Optimization of matching layer design for medical ultrasonic transducer

Matching layers are crucial components in ultrasonic transducers for medical imaging. Without proper matching layers, large acoustic impedance mismatch between piezoelectric resonator and the human body tissue will cause most of the ultrasound energy to lose. For a given frequency, the matching layer thickness should be one quarter of the wavelength and its acoustic impedance should be the geometric mean of the piezoelectric material and the imaging body. There are no natural materials that can meet such requirements, therefore, solid particle/polymer composites are commonly used as matching layer materials. The acoustic impedance of such composites should be in the range of 2-15 MRayls. Therefore, all ultrahigh frequency transducers currently used or under development are not properly matched because the lacking of desired materials. This problem hinders the development of finer resolution ultrahigh frequency ultrasonic imaging. In order to increase the efficiency of matching layer, we have developed a TiO2 nano-structured material with high acoustic impedance and simple processing procedure. The material has porous nano-structure with the volume fraction of voids being controlled by the amount of bonding amorphous phase in the material. Its acoustic impedance can be tuned by controlling the total volume fraction of voids. Using the quarter wavelength thickness characterization method, the acoustic properties of this nano-structured material were accurately characterized. It is found that the acoustic impedance can reach as high as 7.19 Mrayls, which is a solid improvement compared to that of other available nano-composites. Because of recent years?rapid development of the single crystal PMN-PT and PZT-PT materials in medical ultrasonic transducer applications, one of the main tasks of this thesis is to design matching layer with gradient acoustic impedance to achieve broad bandwidth. Wave propagation within multilayer structure has been analyzed and simulated with the principle of finite difference time domain technique by programmed application. By adjusting the acoustic impedance distribution, we get different output spectrums and therefore choose the best distribution functions. The geometry design in matching layer design all also studied.