This paper describes a new system to match the impedance automatically for a re-entrant resonant cavity applicator during brain tumor hyperthermia treatments non-invasively. We have already discussed the effectiveness of the heating method using a manual impedance matching controller with heating experiments of an agar phantom and computer simulations. With this heating method, to generate heating power in the cavity, it is necessary to match the impedance and to tune the resonant frequency. In the initial prototype heating system, both were manually adjusted. However, it was difficult to match the impedance and to tune the frequency especially using a high resonant frequency.
To overcome this problem, we developed the automatic impedance matching system (AIMS). It is known that reflected power is generated when the impedance matching is not complete. In this system, to reduce the reflected power which is fed back to the computer, the stepping motor to turn the dial of the variable capacitors is controlled by developed software. To evaluate the developed AIMS, the heating experiments of the agar phantom, using several electromagnetic modes, were performed. The VSWRs are 1.05 and 1.01 using manual matching and AIMS, respectively. From these results, we found that the temperature rise in the agar phantom using AIMS was about 180% (with TM₀₁₂-like mode) and about 110% (with TM₀₁₀-like mode) more than using the manual type controller under the same heating condition. It was found that the proposed system was very effective for hyperthermia treatment using the resonant cavity applicator even when the resonant frequency was high.