The effect of coupling materials on specific absorption rate distributions at 915 MHz

Deionized water is commonly used to couple microwave applicators to the patient surface in the administration of local hyperthermia. Profiles of the specific absorption rate (SAR) at 1-cm depth show that deionized water coupling significantly distorts the SAR distributions of the Clini-Therm 915-MHz 10 X 10 and 15 X 15 cm2 applicators. Maxima and minima that are discernible in the SAR profiles obtained by direct applicator load contact are amplified producing unexpected hot and cold regions in the heating pattern. An exception is coupling achieved by use of the Clini-Therm cooling pad, oriented such that the direction of deionized water flow is perpendicular to the electric field. The distortion in the SAR distribution can also be eliminated by replacing deionized water with mineral oil, a material having a much lower dielectric constant (epsilon = 2). The SAR profiles for mineral oil coupling are comparable to those obtained for direct contact; however, the efficiency of power transfer is slightly less (70%) and the level of microwave leakage is approximately four times greater.