Background: Mini Annular Phased Arrays (MAPA) provide great versatility in targeted delivery of thermal dose and are therefore frequently used in hyperthermia treatment of deep-seated tumors. In recent years computational methods have emerged to accurately calculate the distribution of electromagnetic parameters (e.g. EM field, SAR) in the 3D volume of antenna applicators. This work concentrates on comparison of the experimental measurements of SAR to corresponding simulation results from HFSS (Ansoft Corporation), in a 4 antenna cylindrical MAPA.

Methods: A 3 dimensional robotic arm designed for SAR measurements was used to position the 3-directional EM field probe in the volume of the MAPA applicator (see Figure). The range of the motion of the arm was about 18 cm in each direction, but for safety reasons only 15x15cm scans were usually utilized. The motion of the probe was controlled by Matlab scripts that took into account the geometry and boundaries of the trajectory; at each stop on the probe’s trajectory a measurement was taken and recorded for analysis. The applicator (27.5 cm long, 23 cm ID) was filled with distilled water, which is usually used as bolus to provide a good refraction match.

Results: The antennas were connected to the calibrated sources of power, and the electric fields from various amplitude settings were measured. Most of the settings resulted in the measurements of a “Gaussian” shaped distribution of the SAR, with size of the hot spot on the order of 7 cm (FWHM). In comparison, the HFSS simulated spot size was on the order of 9 cm. The position of the hot spot was successfully moved in a range of 9 cm and 8 cm in two perpendicular directions in the axial plane of the applicator.

Conclusions: We found good qualitative agreement between measurements and simulations of the SAR. However direct comparison of the simulation parameters (antenna amplitudes and phases) to the input of real applicator is difficult because of the cable losses and impedance mismatch.