Enhancing the focusing properties of a prototype non-invasive brain hyperthermia system: a simulation study.

Aim of this study is the improvement of the focusing properties of a prototype system for deep brain hyperthermia able to provide also passive measurements of temperature distributions inside the human body and especially the brain. One of the main modules of the system which ensures the necessary beamforming and focusing on the body and brain cortex areas of interest is the symmetrical axis ellipsoidal conductive wall cavity. The proposed system operates in a total non-invasive contactless passive manner and is designed to provide hyperthermia treatment and temperature monitoring. Extensive simulations to compute electric field distributions and SAR values at several frequencies inside the human head model and inside the whole ellipsoidal reflector were carried out. One of the main problems that have to be tackled in order to achieve the desired depth and focusing resolution is to reduce back scattering while improving penetration. With this view, the FEM simulations using a commercial tool aimed at improving the system's focusing properties following various approaches. In order to enhance the matching conditions on the air-head interface, layers made of metamaterials (left handed materials) and dielectric materials were placed around the human head model. The results show that the use of a metamaterial layer in conjunction with a layer of lossless dielectric material generates the largest improvement. Measurements using phantoms with the proposed focusing improvement techniques in future studies will complement the present research and reveal the potential practical value of the system.