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 electro-acoustic 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 HIFU CMUT array ( fHIFU = 3 MHz) surrounding a 256-element linear imaging CMUT array. Acoustic characterization of the HIFU array included 3D 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×W ≈ 10 mm × 5 mm). Combinations of dynamic focusing, along with probe rotation and translation produced larger thermal ablations (L×W ≈ 20 mm × 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.