Influence of water bolus temperature on measured skin surface and intradermal temperatures

Temperature measurements utilizing thermometry probes placed on the skin surface are often used clinically to assess temperatures for the purposes of power control and evaluating treatment efficacy. There is a question, however, as to what extent applicator temperature-controlled liquid coupling boluses can create temperature gradients which can cause significant differences between measurements taken by sensors placed on the skin surface and the actual temperature of the tissue beneath. To address this question, experiments were conducted with human subjects instrumented with surface and shallowly implanted temperature sensors. Microwave applicators with circulating bolus water set at 21 and 41 degrees C were used to induce a temperature gradient in depth in the superficial tissue by thermal conduction. No microwave energy was applied. The average measurement offset at 41 degrees C was 15% of the difference in temperature between the interstitially measured skin temperature and the coupling bolus temperature, towards the temperature of the coupling bolus. The corresponding offset with coupling boluses set near 21 degrees C was 32%. Different water bolus types and volumes were observed to induce different percentage offset errors.