P2O-8 Single-Element PLZT Transducer for Wide-Bandwidth Imaging of Solid Materials

Hard tissue medical imaging and other applications such as IC-chip package inspection present a series of challenges for ultrasound imaging, including small feature size, high acoustic clutter, and high acoustic impedances. We report here on the design and performance of a 19 MHz resonant thickness mode, ceramic PLZT (Lead Lanthanum Zirconate Titanate) transducer designed for high acoustic impedance (>10 MRayl) operation in the presence of acoustic clutter. One-dimensional transducer analysis using the Mason model is also presented and is used to predict the expected bandwidth and center operating frequency, and to compare PLZT and PZT-5H in our transducer design. The packaged transducer features a high impedance matching layer and a gallium-indium alloy couplant (17.4 MRayl), which together minimize surface clutter due to impedance mismatches. The transducer has a narrow 8deg -3 dB full beamwidth, a 30% instantaneous bandwidth, and a narrow pulse width (250 ns), resulting in high lateral (210 mum at 1.5 mm) and axial (350 mum in gallium-indium alloy) resolutions for detection of small features. The transducer has been integrated with a match filter receiver, and the system has been demonstrated to obtain a single pulse SNR of 45 dB in detecting a 25 mum thick crack in the highly cluttered environment of a human tooth. The Mason model predicts that the PLZT transducer has a 50% higher bandwidth than an equivalent PZT-5H transducer, in large part because of the increased electromechanical coupling coefficient (PLZT k ap 0.82, and PZT-5H k_t ap 0.65). The tradeoff is ~30% lower acoustic-to-electric responsivity caused primarily by a larger dielectric constant. This is not so significant in the present applications (solid-state imaging), which tend to have clutter-limited environments. However, the improved bandwidth is very useful in improving range resolution and in gating out unwanted echoes, including those from the clutter