Two high-resolution thermal monitoring sheets for clinical superficial hyperthermia

PURPOSE: Temperature measurement during superficial hyperthermia is limited by poor spatial resolution. We investigated two sheets to improve temperature monitoring of the skin surface.

METHODS AND MATERIALS: Two different sheets were studied with a grid of temperature sensors with one sensor per ~5 cm2. The first was a matrix of multisensor thermocouple probes laced through a silicone sheet. The second sheet had rows of thermistors connected by meandering copper leads mounted on stretchable printed circuit board (SPCB). Accuracy, temperature resolution and two hour stability of both sheets were investigated. Furthermore, we determined the ability to follow body contours, thermal conduction errors and electromagnetic (EM) compatibility to clinically used 434 and 915 MHz hyperthermia applicators.

RESULTS: For both sheets the accuracy (≤0.2 °C), temperature resolution (≤0.03 °C) and stability (≤0.01°C hr-1) were adequate for clinical use. Thermal conduction errors ranged from 5.25 - 11.25 mm vs. 2.15 mm for the thermocouple probe and thermistor, respectively. Both sheets could follow body contours, where the ratio air/ water bolus surface was <5%. When aligned perpendicularly to the EM field the meandering copper tracks used on the SPCB did induce self-heating, while the thermocouple probes did not. Self-heating had a linear relationship with the angle of the leads with respect to the EM field direction for both sensors at both frequencies. Self-heating of the thermistor was similar for both frequencies, while it was circa two-fold higher for 915 vs. 434 MHz for the thermocouple.

CONCLUSION: The use of SPCB technology for skin surface monitoring was promising. However, suppressing self-heating induced by the horseshoe shaped copper tracks needed for stretchability of the SPCB requires more in-depth investigation. The thermocouple matrix was the most promising for clinical application, meeting 6/7 of the major requirements for skin surface temperature monitoring when positioned perpendicular to the EM field.