Analysis and testing of a concentric ring applicator for ultrasound hyperthermia with clinical results

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Abstract
A planar ultrasound transducer was modified by etching concentric circles on one surface of a piezoelectric ceramic to create four rings. The 10 cm diameter transducer had four active rings and an unenergized centre. The transducer housing was designed to be completely immersed in fluid, suitable for intraoperative hyperthermia. The transducer was resonant at 1.0 MHz and was tested in a water tank and in an acoustic absorbing medium where the steady-state temperatures were measured. A comparison between a single 10 cm element and the concentric ring modification with all rings at equal power density showed the performance to be nearly identical. In vivo experiments in canine thigh verified the phantom predictions as individual rings were energized. Theoretical intensity calculations were made and compared favourably to water tank test results. Clinical hyperthermia treatments for chest wall and head and neck tumours showed that the temperature distribution could be highly modified by adjusting the power to individual rings while holding the transducer stationary. Automated temperature mapping parallel to the transducer face was used to compare a single element applicator to the concentric ring applicator in clinical treatments on the same lesion. The concentric ring applicator was radially adjustable and was found to be advantageous in lowering the central peak temperatures and flattening the temperature distribution in tumours. A comparison between the single element clinical and operating room series showed that when pain is removed as a treatment limiting factor, higher central tumour temperatures are possible and more of the tumour volume achieves therapeutic temperatures. The concentric ring design improves the temperature distribution such that the higher central temperatures will not be necessary.

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