Power deposition patterns of interstitial microwave hyperthermia applicators having tapered dielectric sheaths

A coaxial dipole inserted into a cylindrical dielectric sheath is widely used as an interstitial microwave hyperthermia applicator. A means is described here of controlling the current distribution along the dipole and consequently controlling the distribution of the radiated power density by tapering the outside diameter of the dielectric sheath. Numerical results have been obtained for nylon catheters having three different cross-sections using the theory of linear, insulated antennas embedded in electrically dense media. The inner diameter of the dielectric sheath is the same in the three cases and is equal to the diameter of the conductor. The outer diameter is uniform in case 1, linearly tapered in case 2 and nonlinearly tapered in case 3. Numerical results were obtained at a frequency of 915 MHz, assuming a relative permittivity in the ambient medium of (51–j31), which is typical of tissue having a high water content. The tapers were selected such that the resonant length was the same in the three cases. It was found that the power density close to the tip of the applicator was increased considerably through the use of a linearly tapered sheath, with further improvement being obtained using the nonlinear taper.