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 connectio n to the antennas of the applicator, and the power was sampled to dete rmine 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 degrees with frequency; (2) be as much as 12 degrees different than the prescribed phase; and (3) be as much as 30 degrees different than the phase measured and displayed by the system. Previous studies indicate that in order to accurately plan and delive r treatments with this type of device, it is necessary to know the pha se relationship of the array to within at least 20 degrees. Since diff erences as great as 20 degrees were obtained under ideal loading condi tions, greater differences could result during normal (clinical) loadi ng conditions, and these may not be compensated by the operator becaus e of the inaccurate values reported by the system itself. These findin gs should be of concern to investigators using these devices since the y could result in SAR distributions different from those planned and/o r optimized for a particular patient. It is therefore recommended that the measurements such as those reported here also be performed on sim ilar clinical devices as a standard quality assurance procedure so tha t the power steering capabilities of these systems can be utilized eff ectively and safely.