The design and evaluation of interstitial ultrasound applicators for controlled thermal treatment of localized tumors

Abstract

This dissertation investigates the design and development of interstitial ultrasound applicators to provide localized and controlled heating of tissue for potential clinical implementation as an alternative thermal treatment of cancerous and benign diseases. Implantable ultrasound applicators were designed and fabricated using cylindrical piezoceramic transducers (1.5–2.5 mm o.d., frequency =6.5–7.5 MHz) in a direct‐coupled configuration. Transducers were longitudinally sectioned to produce specific angular sectors of acoustic output (90°–360°). Applicators were also constructed using multiple transducer elements (3) with individual power control. An internal cooling mechanism (air or water) was implemented to remove excessive thermal energy from the inner transducer surface (h=700–4200 Measurements of the relative acoustic intensity were used to characterize the efficiency, quality, and directivity of the ultrasound beam output. Thermal performance was experimentally evaluated through multiple heating trials in vivo (canine prostate, porcine liver, and thigh muscle), in vitro (porcine liver, muscle, and kidney), and in tissue‐mimicking phantom for both hyperthermia and coagulative thermal therapy. Theoretical capabilities of these applicators were evaluated using biothermal computer simulations to model the acoustic output and resultant heating in tissue. In general, results demonstrated excellent thermal performance and control of heating in tissue. Directed and collimated thermal coagulation was produced in both the angular and axial expanse, and found to correspond to the active acoustic sector of the transducer. The use of multiple transducer elements provided active control of axial heating in the tissue. Increased levels of applied power (12–48 W) with transducer cooling substantially improved radial depths of coagulation (8–22 mm) and decreased sonication times (0.5–5 min). This ability to control and direct heating shows great potential for the treatment of localized tumors in sites such as the prostate, brain, liver, and breast, without damaging the surrounding healthy tissue.