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In this work, a new therapeutic ultrasound device is presented that is designed for performing minimally invasive thermal ablation of brain tumors under guidance with magnetic resonance imaging (MRI). The device consists of an array of ultrasound transducers, oriented on multiple faces of a flexible sheath with an integrated cooling system that can be directly inserted into the brain through a small burr hole in the skull. Heating can be monitored using real-time MRI and conformed to the tumor volume by varying the power to the individual elements on the probe. In this work, preliminary testing of the device was performed and included acoustic characterization, numerical simulations, and experiments in a clinical MRI system. Numerical simulations of the acoustic field and temperature rise during heating were compared with results of in vitro testing using bovine brain samples. The results demonstrate that the device has good MRI compatibility and is capable of generating output surface intensities of greater than 20 W/cm2, which is sufficient to ablate tissue at depths of more than 10 mm from the probe in less than four minutes of heating. [ABSTRACT FROM AUTHOR]

Interstitial therapeutic ultrasound devices are a promising technology for performing thermal ablation in a wide variety of organs. In this study, the use of Capacitive Micromachined Ultrasound Transducers (CMUTs) for interstitial heating applications was investigated. CMUTs exhibit potential advantages for use in therapeutic ultrasound applications in comparison to standard piezo ultrasound transducer technologies as they have good characteristics in terms of miniaturization (cell size: few dozens of microns), bandwidth (several MHz) and high electro-acoustic efficiency. Two designs of CMUT arrays were studied: (1) a 1D 128-element planar-CMUT array originally dedicated to abdominal ultrasound imaging purposes (5 MHz, element size: 0.3 × 8.0 mm2); (2) a 12-element linear-array, 32.4-mm long and 0.8-mm wide, developed specifically for minimally-invasive interstitial therapeutic applications (6 MHz, element size: 2.7 × 0.8 mm2). Simulations were performed to evaluate the ability to generate thermal lesions in soft tissues with: (1) 1 single linear array, (2) a combination of multiple linear arrays positioned on a cylindrical catheter. Experimental investigations performed with the CMUT imaging array showed the ability to generate surface acoustic intensities (Iac) up to 20 W×cm-2 and to generate intense centimetric thermal lesions in in-vitro turkey breast tissues. At 6 MHz, a single element was able to generate in water a maximum peak pressure of >0.5 MPa. In simulations, the ability to use various power levels and frequencies on independent elements, as well as combinations of multiple linear-arrays offered sufficient flexibility to achieve a wide variety of thermal ablation patterns in 3D. Simulated ablation volumes could be controlled to cover accurately non-symmetrical volumes of brain metastases. In conclusion, CMUT arrays show interesting characteristics, which may open new perspectives of spatial control for conformal interstitial thermal therapy with miniaturized multi-element therapeutic ultrasound catheters. [ABSTRACT FROM AUTHOR]