Temperature measurements in high thermal gradients: I. The effects of conduction

Two temperature probes (a fluoroptic sensor and a metallic thermistor), which are both suitable for stereotaxic implantation, were used in comparative thermometry studies during interstitial microwave heating of the brain in vivo. Thermal distributions having large temperature gradients (5-10 degrees C/cm) were routinely observed. The temperature differentials (delta T) between the 2 probes were position dependent within the thermal field. The maximum difference in temperatures measured, using the 2 probes along identical tracks without a catheter, ranged between 0.5 degree C and 1.8 degree C. Near the brain/air surface, the thermistor measured lower temperatures than the optical probe; however, medial to the antenna, the thermistor temperatures were higher than the optic sensor. The measured temperature discrepancies are the result of smearing due to thermal conduction along the axial length of the metallic thermistor probe. These effects are significantly accentuated when the temperature probes are tracked in catheters. Experiments performed in a nonperfused phantom, heated with the interstitial microwave antenna, demonstrated similar conductive effects. Studies in a nonelectromagnetic environment (flow cell-thermal step gradient) additionally confirmed that thermal conductive artifacts were the major source of temperature error.