Multiaxial Ultrasound Transducers for Medical Ultrasound imaging

Medical ultrasound imaging is one of the most popular diagnostic imaging methods worldwide. Recent years have seen renewed interest in applications such as passive ultrasound imaging (for monitoring ultrasound therapies), and ultrasound computer tomography (for high-fidelity diagnostic imaging). Underpinning all of these systems are piezoelectric ultrasound transducers, which convert between mechanical and electrical energy. Current applications of ultrasound transducers exploit the piezoelectric effect in only one direction, which allows them to either generate a fixed ultrasound profile or measure incident acoustic pressure. In imaging, the measurement of only pressure introduces limitations and necessary assumptions to image reconstruction methods that can introduce artifacts. While larger arrays of transducers can mitigate artifacts, this also increases cost and computational expense. Recent developments, however, have introduced multiaxial driving, a technique that generates a steerable ultrasound emission profile by exploiting the piezoelectric effect in two or more axial directions. Preliminary work also demonstrated that this process is reversible: the two or more electrical signals measured from a multiaxial transducer vary according to the Direction of Arrival (DOA) of the wave that generated them. This observation motivated the following objectives of this research: 1) The characterization of measured multiaxial signals relative to a wave's DOA, 2) The demonstration of a biaxial method for passive ultrasound imaging, 3) The demonstration of a biaxial method for Ultrasound Computer Tomography (UCT) For objective 1, finite-element simulations and experimental transducers were used to demonstrate DOA estimation over a range of 48 degrees using the phase difference and amplitude ratio between the two measured biaxial signals. For objective 2, we provided the first demonstration of a single-element passive beamforming algorithm and passive ultrasound imaging without conventional time-of-flight information using an experimental, two-element biaxial sensor array. Finally, for objective 3, we provided the first demonstration of a bent-ray UCT algorithm that uses only DOA information. A simulated 128-sensor array showed improved image sharpness and structural quality compared to conventional methods. Overall, this dissertation demonstrates the utility of multiaxial ultrasound sensors, the novel information they provide, and offers two robust applications that improve on conventional ultrasound imaging methods, enabling new approaches for future systems.