Phase stability of a clinical phased array system for deep regional hyperthermia

Measurements were performed on the amplifiers of a phased array system to evaluate the dependence of phase on frequency. The power from the amplifiers was terminated into a 50 ohm load at the point of connection to the antennas of the applicator, and the power was sampled to determine the phase relation between amplifier channels. It was found that the measured phase difference between two amplifier channels can: (1) change by as much as 20° with frequency; (2) be as much as 12° different than the prescribed phase; and (3) be as much as 30° different than the phase measured and displayed by the system. Previous studies indicate that in order to accurately plan and deliver treatments with this type of device, it is necessary to know the phase relationship of the array to within at least 20° Since differences as great as 20° were obtained under ideal loading conditions, greater differences could result during normal (clinical) loading conditions, and these may not be compensated by the operator because of the inaccurate values reported by the system itself. These findings should be of concern to investigators using these devices since they could result in SAR distributions different from those planned and/or optimized for a particular patient. It is therefore recommended that the measurements such as those reported here also be performed on similar clinical devices as a standard quality assurance procedure so that the power steering capabilities of these systems can be utilized effectively and safely.