DESIGN OF APERTURE SOURCES FOR DEEP HEATING USING ELECTROMAGNETIC ENERGY

Currently, there exists interest in the use of electromagnetic energy (nonionizing) for research. One of the most important problems facing such applications is the proper design of applicators which deliver deep penetration of electromagnetic energy into the tissues. At the present, microwave aperture sources used for these applications often have shallow energy-penetration characteristics. In contrast to aperture sources with deep energy-penetration characteristics, aperture sources with shallow energy penetration require higher input power, allow larger scattered field into the environment, and cause often-unwanted higher heating to the surface tissues. In the present theoretical investigation, source frequency, aperture size and field distribution are parameters used for designing aperture sources which deliver deep penetration of electromagnetic energy into different sizes of spherical tissue-equivalent bodies. The source frequencies used in this calculation range from 10 MHz to 10 GHz. ISM frequencies (for unlicenced use in industrial, scientific and medical applications) in this range are included in the calculation. Three types of sources are used: Plane wave “Cap” aperture, and “Belt” aperture. The “Cap” aperture is a circular aperture shaped like a polar cap in contact with the irradiated sphere. The “Belt” aperture is a loop source that wraps around the irradiated sphere. Four different sizes and three different aperture field distributions of each aperture are used. Twelve sizes of the tissue-equivalent sphere are used with radii ranging from 0.5 to 10.0 cm. For each size of the sphere, four cases are considered: A single-layered muscle sphere, and three triple-layered spheres with muscle, fat and skin layers. The thicknesses of the fat layers of the three triple-layered spheres are respectively 10, 20 and 30% of the radius of the muscle sphere. The thickness of the skin layer is 1.0 mm in each case. The results indicate a strong resonance type of dependence on source frequency for deep electromagnetic energy penetration. This resonance frequency decreases with increased sphere size. The energy penetration is also found to be a strong function of the aperture size. With optimum design, the aperture sources can produce deeper energy penetration characteristics than obtained with plane wave sources. The optimum energy penetration into the smaller size spheres (less than 5.0 cm radius) is found to be much greater than that for the larger size spheres. In general, the optimum deep energy penetration characteristics of the triple-layered spheres are not as good as those of the single-layered spheres. An improvement over the energy penetration characteristics of some of the existing applicators can be obtained by using the optimum aperture frequency, size, shape and field distribution reported in this investigation. © 1979 Health Physics Society.