Multilayered PZT/polymer composites to increase signal to noise ratio and resolution for medical ultrasound transducers

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 multilayer 1-3 PZT/epoxy composites to increase both, but have encountered significant fabrication challenges. These difficulties include making the bond thickness between the layers small relative to the ultrasound wavelength and aligning the posts of the composite to increase coupling coefficient. We have routinely achieved a bond thickness of less than 5 microns but aligning the posts is more complicated. Finite element (PZFlex) 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 10-20 μm) 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 multilayer 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. 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 22% to 30%. In vitro and in vivo images showed corresponding improvements