Capacitive Micromachined Ultrasonic Transducers for Therapeutic Ultrasound Applications

Abstract

Therapeutic ultrasound guided by magnetic resonance imaging (MRI) is a noninvasive treatment that potentially reduces mortality, lowers medical costs, and widens accessibility of treatments for patients. Recent developments in the design and fabrication of capacitive micromachined ultrasonic transducers (CMUTs) have made them competitive with piezoelectric transducers for use in therapeutic ultrasound applications. In this paper, we present the first designs and prototypes of an 8-element, concentric-ring, CMUT array to treat upper abdominal cancers. This array was simulated and designed to focus 30–50 mm into tissue and ablate a 2–3 cm diameter tumor within 1 hour. Assuming a surface acoustic output pressure of 1 MPa peak to peak (8.5 W/cm2) at 2.5 MHz, we simulated an array that produced a focal intensity of 680 W/cm2 when focusing to 35 mm. CMUT cells were then designed to meet these frequency and surface acoustic intensity specifications. These cell designs were fabricated as 2.5 mm by 2.5 mm test transducers and used to verify our models. The test transducers were shown to operate at 2.5 MHz with an output pressure of 1.4 MPa peak to peak (16.3 W/cm2). With this CMUT cell design, we fabricated a full 8-element array. Due to yield issues, we only developed electronics to focus the four center elements of the array. The beam profile of the measured array deviated from the simulated because of crosstalk effects; the beamwidth matched within 10% and sidelobes increased by 2 times, which caused the measured gain to be 16.6 compared to 27.4.

Increasing the sonoporation efficiency of targeted polydisperse microbubble populations using chirp excitation

“…This chirp rate would be defined by the measured resonant response of the polydisperse microbubble population used and would ensure that a greater population of microbubbles at a specific size would experience excitation at resonance for longer in an effort to maximise the shear stress generated on the cells. In addition, transducers with a broader bandwidth, such as capacitive micromachined ultrasonic transducers (CMUTs) [63] could be used to increase the population of microbubbles excited at resonance without the need to sacrifice output power. This would allow for higher acoustic pressures to be tested, which is a limitation of this study.…”

Evaluation of an Ultrasound-Guided Focused Ultrasound CMUT Probe for Targeted Therapy Applications

“…Several studies have also assessed the potential benefits of CMUT arrays for therapeutic intents, but all have been strictly circumscribed to high-intensity focused ultrasound (HIFU) [17] [18] [19] [20] [21]. Additionally, these studies have generally considered only the early stage of development and characterization, whereas the complete functionality of a fully packaged CMUT-based probe for FUS applications still needs to be demonstrated.…”

A 1-MHz 2-D CMUT array for HIFU thermal ablation

“…In previous years, we have demonstrated the feasibility of capacitive micromachined ultrasonic transducers (CMUTs) for thermal ablation using a concentric ring array [1] and an unfocused single-element transducer [2]. For high intensity focused ultrasound (HIFU) applications, CMUTs have advantages as opposed to the conventional piezoelectric transducers.…”