A comparative heating-pattern study of direct-contact applicators in microwave diathermy

Heating patterns, induced in planar and limb phantoms of simulated fat and muscle tissue by large and by small, direct-contact microwave diathermy applicators operating at 2.45 GHz, were studied. Teflon slab-loaded rectangular waveguides, circularly polarized horns, and other large radiators were investigated as well as the only two low-power direct-contact emitters presently in noninvasive clinical use. The integrity of temperature measurements by a thermographic camera was established and probe thermometry was used to compare polyethelene with a silk screen as the midplane phantom film. The width and depth of penetration of microwave heating were determined to evaluate energy deposition inside simulated muscle tissue. The results of temperature profiles show that a Teflon slab-loaded rectangular waveguide and a circularly polarized horn provide uniform heating in the center of the phantom's midplane and that the focusing effects of limb phantoms enhance depth of penetration. Measurements of the electric field demonstrated that direct-contact applicators can be designed to maintain leakage levels below 5 mW/cm per 100 watts of forward power for large applicators, and below 5 mW/cm² per 10 watts of forward power for small applicators at a distance of 5 cm. Leakage of a circular aperture horn can be minimized with an output flange choke.