Passive Elastography for Clinical HIFU Lesion Detection.

High-intensity Focused Ultrasound (HIFU) is a promising treatment modality for a wide range of pathologies including prostate cancer. However, the lack of a reliable ultrasound-based monitoring technique limits its clinical use. Ultrasound currently provides real-time HIFU planning, but its use for monitoring is usually limited to detecting the backscatter increase resulting from chaotic bubble appearance. HIFU has been shown to generate stiffening in various tissues, so elastography is an interesting lead for ablation monitoring. However, the standard techniques usually require the generation of a controlled push which can be problematic in deeper organs. Passive elastography offers a potential alternative as it uses the physiological wave field to estimate the elasticity in tissues and not an external perturbation. This technique was adapted to process B-mode images acquired with a clinical system. It was first shown to faithfully assess elasticity in calibrated phantoms. The technique was then implemented on the Focal One® clinical system to evaluate its capacity to detect HIFU lesions in vitro (CNR = 9.2 dB) showing its independence regarding the bubbles resulting from HIFU and in vivo where the physiological wave field was successfully used to detect and delineate lesions of different sizes in porcine liver. Finally, the technique was performed for the very first time in four prostate cancer patients showing strong variation in elasticity before and after HIFU treatment (average variation of 33.0 ± 16.0 % ). Passive elastography has shown evidence of its potential to monitor HIFU treatment and thus help spread its use.

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 ( [Formula: see text] = 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/cm2 that allowed producing elementary ex vivo ablations in depth of liver tissue ( L ×W ≈ 10×5 mm). Combinations of dynamic focusing, along with probe rotation and translation produced larger thermal ablations ( L ×W ≈ 20×20 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.