Development of small high intensity focused transducers for ultrasound endotherapy

The potentialities of HIFU for selective non-invasive tissue destruction have demonstrated experimentally and clinically for extracorporeal or transrectal applications. We have now developed small HIFU transducers for endoscopic purposes. Two probes were built and tested with in vivo tissues. The transducers were piezocomposite spherical strips with a working frequency of 3.0 MHz. Targeting was achieved with a ultrasound linear array (5 MHz). Thirty New Zealand hybrid rabbits were used for therapeutic parameter determination. The beam was directed transcutaneously to the liver at depths of 22 mm from the abdominal wall. The endoscopic feasibility was assessed by using rabbits bearing a percutaneous gastrostomy and targeting volumes of liver through the gastric wall. Autopsies were performed 7 hours after treatment. For transabdominal application, a 3.15 cm³ mean volume of coagulation necrosis were reproducibly obtained in the liver at the desired site with a reference ultrasound power of 10 W (Focal Power without absorption) and a timespan of 12 s. For transgastric application, the mean volume was 5.5 cm³ with 7.5 W and 5 s, respectively. No gastric wall burns were observed with the treatment parameters defined. HIFU can be applied successfully with small transducers readily adaptable to the tip of an endoscope, thus opening a new perspective for endotherapy