Research On Microwave Hyperthermia Machine Output Power Control And Calibration

Microwave hyperthermia in combination with chemotherapy and radiotherapy for treatment of cancer has achieved significant clinical effect. It has therefore gained more and more popularity in clinical applications. The research and development of the high power microwave hyperthermia apparatus related to microwave power measurement and control technology, and new sensor technology, as well as analog and digital circuit technology with a combination of microcontroller and PC computers and so on. The microwave output power is an important parameter in microwave hyperthermia. Usually, the output power of microwave source has one to one relationship with the anode current of magnetron. The measurement and control of the microwave power can be achieved by measuring and controlling the magnetron anode current, but the power calibration is very difficult. At present, the output power calibration of microwave hyperthermia apparatus has commonly been done by measuring the output microwave power from the microwave cavity coupled to the coaxial cable using a microwave low-power meter or a dedicated water load device, and simultaneitly recording the corresponding magnetron anode current using a current meter. The measured output microwave power and the corresponding magnetron anode current are utilized as the benchmark of the output power of microwave hyperthermia apparatus. However, in practical application, the microwave hyperthermia apparatus needs a radiator connected in the coaxial cable terminals for radiating microwave. As there is a loss after microwave through the radiator, the current method for calibration output power of microwave hyperthermia apparatus is defective. It dose not consider the conversion efficiency of radiator, so in that method, the measured power is not the actual output power of microwave hyperthermia apparatus.To improving the method of power calibration of microwave hyperthermia apparatus, it is theoretically studied in this paper the properties of water on the microwave radiation and the interaction of microwave with water. The reflection and transmission coefficients of microwave (2450MHz) on the water surface is calculated. It is confirmed that the water can serve as a load to absorb microwave in the experiment device described in this paper. A new type microwave power calibration system has been designed, in which a certain amount of water is taken as load. The system consists of medical microwave source, microwave control systems, microwave power measurement device and a thermocouple for temperature measurement.The frequency of the used microwave is 2450MHz which present microwave hyperthermia apparatus uses. The control system consists of ARM9 MCU S3C2440 core coupled with the external magnetron anode current sampling and controling circuit, the temperature measuring circuit, RS232 to TTL level converter circuit, JTAG interface circuit and so on. The driver programes and application programs of the control system are writed in C language and compiled on the platform of linux2.6. The JTAG port and the RS232 serial port are designed to program in-situ. In teams of them, it is easily to adjust the circuit's working parameters or to update the control program. The RS232 serial port is also used for communication between embedded systems and PC, and the control interface on PC is designed by means of Labview. The microwave power measurement device consists of the glass container packaged with insulation material (asbestos, foam), the foam plugs, the blender, the plastic covers and a certain amount of water. The water temperature change in glass container is measured with a temperature sensor. Through a large number of experiments using this calibration system, the relationship between the microwave magnetron anode current and the microwave power radiated by the radiator is final determined. Thus a new and proper method for actual output power calibration of microwave hyperthermia apparatus is developed. It will play an active role in standardization the use of all kinds of microwave hyperthermia apparatus which are more and more popular in clinical practice.