EUS-guided High-Intensity Focused Ultrasound

Background/Rationale: High-intensity focused ultrasound (HIFU) is a rapidly developing technology that is becoming more widely used for non-invasive and minimally invasive ablation of benign and malignant tumors. In addition, recent studies suggest that unique mechanical effects of HIFU may help to enhance targeted drug delivery and stimulate an anti-tumor immune response in many tumors including pancreatic tumors. However, targeting of pancreatic tumors using an extracorporeal source is often not possible due to the lack of an adequate acoustic window because of the presence of overlying bowel gas. The development of an EUS-guided HIFU transducer has many potential benefits including improved targeting, decreased energy requirements and decreased potential for injury to intervening structures.

Aims: To design, develop and test an EUS-guided HIFU transducer designed for endoscopic applications.

Methods: Specifications regarding the dimensions of the HIFU transducer were initially determined based on current therapeutic EUS specifications (diameter and rigid tip length). A metal housing was designed to house an EBUS imaging transducer for image guidance, a spherically curved PZT element (radius of curvature=35mm, frequency=3.9MHz), two passive cavitation detectors and circulating water channels. The metal housing is fitted with a balloon that is filled with water for ultrasound coupling. The acoustic output of the transducer output was tested with radiation force and hydrophone measurements. Characterization of lesion formation was performed using polyacrylamide gel phantoms and in ex vivo bovine livers. Preliminary in vivo studies were performed to assess the ergonomics of the transducer design and demonstrate the feasibility of targeting and creating lesions in the porcine pancreas.

Results: The focal distance was measured to be 35mm from the surface of the transducer with focal dimensions of 2mm (diameter) x 10mm (axial). The transducer successfully created lesions in gel phantoms and ex vivo bovine livers with focal acoustic intensities of 250 W/cm². In vivo studies demonstrated that targeting and creating lesions in the porcine pancreas was feasible.

Conclusions: An EUS-guided HIFU transducer was successfully designed and developed with dimensions that are appropriate for endoscopic use. Further development of this technology will allow endoscopists to perform therapeutic ablation of peri-lumenal lesions, expanding the capabilities of the therapeutic endoscopist.