Angular directivity of thermal coagulation using air-cooled direct-coupled interstitial ultrasound applicators

The performance characteristics and thermal coagulation of tissue produced by directional air-cooled, direct-coupled interstitial ultrasound (US) applicators were evaluated. Prototype applicators (2.2 mm o.d.) were constructed using cylindrical transducers sectored into angular active zones of 90 degrees, 200 degrees, 270 degrees, and 360 degrees. Acoustic characterization of the applicators showed the beam output to be angularly directed from the active sector of the transducer and collimated within the axial extent. Empirical determination of the average convective heat transfer coefficient, resulting from airflow cooling the inner surface of the transducer, showed significantly high levels of transfer (> 700 W m(-2) degrees C(-1)) with a flow rate of 5.6 L min(-1). Thermal performance of the applicators was characterized through high temperature heating in vivo (porcine thigh muscle, 11 trials) and in vitro (bovine liver, 46 trials). Results demonstrated directional coagulation of tissue, with good correlation between the angular extent of the lesions and the active acoustic sector. Radial depth of coagulation with a 200 degrees applicator extended 8-17 mm, with a heating time of 1-10 min, respectively. Angular and axial lesion shape remained similar over the course of 1-10 min heating trials. Implementation of air-cooling within direct-coupled interstitial US applicators provided enhanced directivity of heating in angular and axial dimensions, and significantly increased the power handling and radial depth of tissue coagulation.