SAR characteristics of the Sigma-60-Ellipse applicator

Purpose: To characterize the basic performance of the Sigma-60-Ellipse applicator. Materials and methods: The E-field distributions were measured using Schottky diode sheets in a cylindrical phantom (diameter 26 cm, length 50 cm), filled with saline-water (2 g NaCl/L). The phantom was positioned symmetrically in the Sigma-60-Ellipse applicator. The stability of the SAR distribution was assessed as a function of power and frequency. Furthermore, the accuracy of target steering was evaluated at various frequencies. Finally, the SAR characteristics were compared with those of the Sigma-60 and the Sigma-Eye applicators. Results: The average 50% iso-SAR area increased from 241 to 296 cm(2) when the RF power increased from 100 to 1600 W. The SAR maximum was located in the centre of the applicator for the frequencies of 75-80 MHz and it moves towards the feet for higher frequencies (up to 3.5 cm at 120 MHz). The average 50% iso-SAR area decreased from 268 to 161 cm(2) With increasing frequency from 75 to 120 MHz. The 50% iso-SAR longitudinal length was almost stable (mean 21.3 cm) at 75-120 MHz for both power outputs of 400 and 800 W. As expected the 50% iso-SAR radial length decreased with frequency from 14.9 cm at 75 MHz to 8.4 cm at 120 MHz. There was a fair agreement between requested and measured target settings. At the lower frequencies of 75-90 MHz and at 100 MHz the SAR characteristics were almost identical to those of the Sigma-60 and Sigma-Eye applicators, respectively. Conclusion: At the frequency range of 75-90 MHz the Sigma-60 and at 100 MHz the Sigma-Eye can safely replaced by the Sigma-60-Ellipse applicator.