The development of a high impedance all ceramic thermistor resistant to microwave irradiation: its possible medical application.

We developed a prototype all ceramic thermistor (ACT) composed of a silicon carbide (SiC) ceramic thermistor and SiC ceramic fibre-lead wires in order to accurately measure tissue temperature during microwave heating treatment. High density SiC ceramics reflects electromagnetic waves and manifests microwave resistant properties. The resistance of the SiC ceramic thermistor is approximately 15 M Ohm at 20 degrees C and has a sensitivity of approximately 200 K Ohm/ degrees C. SiC ceramic fibre resistance is approximately 40 K Ohm/cm, whereas its sensitivity is approximately 10 Ohm/ degrees C-cm, significantly lower than that of the SiC ceramic thermistor. The resistance of the SiC ceramic fibre is stable. This enables the ACT to be designed with the two-probe method instead of the four-probe method which was adopted in the Bowman type thermistor, resulting in a greatly simplified model. The measurement error caused by the temperature dependence of SiC ceramic fibre is < 0.02 degrees C. The ACT was inserted into muscle-equivalent agar phantom and into gluteal muscles of rabbits, and microwave irradiation was carried out. We recorded the phantom and tissue temperature without disturbance by the electromagnetic field for heating. This prototype ACT appears promising for its possible clinical applications in such settings as hyperthermic cancer therapy in man.