**7. Microwave imaging radar antenna element**

Recently microwave imaging was considered as an alternative promising technology for medical imaging especially since the required cost is much lower than the most prominent medical imaging methods such as computational tomography (CT) or magnetic resonance imaging (MRI). For the use of microwave imaging devices like the ones used for breast tumor detection in clinical trials [19, 20], the number or radiating elements is important. Generally, it is desired to use high number of elements which should have unidirectional radiation patterns, while the crosscoupling among the radiating elements must be as small as possible. As a result, the use of compact unidirectional UWB radiators is considered. An effective solution is the use of cavity-backed slot antennas. Such an element is presented in **Figure 17**. An implementation of a microwave imaging device consisting of similar such radiators is described in [21, 30]. The front face can be made conformal to better match the non-planar surface, while the feeding cable can pass through the metallic cavity that is used to cancel the back radiation. The desired low profile of the cavity and the intermediate gap between the radiating antenna elements and the target make the design of such an antenna a challenging task. In order to avoid additional reflections, the target is immersed in a liquid with controlled dielectric constant since the UWB antennas' matching can be severely degraded when the antenna radiates in close proximity or in touch with the human body. The high effect of the human body with the complex electromagnetic characteristics on the UWB antenna performance necessitates the use of accurate human phantoms in the full-wave simulations. **Figure 17c** demonstrates a cavity-backed slot UWB antenna in close proximity with a realistic breast phantom in a setup used in a full-wave simulator in order to ensure the good performance of the antenna when it radiates in close proximity with human body parts. Although the metallic cavity increases the profile of the receiver, it is very useful since it blocks signals which are not coming directly from the target.

Different vendors [19, 20] use customized UWB antennas which serve better the preferred reconstruction algorithms that they use; however, the presented cavitybacked slot radiator is one of the best candidates for medical microwave devices.
