A kerfless eight element high frequency ultrasound annular array transducer using 9 μm P(VDF-TrFE) bonded to a high density flexible interconnect was fabricated. The flexible circuit composed of Kapton polyimide film with gold electrode pattern of equal area annuli apertures on the top side of a 50 μm thick Kapton polyimide film. Each element had several 30 μm diameter electroplated vias that connected to electrode traces on the bottom side of the Kapton polyimide film. There was a 30 μm spacing between elements. The total aperture of the array was 3.12 mm. The transducer's performance has been modeled by implementing the Redwood version of the Mason model into PSpice and using the Krimholtz, Leedom and Matthaei (KLM) model utilized in the commercial software PiezoCAD. The transducer’s performance was evaluated by measuring the electrical impedance with a HP 4194 impedance analyzer, pulse echo response using a Panametrics 5900 pulser/receiver and crosstalk measurement for each element in the array. The measured electrical impedance for each element was 540 Ω and -84° phase. In order to improve device sensitivity an inductor was attached in series with each element to reduce the insertion loss to 33 dB. The measured average center frequency and bandwidth of each element was 55 MHz and 50% respectively. The measured crosstalk at the center frequency was -45 dB in water.
Ultrasound phantoms are used as a quality assurance mechanism for assessing the imaging performance of ultrasound transducers. Several phantoms have been fabricated specifically for imaging by means of high frequency transducers with a center frequency ranging from 20 to 100 MHz. To quantify the transducers imaging performance the spatial resolution, dead zone, linear fidelity, depth of penetration and image uniformity are measured from ultrasound images created by scanning specially designed phantoms. Eight micron diameter tungsten (high acoustic reflectivity and diameter size less than (lambda) /2) wire targets are used for all the phantoms. Transducer characterization consists of a standard pulse echo analysis and insertion loss measurement for each transducer. Imaging of quality assurance phantoms and transducer characterization provide a practical means for evaluating the performance of high frequency transducers.
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