The concept of using multifrequency energy transmission to reduce hot spots during deep-body hyperthermia

We propose a multifrequency energy transmission scheme for annular phased arrays in deep-body hyperthermia. The problem of frequency dependent resonance effects reported in hyperthermia phantom calibration studies, leading to skewed energy deposition patterns and unwanted energy maxima (hot spots), is addressed by comparing broadband energy transmission with conventional monofrequency excitation. A heating system, including microstrip spiral applicators placed at a cylindrical surface in four quadrants, was designed to study this new concept in a homogeneous phantom. The system includes an option to select three cw tones for simultaneous transmission within a frequency range from 232 to 936 MHz. Quantitative analyzes of E-field probe-scan data, generated in a 6 g/L saline phantom and evaluated by three figures of merits defined, demonstrate the possibility of adding up power in the focus site while smearing secondary maxima outside this volume. Random selection of the three sinusoids within this band shows a probability of 0.7 to improve performance compared to the conventional single-frequency heating. A numerical study of the electromagnetic problem was also implemented using the FD-TD method. Analysis shows acceptable agreement between measured and simulated two-dimensional energy deposition patterns.