Miniaturized Ultrasound Transducers for the Acoustic Excitation of Microbubbles and Microgel

Microbubbles and microgels have been widely utilized for both diagnostic and therapeutic ultrasound applications. Owing to their nonlinear characteristics of harmonic responses, coalescence, and cavitation, these microparticles have enabled contrast-enhanced harmonic vascular imaging, spatiotemporal drug-delivery, and enhanced thrombolysis efficiency. Despite their advantages, a crucial limitation of these microbubble-aided ultrasound techniques is the lack of the suitable custom ultrasound transducers. The main theme of this dissertation is the development of new small-aperture ultrasound transducers focusing on the efficient acoustic excitation for microbubbles/gels and associated data acquisition capability. Three different types of the ultrasound transducers were aimed to be developed for both diagnostic and therapeutic applications: 1) stacked-type, dual-frequency, 1-3 piezoelectric composite transducers for intracavitary acoustic angiography, 2) Forward-looking, lowfrequency, intravascular ultrasound transducer for microbubble-mediated sonothrombolysis, 3) Laser-generated-focused-ultrasound transducer for microbubble-mediated ultrasound thrombolysis and spatiotemporal drug delivery. Prototype transducers were designed, fabricated, acoustically characterized, and the performance for each application was tested in vitro. The results indicated that the new miniaturized prototype transducers and design guidelines provided in this study can be useful for developing versatile clinical devices for advanced, ultrasound-based diagnosis and treatment for blood vessels and tumors.;The stacked-type dual-frequency transducers for intracavitary acoustic angiography was developed using 1-3 piezoelectric composite for both transmitter and receivers. In comparison with the previously designed intravascular transducers, the larger aperture (∼2 mm x ∼8 mm), higher pressure output (mechanical index > 1.0), lower frequency (2 MHz for transmitter and 14 MHz for receiver) of the custom dual-frequency transducer enabled to achieve contrast-to-tissue ratio of 16 dB at the tissue penetration depth of 8 mm.;Forward-looking, stacked-type, focused intravascular transducers demonstrated the thrombolysis efficiency close to 0.7%/min in intravascular sonothrombolysis approach without use of thrombolytic agents. Microbubble-mediated approach was considered in this study, and the enhanced cavitation effects were clearly observed during in vitro thrombolysis tests. With the small size (∼1.5 mm diameter), and low frequency (620 kHz), it was demonstrated that the custom concave lens can realize the focusing effect. The developed transducer exhibited sufficient pressure output to induce the inertial cavitation, and the microstreaming and microjets caused by the bubble destruction were able to induce shear stress on the target blood clots.;Laser-generated-focused-ultrasound (LGFU) transducer showed its capability to induce inertial cavitation of microbubbles and stable cavitation of microgels. The developed carbon-black- polydimethylsiloxane LGFU transducer exhibited high frequency (14 MHz), high pressure (> 10 MPa) shock waves at the tight focal width (< 500 mum). It was demonstrated that the short pulse generated by LGFU transducer can realize sufficient physical effects on microgel-involved drug delivery and microbubble-mediated sonothrombolysis.