Optimal Control Of The Effective Thermal Field Of Invasive Antenna For Microwave Tumor Hyperthermia

Microwave hyperthermia called"green therapy"by the international medical community can effectively kill tumor without damaging normal tissues. Invasive antennas are inserted into the tumor or cavity for invasive microwave hyperthermia. The advantage of this method lies in the fact that it avoids the epidermis of microwave energy absorption. Tumor is heated by microwave energy directly. This method also has high efficiency and controllable power. Hence, how to design safe and efficient invasive antenna becomes one of the urgent topics . The main contents of this paper are as follows:1. The electromagnetic properties of biological tissues and the propagation characteristics of microwave in biological tissues are described in this paper. According to the propagation parameters in tumor of the electric field at different microwave frequencies, the operating frequency of the invasive antenna is obtained.2. The impedance-matching performance of the choke antenna is not good and the top of the choke antenna has little energy. To solve these problems, the choke-umbrella antenna operated at 2450MHz is designed in this paper. The structural parameters of chock-umbrella antenna and the specific absorption rate (SAR) distribution in tumor are obtained by transmission line theory and finite element numerical simulation. Multi-layered block of tissue-equivalent phantom is heated with the fabricated invasive antenna. The three-dimensional effective thermal field (three-dimensional area whose temperature greater than 42℃) in phantom as well as the distribution of the temperature field in the invasive layer is obtained with an infrared thermo graphic technique. Research results show that the improved choke-umbrella antenna with lower reflection coefficient can generate an approximately spherical three-dimensional effective thermal field. The high-temperature area is mainly concentrated on the top of the antenna. This indicates the choke-umbrella antenna is a better choice for invasive microwave hyperthermia.3. To heat the large tumor efficiently, two-antenna array is used to expand the thermal field. The relationship between microwave power, antenna spacing and SAR distribution is obtained by numerical simulation. The SAR distribution similar to spherical is obtained by optimizing the power and spacing with genetic algorithms, which provides theoretical basis for heating experiments. The three-dimensional effective thermal field in phantom as well as the distribution of the temperature field in the invasive layer is obtained with heating experiments. Research results show that the effective thermal field distribution of the two-antenna array changes from ellipsoid to sphere as the heating time increasing. The two-antenna array can effectively heat larger tumors of about 3cm in diameter. The highest temperature of the invasive layer lies in the middle of two antennas rather than the top of the single antenna. This indicates the energy distribution induced by the array is not equal to the simple linear superposition of the two isolated energy distributions. With the same microwave power and heating time, interaction between the two thermal fields is utilized to obtain spherical area which is larger than two isolated fields.