The development of improved ultrasound heaters suitable for superficial tissue heating.
The acoustical fields generated by several ultrasound heaters have been measured to determine their suitability for heating superficial lesions in cancer patients. Three different acoustical sensors were used in the study, and one sensor, consisting of a thermocouple embedded in a sound-absorbing plastic, proved to be the most practical device for routine use. The method employed to mount the piezoelectric crystal in the heater was found to have a profound effect on the acoustical field generated by the heater. Clamping the crystal close to its edge resulted in a field with a prominent acoustical maxima on the crystal axis. A flexible attachment at the very edge of the crystal resulted in a much more uniform field which could be further improved by rapidly sweeping the frequency of the signal used to excite the piezoelectric crystal. A heater in which the rear electrode of the piezoelectric crystal is sectioned into a series of concentric rings has also been developed. This heater can deposit more acoustical energy at the edges of a treatment volume than at the center, and the diameter of the ultrasound beam can also be adjusted. The results demonstrate that (i) the acoustical fields of ultrasound heaters must be accurately measured before using the heaters clinically, and that (ii) by careful design, both uniform or controlled nonuniform acoustical fields can be generated.