Dottorato in TECNOLOGIE DELL'INFORMAZIONE E DELLE COMUNICAZIONI (ICT)

Titolo della tesi: Electromagnetic device for imaged guided microwave ablation

This dissertation demonstrates the design of a microwave imaging system for monitoring liver thermal ablation treatments. Liver cancer is the third most deadly cancer worldwide and has an increasing yearly fatality rate. Liver thermal ablation is considered to be an effective alternative to conventional treatment methods such surgery. However, over the years, real-time monitoring of liver thermal ablation has become a big challenge because the existing modalities like computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and Ultrasound imaging (US) have limitations in the applications at hand, that make them incapable or not suitable of providing real-time temperature values. Therefore, the assessment of the ablation procedure heavily relies on the clinician’s experience. Microwave imaging (MWI) is a potential candidate for this clinical need, since it provides a map of the dielectric properties of an unknown target from the knowledge of the scattered electromagnetic field. In fact, during liver thermal ablation treatments, the water molecules in the ablation zone dramatically reduce due to the heating. This process yields a change in dielectric properties values in the ablation zone as compared to the un-treated liver. The principle of microwave imaging for thermal ablation monitoring is to take advantage of the dielectric properties contrast between the ablated zone and the un-treated liver tissue. In fact, by recording and processing the scattered field at different stages of the treatment, it should be possible to image the evolution of the dieletric properties in the domain of interest. According to existing knowledge of the correspondence between liver’s dielectric properties values and temperature, it would then be possible to derive the local temperature in the ablation zone and hence determine the ablation stage. Among the advantages of MWI for thermal ablation monitoring it could be cited the low-cost, portability, capability of real-time imaging, harmless nature, as exploitation of low-power, non-ionizing radiation. Thanks to these circumstances. MWI has been considered for a number of biomedical applications, such as breast imaging, brain imaging, bone imaging, etc. A microwave imaging system for monitoring liver thermal ablation treatment would be made by an array of antennas embedded into a coupling medium and located in close proximity to the human abdomen, in front of the area to be treated. In this research, firstly, a numerical analysis was performed to determine the optimal working conditions in terms of operating frequency and coupling medium dielectric properties. Additionally, the dielectric properties of healthy ex vivo liver, as well as thermally ablated one were measured. Secondly, the antennas in the microwave imaging system were designed within the proposed working condition. Studies were performed with different antenna substrate materials looking for the most compact design. Three different antipodal Vivaldi antennas were designed and compared. After the microwave antenna design was completed, an in-silico assessment of the experimental set-up was performed, to define the optimal number of antennas and their spacing. The optimized set-up consists of eight antennas arranged in a staggered two-rows antennas array configuration immersed inside the coupling medium. Then, a simple yet representative experimental set-up for the validation of the imaging system was studied. The set-up foesees the 8 antennas inserted inside a tank filled with the coupling material; in front of the antenna array, a 3-D printed ellipsoidal phantom filled with tissue-mimicking liquid represents the thermally ablated zone. The retrived images inside the domain of interest show that the designed system can detect the position of the ablated zone and identify it at different ablation stages. Finally, the chosen antenna was realized and experimentally verified, and a recipe to realize the coupling material was proposed and experimentally tested. Ultimately, the system was experimentally assessed with one antenna mechanically moved in a linear motion measuring the signal in front of the 3-D printed phantom. The numerical and experimental assessment of the microwave imaging system verify the feasibility of such a system for liver ablation monitoring. This study paves the way for the real-time monitoring of liver thermal ablation through microwave imaging techniques.