisation. The effect of noise on the reconstructed head images is also explained. In addition, a stroke classification is investigated. The imaging system provides confident results that in the future this system can be developed to be portable used in any clinic to detect brain strokes. The fifth contribution of the thesis is the use of the optical link in the design of the microwave imaging systems to overcome the losses in the coaxial cables. A simple optical link is designed to transfer the microwave signal to the antenna array of the head imaging system. A calculation of the parameters that are used to evaluate the characteristics of the link is presented and a comparison is made between the characteristics of the coaxial and optic links. The initial results show the possibility of designing an analog over fibre link to transfer the signal to the antenna and receive the backscattered data with much lower signal loss and at low cost. This makes an important improvement to the type of transmission line that can be used to transmit and receive the data of the microwave imaging system. Overall, thesis contributions to the development a couple of microwave medical imaging systems, which have to be used by medical professionals as an alternative low cost portable diagnosis technique. Both developed and developing contain will be benefited the thesis outcome.