Background: The purpose of this research was to determine to what extent the SAR distribution within a water-filled APA applicator can be shifted around the interior by adjusting the phase of four input signals at a specific frequency.

Methods: A water-filled APA applicator was investigated for RF-induced hyperthermia in extremity tumors (Figure 1).
Fig. 1. (a) A schematic diagram of the APA.
(b) Centrally focused pattern. (c) Off-center focused pattern
Return loss (S₁₁) and local SAR were simulated over 100-180 MHz using HFSS (Ansoft Corp). A specific frequency was chosen to provide a good impedance match and appropriate focus of energy in the desired regions. At that frequency, the relative phases (0^o-180^o) of first one channel, then two channels were investigated with 50 Watts on each channel. The solution data of local SAR were exported into a matlab program, which shows the location, maximum value and size of the focal hot spot of each case (Figure 2).
Fig. 2. Hotspot Region in the xy_Plane with different 4 Channel Phases.
Phases are labeled as subtitles.

Results: Figure 2 shows that at 140 MHz, when the four inputs are in phase, the local SAR is concentric within the applicator and approximately 9 cm diameter, with a maximum of 24.3 W/kg at the center. The SAR peak shifted away from the antennas with increased phase. Further increases in phase caused the energy to split into two separate regions. In all simulations, the maximum local SAR was produced when all 4 inputs were in phase.

Conclusions: At 140 MHz, SAR can be adjusted to move a single 9 cm diameter focal hot spot within 75% of the array diameter by adjusting relative phase of four inputs. Attempts to shift the focus further off-center breaks the SAR into multiple hot spots.