Changes in heating patterns of interstitial microwave antenna arrays at different insertion depths.

The changes in heating patterns of interstitial microwave antennas at different insertion depths were investigated in a static phantom at 915 MHz. Antennas for the Clini-Therm Mark VI system were inserted 5-15 cm into muscle-equivalent material, through nylon catheters. The phantom was heated with arrays of antennas at 2 cm spacings for 60 s at 15 W per antenna. Midplane and transverse heating patterns were determined thermographically with the antennas inserted parallel or perpendicular to the split of the phantom. Hot spots, independent of heating near the junction plane, were observed in the midplane of the 2 x 2 and 2 x 4 arrays at 2.8 cm from the insertion end. The magnitudes of these hot spots were reduced by 40-45 per cent as insertion depth was increased from 7 to 10.5 cm. With insertion depths of more than 11.5 cm the hot spots gradually diminished and heating occurred mostly near the junction plane. The observed heating patterns were caused by changes in impedance of the antenna arrays at different insertion depths. The impedance mismatch had resulted in different wave propagation within the tissue material which produced different radiation patterns. During treatments, because heating varies with insertion depth, great care must be exercised in monitoring temperatures.