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

The performance characteristics and thermal coagulation of tissue produced by directional air-cooled, direct-coupled interstitial ultrasound (US) appl icators were evaluated. Prototype applicators (2.2 mm o.d,) were constructe d using cylindrical transducers sectored into angular active zones of 90 de grees, 200 degrees, 270 degrees, and 360 degrees, Acoustic characterization of the applicators showed the beam output to be angularly directed from th e active sector of the transducer and collimated within the axial extent. E mpirical 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 f low rate of 5.6 L min(-1). Thermal performance of the applicators was chara cterized through high temperature heating in vivo (porcine thigh muscle, 11 trials) and in vitro (bovine liver, 46 trials). Results demonstrated direc tional coagulation of tissue, with good correlation between the angular ext ent of the lesions and the active acoustic sector, Radial depth of coagulat ion 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 ove r the course of 1-10 min heating trials. Implementation of air-cooling with in direct-coupled interstitial US applicators provided enhanced directivity of heating in angular and axial dimensions, and significantly increased th e power handling and radial depth of tissue coagulation. (C) 1999 World Fed eration for Ultrasound in;Medicine & Biology.