Changes in heating patterns due to perturbations by thermometer probes at 915 and 434 MHz

The changes in heating patterns due to perturbations by thermometer probes in microwave fields were investigated in static phantoms at 915 and 434 MHz. Thermograms taken parallel to the plane of E and H fields, at depths of up to 2 cm, indicated heating changes of +25 to -45 per cent at 915 MHz and +/- 15 per cent at 434 MHz. The amount of perturbation is dependent on the length, size and location of the probes in the RF fields and their orientations to the electric field. If proper probe placement techniques are not observed when metallic probes are involved, hot and cool spots can be generated and shifted to sites that are not measured. Therefore misleading temperatures can result when changes in heating patterns are not detected. Perturbation also varies with applicator designs and phantom geometry. If thermistors and thermocouples are used, the effects of perturbation should be investigated with individual applicators under applicable clinical conditions.