The development of intracavitary ultrasonic applicators for hyperthermia: a design and experimental study

This study investigated the design concepts and development of a multielement intracavitary ultrasound applicator for use in hyperthermia. A necessary condition imposed on these applicators is that each transducer element be separately powered and produce collimated beams. This way, the power deposition within the target volume can be controlled by varying the power to each element. Theoretical computer simulations (acoustic and thermal) and bench experiments were used to determine the constraints on the transducer element size and the spacing between them. These have shown that the length of the cylindrical segments (or subsections of) must be greater than approximately 10 lambda for proper collimation and that the spacing between them must be less than approximately 1.5 mm for uniform heating. With these design principles in mind, applicators were constructed using sections of cylindrical transducers (wall-thickness resonance). These were surrounded by temperature-controlled circulating water which was enclosed by a latex membrane. This allowed for acoustic coupling and additional control over the depth of the maximum temperature from the cavity wall. This depth could be varied between the cavity surface and up to 1.5 cm for circulating water temperatures between 5 and 42 degrees C, respectively. These applicators were tested in vivo and were able to induce controlled transrectal heating, at depths of 2-3 cm, in the canine rectum and prostate gland.