Cavitation enhances treatment depth when combined with thermal effect using a plane ultrasound transducer: an in vivo study

Minimally invasive techniques using miniaturised ultrasound applicators have been developed for surgical and medical applications. However, the volume of treatment remains limited. Therefore, we propose to combine cavitation with a thermal effect to enhance the treatment depth. This study was conducted in vivo on pig liver, 51 exposures were performed on the surface of the liver, with a miniaturised applicator using a plane transducer (3×10 mm²) operating at 4.75 MHz. This study aimed to compare two methods of treatment. The first one produced the thermal effect exclusively. The second method used cavitation in addition to the thermal effect. Cavitation bubbles were initiated along the beam axis by a 60 W/cm² pulse (0.5 s) followed by a continuous exposure, which generated the thermal effect. The method using cavitation produced the deepest lesions. At an intensity of 30 W/cm² applied for 30 s, the lesions induced with cavitation bubbles were 1.7 times deeper than those produced without cavitation bubbles. Histological analysis shows that they correspond to coagulation necroses. Thus, these results show that the initiation of cavitation bubbles prior to the exposure increases the therapeutic depth significantly. Consequently, this method might open the way to the treatment of large diameter tumours.