Investigations into optimum characteristics for the coupling medium in UWB breast cancer imaging systems

Abstract

Recent years have seen an increased interest in ultra wideband (UWB) microwave imaging systems as an important alternative for mammography and other techniques such as magnetic resonance imaging or ultra sound in breast cancer detection. The reason is that microwaves offer a safe and low cost option for patients screening in comparison with these alternatives [1, 2]. The feasibility of UWB microwave imaging relies on two fundamental properties: Firstly, tumours have higher water content and hence a higher dielectric constant compared with the normal breast tissues, which is mainly fat. Therefore, a strong scattering takes place at the boundary between the normal tissue and lesions. Secondly, microwave attenuation in normal breast tissue is less than 4.5dB/cm across an UW frequency band of 3.1 to 10.6 GHz. This value of attenuation permits the microwave imaging system to detect small size, early-stage tumours. In order to enhance the detection process for a given dynamic range of the breast radar, recent approaches to UWB microwave imaging tend to employ a coupling medium between an imaged object (the breast) and an illuminating device (an antenna). In this case, both the antenna and the breast are immersed in a coupling medium. The reflected/scattered signal is collected and analysed to get a three dimensional image of the object depending on its dielectric profile. The use of a coupling medium minimizes the reflection at the skin interface, thereby reducing the propagation loss, and thus increases the dynamic range and resolution of the imaging system. The reviewing of the literature related to this important topic shows that the researchers use a coupling medium with a dielectric constant which is either equal to that of the skin or the normal tissue [3-5]. Neither of these two choices addresses the question of the optimal characteristics of the coupling medium. In this paper, a theoretical analysis, supported by computer simulations, is conducted to work out the optimal characteristics of the coupling medium.