Research On Acoustic Metamaterials In Improvement Of Medical Ultrasonic Transducers’ Focusing Performance

With the rapid development of medical ultrasound, the design and fabrication of acoustic devices have been challenged by higher criteria, which is difficult to be satisfied with the utilization of traditional materials. As a result, acoustic metamaterials have attracted widespread attention due to their capability of achieving novel physical effects that natural materials cannot. With the development of acoustic metamaterials, their potential applications in the field of medical ultrasound have become hot topics both in laboratories and in clinics.This thesis focuses on the potential applications of the novel physics phenomena of acoustic metamaterials and the related applications in biomedical-oriented focused ultrasound. Specifically, three sub-topics are discussed here:(a) sub-wavelength focusing lens, (b) non-diffracting airy beam focusing, and (c) acoustic focusing superlens.In traditional ultrasound imaging methodologies, ultrasonic waves scattered from the target tissue/organs are collected with a receiving transducer or a transducer array, hence the geometric pattern of the objective area could be reconstructed after algorithm-based signal processing. In principle, the scattered waves include not only the traveling wave components, but also evanescent waves. However, it is quite difficult to detect the evanescent waves since these components decay exponentially along the propagation distance. As a consequence, the resolution of ultrasonic imaging is highly limited due to the diffraction limit. In existing studies, researchers have found that some kinds of acoustic metamaterials could be adopted to amplify evanescent waves and the diffraction limit could therefore be overcome. Here, based on the amplification of evanescent waves using acoustic metamaterials, a multi-channel structure was designed to achieve acoustic sub-wavelength focusing. This planar acoustic lens could have its potential in improving the quality of ultrasonic imaging, as well as enhancing the spatial precision of ultrasonic therapies.To better control acoustic focusing beams, we further explored the realization of Airy beam, which is a bending non-diffracting beam and is of great significance in the applications of medical ultrasound. Due to the complexity of Airy function and the limitation of mechanical fabrication, it is very difficult to generate an acoustic Airy beam in a traditional way. The study of acoustic metamaterials has shown that acoustic metasurface holds the ability of controlling the spatial phase distribution of acoustic wave fronts. In the current work, a simple design was proposed to generate acoustic Airy beam. In the proposed scheme, a piston transducer was corrugated to construct a spatial phase pattern described by Airy function. The surface of the piston was groove-patterned so that the width of each consecutive groove equal to the corresponding segmented length of a "binary spatial Airy function". Numerical simulations were carried out, demonstrating the designed piston capable of generating multi-frequency and broad-band acoustic Airy beams. Analyses were also carried out to discuss the resulted frequency characteristics and its dependence on the size of the piston source, as well as the application of Airy beam in ultrasound focusing.Another problem that is concerned here is to improve the acoustic pattern around the focal point. Acoustic focusing lens is usually used to concentrate acoustic energy, taking advantage of its simplicity and low-cost. In order to improve the performance of acoustic focusing lens, we developed an acoustic focusing lens with periodically aligned sub-wavelength grooves corrugated on its spherical surface. Both theoretical and experimental studies were performed to demonstrate that this kind of acoustic metamaterial is capable of achieving acoustic collimation effect, suppressing the relative side-lobe amplitudes, enhancing the focal gain, and minimizing the focal shift. Considering its future applications in High Intensity Focused Ultrasound (HIFU) therapies, the lens we designed would offer the benefits of improving safety, enhancing efficiency and elevating accuracy.Acoustic metamaterials could be utilized to achieve new physics insights. The metamaterials we designed may have promising applications in medical ultrasound diagnosis and therapy. For instance, the quality of ultrasonic imaging can be significantly improved with the application of sub-wavelength acoustic focusing lens, while the safety of ultrasound therapy can be enhanced with the help of non-diffracting Airy beam and acoustic focusing superlens. Therefore, the obtained results demonstrated here may play important roles in the future developments of acoustic metamaterials as well as medical ultrasound equipment.