Design and in vitro results of a high intensity ultrasound interstitial applicator

Interstitial technologies were proposed for the treatment of deep-seated and unresectable tumours. This study was designed to demonstrate the possibility of producing coagulation necrosis for a short exposure duration (20s) and a large volume with an interstitial ultrasonic applicator equipped with a plane transducer. The applicator was evaluated in terms of electroacoustic efficiency (58% at 10.7 MHz), temperature pattern and in vitro pig liver tissues destruction. The temperature elevation reached 52, 39, 22 and 15 degrees C at, respectively, 2.5, 7.5 and 10 mm from the applicator surface along the propagation axis. The evident cigar-shaped lesions spread on a depth of (8 +/- 2) mm from the surface applicator, (10 +/- 1) mm high and (3 +/- 1) mm high and (3 +/- 1) mm wide. The temperature measurements showed the limitations encountered with a 10 MHz transducer. The high increase in temperature of tissues close to the transducer can induce vaporisation and a gaseous barrier opaque to ultrasound. Owing to the shape and the operating frequency of the transducer, we hope to obtain the same satisfactory performances in vivo with weakly perfusion-dependent lesions. Axial and longitudinal applicator motions will enable us to treat more important volumes.