Dynamic Ultrasound Focusing and Centimeter-Scale Ex Vivo Tissue Ablations With a CMUT Probe Developed for Endocavitary HIFU Therapies

Thermal ablation of localized prostate tumors via endocavitary ultrasound-guided high-intensity focused ultrasound (USgHIFU) faces challenges that could be alleviated by better integration of dual modalities (imaging/therapy). Capacitive micromachined ultrasound transducers (CMUTs) may provide an alternative to existing piezoelectric technologies by exhibiting advanced integration capability through miniaturization, broad frequency bandwidth, and potential for high electroacoustic efficiency. An endocavitary dual-mode USgHIFU probe was built to investigate the potential of using CMUT technologies for transrectal prostate cancer ablative therapy. The USgHIFU probe included a planar 64 element annular high-intensity focused ultrasound (HIFU) CMUT array (f(HIFU) = 3 MHz) surrounding a 256-element linear imaging CMUT array. Acoustic characterization of the HIFU array included 3-D pressure field mapping and radiation force balance measurements. Ex vivo proof-of-concept experiments consisted in generating HIFU thermal ablations with the CMUT probe on porcine liver tissues. The planar CMUT probe enabled HIFU dynamic focusing (distance range: 32-72 mm) while providing acoustic surface intensities of 1 W/cm(2) that allowed producing elementary ex vivo ablations in depth of liver tissue (L x W approximate to 10 x 5 mm). Combinations of dynamic focusing, along with probe rotation and translation produced larger thermal ablations (L x W approximate to 20 x 20 mm) by juxtaposing multiple elementary ablations, consistent with expected results obtained through numerical modeling. The technical feasibility of using a USgHIFU probe, fully developed using CMUTs for tissue ablation purposes, was demonstrated. The HIFU-CMUT array showed tissue ablation capabilities with volumes compatible with localized cancer targeting, thus providing assets for further development of focal therapies.