Wearable Microwave Antennas for Brain Stroke Imaging

The following thesis deals with the design and realization procedure of two wearable antenna prototypes, to be included in a microwave system for brain stroke diagnosis and monitoring. This type of system use one antenna to transmit microwaves into the brain, in order to collect then the field scattered by the tissues with the others antennas of the array. The scattered signal contains the information about the contrast between the electrical properties of the stroke and those of the surrounding tissues, permitting to reconstruct an image of the object. What distinguishes this technique for stroke diagnosis from the others is that it does not use ionizing radiations and it does not involve high costs. Furthermore, it offers the possibility to have a device that is not only transportable, but also wearable. In order to have a wearable device that is also as comfortable as possible for the patient, it must be equipped with flexible antennas, adaptable to the head shape, thus allowing not only the diagnosis of the disease, but also continuous monitoring after its occurrence. The purpose of this thesis is therefore to investigate the state of the art regarding the materials and technologies used for the production of wearable antennas, to design flexible antennas and to develop a possible procedure for their realization. During the antenna design phase, the main requirement to be met was to chose materials that have a good compromise between electrical and mechanical properties. In fact, they must allow good bending behavior to the antenna without changing its performance. Taking into account this and others project specifications, two materials were identified to be used as antenna substrate and thus two prototypes, with two different antenna geometries, were developed. Both are wideband monopole antennas, optimized to have 1 GHz as the central frequency of the bandwidth and to work in direct contact with a material that mimics dielectric properties of average brain tissues. The CST Microwave Studio software was used for the design and through a time domain analysis the following antenna parameters were mainly evaluated: reflection coefficient (s11), electric near-field, surface currents and far-field radiation pattern. Subsequently, others simulations were done to evaluate the antenna bending and to consider a more realistic operation conditions. For that, the geometry of the object in contact with the antenna in the initial tests (parallelepiped) was replaced with a CAD of the head phantom. This CAD corresponds to the same phantom, 3D-printed plastic object, then used for measurements in laboratory. This involved the insertion of a plastic layer between the antenna and the material representing the brain and consequently changes in the antenna performance. The modifications have been analyzed and a possible solution has been proposed. The antennas were then built and tested, making measurements of different antenna parameters and in different conditions. This last step was fundamental to obtain a comparison between the simulated and measured data, to understand which critical issues emerge in the construction process and how to overcome or manage them in a future development of the project. Besides this, the thesis proposes an antenna prototype that with subsequent improvements could be used in the mentioned microwave imaging system.