Study of high-frequency ultrasound transducers for advanced biomedical imaging

Abstract

Ultrasound imaging is one of the main medical diagnostic tools for clinical use due to its non-radioactive and harmless nature. In recent decades, medical ultrasound technologies are being widely developed for not only improving the image quality in convectional clinical use but also applying on new aspects such as clinical therapeutic and preclinical pathology or physiology research applications. New designs of transducer are desired to accomplish those application, and the performance improvement of those assistance material is also required. In this study, analysis is conducted on the synergistic effect of preclinical biomedical imaging through the usage of the high resolution clinical imaging technique. Therefore, high frequency ultrasound transducers accompanied by high performance, such as high spatial resolution and sensitivity, is investigated due to its finer detail with higher spatial frequency in the imaging targets, such as small mammalian and human tissues. Meanwhile, development of fabrication process and fabrication of advanced acoustic materials aiming at improving transducer's performance are also carried out. Matching layer is a critical component that determines the performance of piezoelectric-based ultrasound transducers. For most piezoelectric materials, their acoustic impedances are significantly higher than human tissues and organs (around 1.6 MRayl), so a tunable matching layer with a high acoustic impedance is required for optimizing the acoustic wave transmission. In this thesis work, a high compression fabrication method is invented, with which the acoustic impedance of alumina-epoxy composite matching layer can be tuned from 6.50 to 9.47 MRayl by controlling the applied compression pressure and ratio of alumina to epoxy. This enhancement of acoustic impedance is attributed to the increased acoustic longitudinal velocity when alumina content reaches a critical value causing coalitions and domination of the acoustic wave propagation in the matching layer. Furthermore, the attenuation of this newly developed matching layer is only -10 dB/mm at 40 MHz. The very high acoustic impedance value and very low attenuation make this matching material superior than all reported matching materials, and therefore, can enhance the performance of the ultrasound transducers, especially for medical imaging applications at high to ultra-high frequency regime. In vivo ultrasound imaging with phase array transducers is of great importance for both clinical application and biomedical research. In this work, relaxor ferroelectric PMN- 0.28PT single crystal with very high piezoelectric constant (d33≥2000 pC/N) and a higher electromechanical coupling factor (k33~0.92), was used to develop high-frequency phase array transducers. A 128-element 20-MHz phased-array ultrasound transducer was successfully fabricated with optimized performance of an average ~84% bandwidth at -6 dB and insertion loss of -43 dB. The axial and lateral resolutions of this transducer were determined to be 40.5 µm and 121.5 µm, respectively. With this transducer and Verasonics image platform, in-situ images of a fisheye and in-vivo laboratory mice's cardiac images were acquired, demonstrating successful application of our developed high-frequency phase array transducer for biomedical research of small animals. Cranial ultrasound is limited at infant and temple brain imaging because the rest parts of skull bone are too thick for penetrating. A new 2D annular array transducer idea based on the structure of Fresnel annular array is raised, designed and developed, aiming at transcranial brain imaging and stimulating through the pterion. The simulation result shows a pair of separated sectors in a modified structure produces an on-axis focal point at 34 mm with maximum 20 kPa pressure. The focal spot size is 12 mm length along axial direction and 0.045 mm width along lateral direction. The developed PZT 2D annular array transducer contents total 48 elements distributed in 8 sectors is also measured. The represented elements along the sector show a 16.10 MHz average center frequency and a -6 dB bandwidth around 80%. By further using an electrical signal control, a 3D conical shape image may be obtained. More work needs to be done to include more elements and develop new imaging method.