Infrared thermographic SAR measurements of interstitial hyperthermia applicators: errors due to thermal conduction and convection

Thermal conduction and convection were examined as sources of error in thermographically measured SAR patterns of an interstitial microwave hyperthermia applicator. Measurements were performed in a layered block of muscle-equivalent phantom material using an infrared thermographic technique with varying heating duration. There was a 52.7% reduction in maximum SAR and 75.5% increase in 50% iso-SAR contour area for a 60-s heating duration relative to a 10-s heating duration. A finite element model of heat transfer in an homogeneous medium was used to model conductive and convective heat transfer during the thermographic measurement. Thermal conduction artefacts were found to significantly distort thermographically measured SAR patterns. Convective cooling, which occurs when phantom layers are exposed for thermal image acquisition, was found to significantly affect the magnitude, but not the spatial distribution, of thermographically measured SAR patterns. Results from this investigation suggest that the thermal diffusion artefacts can be minimized if the duration of the applied power pulse is restricted to 10 s or less.