Ferrite Sleeve as a Choke for Intracavitary Microwave Hyperthermia Treatment Using Coaxial Antennas

The study of ferrite sleeve for realizing electrically short choke is reported in this work for coaxial antennas used in intracavitary microwave hyperthermia treatment. Electromagnetic simulations of intracavitary applicators with the optimized ferrite sleeve operating at 700, 915, and 2450 MHz demonstrated stable resonance and targeted power deposition independent of the insertion depth when compared to their unbalanced counterparts. Thermal simulations indicated spherical heating volume for the applicators with the ferrite sleeve. Intracavitary applicator prototypes of \boldsymbol {\lambda }/\mathbf {4} and \mathbf {3\lambda }/\mathbf {8} coaxial monopoles with the optimized ferrite choke indicated return loss more than 22, 27, and 22 dB at 700, 915, and 2450 MHz, respectively, for varying insertion depths (70-130 mm) in agreement with the simulations. Local electric field measurements at 915 MHz in phantom indicated 51% and 58% reduction in 25% field contour along the antenna axis for the \boldsymbol {\lambda }/\mathbf {4} monopole with \mathbf {0}.\mathbf {24\lambda } ferrite sleeve and \mathbf {3\lambda }/\mathbf {8} monopole with \mathbf {0}.\mathbf {12\lambda } ferrite sleeve, respectively. Specific absorption rate and thermal measurements in phantoms confirm that ferrite sleeve < \boldsymbol {\lambda }/\mathbf {4} is sufficient to choke the secondary current on coaxial monopole antennas for delivering targeted power deposition at varying tissue insertion depths.