2001 Volume 17 Issue 1 Pages 33-43
In hyperthermic treatment of malignant tumors, thermal tissue injury increases dramatically with every degree that the tissue increases above 42.5 °C. Accurate temperature monitoring during hyperthermia is important. Generally, tissue temperature is monitored by inserting sensors into the target tissue ; however, the painful procedure is not tolerated by some patients. We devised a noninvasive method to monitor tissue temperature during radio-frequency hyperthermia. The method functions by detecting the magnetic field induced by the radio-frequency currents that flow through the heated tissue. This technique uses small multi-channel coil antennas to detect radio-frequency currents and generates a two-dimensional image of their distribution in the tissue. The temperature distribution was estimated from the temperature increase and the intensity of the current of the radio frequency. A 4 % agar phantom was used as a model for target tissue receiving hyperthermic treatment. Around the agar phantom, 16 small coils were arranged in a ring. A rectifying circuit and a leveling circuit were connected to each coil antenna, and the current was converted with a fixed resistance into voltage. Since the voltage output from each antenna was attenuated at 1/2πr (r : distance from the radio-frequency current), single-peaked projection data were prepared. After treatment with various signals, radio-frequency currents that flowed through the heated object were determined as a two-dimensional current distribution profile by back-projection. When the insertion of an iron rod into the agar phantom produced a hot spot, the current distribution was examined and compared with the two-dimensional temperature distribution evaluated by thermography. A good correlation was observed between the distribution of radio-frequency currents detected by the coil antennas and the temperature distribution detected by thermography. The linear relationship observed in this study between the two-dimensional distribution of radio-frequency currents and the thermographic temperature distribution suggests the possibility of non-invasive evaluation of temperature distribution in the hypothermic target and the clinical usefulness of this method for temperature monitoring during hyperthermia.
