Axial control of thermal coagulation using a multi-element interstitial ultrasound applicator with internal cooling

A multi-element, direct-coupled ultrasound (US) applicator with internal wa ter cooling was investigated for axial control of interstitial thermal coag ulation. A prototype implantable applicator was constructed with a linear a -ray of three tubular PZT ultrasound transducers teach 2.5 mm OD, 10 mm len gth, 360 degrees emittance). Acoustic beam distributions from each element were measured and found to be collimated within the transducer length. The internally cooled applicator could sustain high levels of applied power to each transducer (0 to 40 W) and maintain acceptable applicator surface temp eratures (<100 degrees). Thermal performance of the applicator was investig ated through heating trials in vivo (porcine thigh muscle and liver) and in vitro (bovine liver). The radial depth of thermal lesions produced was dep endent on the applied power and sonication time and was controlled independ ently with power levels to each transducer element. With 18 W per element ( applied electrical power) for 3 min, cylindrical thermal lesions were produ ced with a diameter of similar to 3 cm and a length ranging from 1.2 cm (wi th one element) to 3.5 cm (three elements). Higher powers (24 to 30 W) for 3 to 5 min provided increased depths of coagulation (similar to 4 cm diamet er lesions). Analysis of axial lesion shapes demonstrated that individual v ariation of power to each transducer element provided control of axial heat ing and depth of coagulation (for custom lesion shapes); lesion lengths cor responded to the number of active transducers. This ability to control the heating distribution dynamically along the length of the applicator has pot ential for improved target localization of thermal coagulation and necrosis in high temperature thermal therapy.