High Acoustic Impedance and Attenuation Backing for High-Frequency Focused P(VDF-TrFE)-Based Transducers

Abstract

Backing materials with tailored acoustic properties are beneficial for miniaturized ultrasonic transducer design. Whereas piezoelectric P(VDF-TrFE) films are common elements in high-frequency (>20 MHz) transducer design, their low coupling coefficient limits their sensitivity. Defining a suitable sensitivity–bandwidth trade-off for miniaturized high-frequency applications requires backings with impedances of >25 MRayl and strongly attenuating to account for miniaturized requirements. The motivation of this work is related to several medical applications such as small animal, skin or eye imaging. Simulations showed that increasing the acoustic impedance of the backing from 4.5 to 25 MRayl increases transducer sensitivity by 5 dB but decreases the bandwidth, which nevertheless remains high enough for the targeted applications. In this paper, porous sintered bronze material with spherically shaped grains, size-adapted for 25–30 MHz frequency, was impregnated with tin or epoxy resin to create multiphasic metallic backings. Microstructural characterizations of these new multiphasic composites showed that impregnation was incomplete and that a third air phase was present. The selected composites, sintered bronze–tin–air and sintered bronze–epoxy–air, at 5–35 MHz characterization, produced attenuation coefficients of 1.2 and >4 dB/mm/MHz and impedances of 32.4 and 26.4 MRayl, respectively. High-impedance composites were adopted as backing (thickness = 2 mm) to fabricate focused single-element P(VDF-TrFE)-based transducers (focal distance = 14 mm). The center frequency was 27 MHz, while the bandwidth at −6 dB was 65% for the sintered-bronze–tin–air-based transducer. We evaluated imaging performance using a pulse-echo system on a tungsten wire (diameter = 25 μm) phantom. Images confirmed the viability of integrating these backings in miniaturized transducers for imaging applications.

Development of an Artificial Soft Solid Gel Using Gelatin Material for High-Quality Ultrasound Diagnosis

“…The characteristic impedance (Z T ) of the transducer, elastic modulus (E), and density (ρ) of the gelatin material were determined as 30.0 MRay1, 14.26 MPa, and 1.196 g/cm 3 , respectively [28][29][30][31]. Here, elastic modulus (E) refers to the ratio of the crosschange rate of transverse and longitudinal waves when ultrasonic waves mechanically vibrate in the transducer [32]. At this time, density (ρ) means a certain density that can maintain elasticity to enable mechanical vibration to alternate transverse and longitudinal waves in the transducer.…”

“…The ultrasound transmission velocity (c T ) and attenuation coefficient (Att) of the probe were determined as 1544 m/s and 0.2 dB/cm•MHz [31], respectively. Here, acoustic wave velocity refers to the inherent speed at which ultrasonic waves pass from the transducer to the phantom [32]. At this time, the transmission space of ultrasonic waves to pass through the transducer and the phantom must match each other, and this definition can be expressed as acoustic impedance.…”

“…At this time, the transmission space of ultrasonic waves to pass through the transducer and the phantom must match each other, and this definition can be expressed as acoustic impedance. Therefore, it is important that ultrasonic waves are perfectly transmitted from the transducer to the pantor; but at this time, some loss of ultrasonic waves may occur due to differences in the characteristics of the medium (lattice, density, elastic modulus, and characteristic impedance difference) [32]. At this time, the lower the ultrasonic transmission loss, the better, and the measure of minimizing ultrasonic transmission loss can be defined as the attenuation coefficient.…”