Preliminary Investigation of a 64-element Capacitive Micromachined Ultrasound Transducer (CMUT) Annular Array Designed for High Intensity Focused Ultrasound (HIFU)

Abstract

BackgroundTreatment of prostate cancer using endocavitary High Intensity Focused Ultrasound (HIFU) has become more commonplace since the first treatments in the 1990s. The gold standard HIFU strategy to treat prostate cancer is the complete thermal ablation of the entire prostate gland under real-time ultrasound (US) image guidance. A more desirable treatment and the current trend, however, is towards a focal treatment but more accurate and finely tunable thermal lesions are needed along with improved US imaging guidance. In this study, Capacitive Micromachined Ultrasound Transducer (CMUT) technology is being investigated, as they have shown recent promise for US imaging and potential to be used for HIFU therapy. They offer potential advantages over current piezoelectric designs in the context of ultrasound-guided HIFU (USgHIFU) focal therapies. ObjectiveThe presented study evaluates the ability of a planar annular array CMUT design to achieve HIFU dynamic focusing and feasibility of generating thermal lesions in biological tissues. MethodThe proposed CMUT design consists of a 64-element annular array for HIFU delivery with a space in the center that accommodates a high-resolution 256-element linear imaging array. The pressure field simulations of the HIFU portion of the array were performed using the Rayleigh integral method. The bioheat transfer equation was then used to predict lesion formation. The HIFU performances of the proposed CMUT phased-array design were compared to those of the device currently used in the clinic. Partial CMUT prototypes, including the therapeutic part only, were fabricated and experimentally characterized (electromechanical CMUT behavior, ultrasound pressure field distribution and acoustic intensity). ResultsThe planar 64-element annular CMUT design is capable of dynamically focusing a 3 MHz ultrasound beam at distances ranging from 32 to 72 mm, comparable in size and shape to the ones obtained with the clinical device. The simulated ultrasound fields correlated well to experimental measurements. Visual observation and impedance measurements of the CMUT cells allowed direct estimation of the collapse and snapback voltages of the ring-elements. The surface acoustic intensity of the CMUT ring-elements with both AC driving and DC bias voltages can achieve over 6 W/cm2, shown in simulation to be compatible with the generation of thermal lesions. The electro-acoustic efficiency of the CMUT elements increased with increasing DC bias voltages to reach 31%, and remained stable with increasing AC driving voltages. The ultrasound energy could be dynamically focused from this planar CMUT array during several dozen of minutes. ConclusionThis work demonstrates the feasibility of utilizing a planar CMUT probe for generating dynamic HIFU focusing and lesioning compatible with the ablation of prostate tissues under endocavitary treatment approach. Future investigations will consist of validating the lesioning capability experimentally both in vitro and in vivo.