Capacitive Micromachined Ultrasound Transducers for Interstitial High-Intensity Ultrasound Therapies

Capacitive micromachined ultrasound transducers (CMUTs) exhibit several potential advantages over conventional piezo technologies for use in therapeutic ultrasound(US) devices, including ease of miniaturization and integration with electronics, broad bandwidth (>several megahertz), and compatibility with magnetic resonance imaging (MRI). In this paper, the electroacoustic performance of CMUTs designed for interstitial high-intensity contact US (HICU) applications was evaluated and the feasibility of generating US-induced heating and thermal destruction of biological tissues was studied. One-dimensional CMUT linear arrays as well as a prism-shaped 2-D array composed of multiple 1-D linear arrays mounted on a cylindrical catheter were fabricated. The electromechanical and acoustic characteristics of the CMUTs were first studied at low intensity. Then, the acoustic output during continuous wave (CW) driving was studied while varying the bias voltage ( $V_{DC}$ ) and driving voltage ( $V_{AC}$ ). US heating was performed in tissue-mimicking gel phantoms under infrared (IR) or MR-thermometry monitoring. Acoustic intensities compatible with thermal ablation were obtained by driving the CMUTs in the collapse- snapbackoperation mode ( $I_{ac} = 10 - 30\text {W}\cdot \text {cm}^{-2}$ ). Hysteresis in the acoustic output was observed with varying $V_{DC}$ . IR- and MR-thermometry monitoring showed directional US-induced heating patterns in tissue-mimicking phantoms (frequency: 6–8 MHz and exposure time: 60–240 s) extending over 1.5-cm depth from the CMUT surface. Irreversible thermal damage was produced in turkey breast tissue samples ( $\Delta T_{\mathrm {max}} > 40^{\circ }\text{C}$ ). Multidirectional US-induced heating was also achieved in 3-D with the CMUT catheter. These studies demonstrate that CMUTs can be integrated into HICU devices and be used for heating and destruction of tissue under MR guidance.