Microwave interrogation of biological tissues involves measuring very weak signals from a highly reflective object. This requires special ways to collect the signals and methodology to improve sensitivity. In the literature, numerous designs for ultra-wideband antennas intended for biological sensing applications have been reported [1-4]. When deploying sensors in prototype systems, an additional level of complexity arises when the system needs to adapt to the human body, which varies in size and shape from patient-to-patient [5]. At the University of Calgary we have developed two different systems targeted to breast imaging using ultra-wideband (UWB) signals [6,7]. Based on our experience in developing these systems and our initial testing with breast cancer patients and volunteers, we identified the need for different approaches to data collection. We have developed a mono-static radar system [6], and used this system in a study of 9 patients. This experience has led to the development of a second-generation system (Fig. 1) that permits positioning of the sensors with four degrees of freedom. The additional two degrees of freedom (compared to the first prototype) provide the capability to adapt to different breast sizes and shapes while orienting the sensor normal to the breast skin as shown in Fig. 2. The outline of the breast is measured using a laser system before positioning the sensor at the desired location and orientation. We also developed a system to measure transmitted signals over a large band of frequencies in order to estimate average electrical properties of the breast [7]. The results of the transmission measurement study indicated the value of collecting measurements with multiple sensors (i.e. more than one transmit and one receive element). This has led to the development of a new sensor, dubbed the Nahanni [8], shown in Fig. 3, which is implemented into our new transmission measurement system, shown in Fig. 4. This system is composed of two arrays each containing 5 sensors. The breast is placed between the two array assemblies while the upper element can be lowered to come into contact with the breast skin. While being similar to a mammography system, only slight deformation of the breast tissues will be necessary for the examination. Thanks to our new sensor, no immersion medium is required. This permits to realize a system that is drastically easier to deploy. In this contribution, we will discuss our experiences with the patient studies that we have conducted, focusing on challenges that are encountered in measuring meaningful signals. The latest systems designed to address these challenges and their applications will also be discussed.
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