HEATING PATTERNS IN BIOLOGICAL TISSUE PHANTOMS CAUSED BY MILLIMETER-WAVE ELECTROMAGNETIC IRRADIATION

Distribution of millimeter wavelength electromagnetic energy absorptio n in surface layers of biological tissue models was studied using meth ods of Infrared Thermography. 0.1 mm thin-layer phantoms were irradiat ed in the near field using different types of horn antennas in the 37- 78 GHz frequency range. Heating patterns were recorded during microwav e irradiation, and surface SAR distributions were calculated. The temp erature resolution was better than 0.05 K. It was found that horn ante nnas produced nonuniform heating patterns in irradiated objects. These nonuniform patterns were due to a geometrical resonance resulting fro m a secondary wave-mode interaction between an irradiated object and t he corresponding critical cross-section of the horn antenna. Local SAR values in hot spots exceeded the spatially averaged values by over 10 times, and the widths of these hot spots at 5 times the average SAR w ere often 1 mm or less. The location, quantity, number and size of the local field absorption maxima of irradiated objects strongly depended on the frequency of electromagnetic irradiation, with equivalent Q-fa ctors of 500 or more. These findings provide an explanation for a numb er of frequency-dependent effects of millimeter wave electromagnetic i rradiation.