Use of thermocouples in the intense fields of ferromagnetic implant hyperthermia

Thermocouples have been used in the intense fields of ferromagnetic implant hyperthermia for extensive temperature measurements. Electromagnetic interference at 100 kHz is minimized with feed-through filters mounted on a copper plate in a shielded assembly. The thermocouples are temperature compensated in the software by monitoring the temperature of the copper plate. D-shell connectors are used for multiple sensor thermocouple connections and to provide proper shield termination of external cables. The technical approaches of grounding, shielding and filtering are examined, and experiments have been conducted to determine the proper usage of thermocouple during ferromagnetic implant treatments. The dependence of filter performance on thermocouple impedance and the effect of D-shell connections on temperature accuracy have been studied. The results indicate that self-heating due to electromagnetic energy coupled into 51 microns thermocouple leads and unbalanced current along 76 microns leads are insignificant at 100 kHz. But severe heating has been observed in 511 and 813 microns leads, especially in high-conductivity copper wires. By using 25-50 microns thermocouple leads, better than 0.1 degree C accuracy and 0.05 degree C resolution have been achieved during ferromagnetic hyperthermia. At field strength of 1500 A/m, artifacts of 0.03-0.05 degree C are observed.