ABSTRACT Increasing transducer bandwidth and signal-to-noise ratio (SNR) is fundamental to improving the quality of medical ultrasound images. In previous work, we have proposed the use of multi-layer 1-3 PZT/epoxy composites to increase both but have encountered significant fabrication challenges. These difficulties include making the bond thickness between the layers extremely small relative to the ultrasound wavelength and aligning the posts of the composite to increase the coupling coefficient. We have routinely achieved a bond thickness of less than 5 mu m but aligning the posts is more complicated. Finite element (PZFlex; Weidlinger, Assoc., New York, NY and Los Altos, CA) simulations show that the pulse-echo SNR and bandwidth degrade significantly with misalignment of the posts. Alignment of greater than 90% of the post pitch (i.e., tolerance of 10 to 20 mu m) is required to obtain significant increases in SNR and bandwidth relative to conventional transducer arrays. This will be a difficult tolerance for large-scale production.
Thus, we have developed a multi-layer composite hybrid array that will not require post alignment. This structure consists of a layer of 5 MHz 1-3 composite material on top of conventional 5 MHz PZT, which will provide greater SNR relative to conventional composites and increased bandwidth over multi-layer PZT. PZFlex simulations show that for a 2 MHz linear array element, the 2 layer hybrid structure increases the pulse-echo SNR by 7.5 dB over that from a single layer PZT element. Even without a matching layer, an increase in the -6 dB pulse-echo fractional bandwidth from 22% for the PZT element to 35% for the hybrid element was also predicted. Experimentally, in a 32 element array, we achieved an increase of 5.2 dB in SNR and an increased -6 dB bandwidth from 23 to 30%. In vitro and in vivo images showed corresponding improvements.
1999 IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control Vol. 46, pp. 961-971, 1999
© 1999, by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved.