In the field of high-frequency ultrasound imaging (≥ 20 MHz), tools for characterizing the performance of imaging systems are lacking. Indeed, commercial phantoms are often inadequate for this frequency range. The development of homemade phantoms on the laboratory scale is often required but is hindered by the difficulty in making very small structures that must be distributed with high accuracy in 3-D space. We propose investigating the use of 3-D photopolymer printing to create resolution and calibration phantoms designed for high-frequency ultrasound imaging. The quality and importance of these phantoms are discussed from the point of view of ultrasound parameters and imaging. First, the compressional wave group velocity, acoustic impedance, and attenuation of six photopolymerized materials were measured using temporal and spectral methods in a substitution experimental setup. Measurements were performed on printed samples using a broadband-focused single-element transducer covering a large frequency range (15-55 MHz). Two 3-D phantoms incorporating different shapes and dimensions were designed and printed. Finally, 3-D acoustic images were obtained using either a mechanically driven single-element transducer or a high-frequency commercial imaging system. Three-dimensional printing enabled us to generate phantoms suitable for high-frequency imaging with complex geometry inclusions and with a surrounding material having acoustic properties close to those of human skin. The calculated SNR between the inclusion and surrounding media is approximately 50 dB. In conclusion, 3-D printing is a useful tool for directly, easily, and rapidly manufacturing ultrasound phantoms for ultrasound imaging system assessments and computational calibration or validation.

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.

For transducer design it is essential to know the acoustic properties of the materials in their operating conditions. At frequencies over 15 MHz, standard methods are not well adapted because layers are very thin and backings have very high attenuation. In this paper we report on an original method for measuring the acoustic properties in the 15 to 25 MHz frequency range, corresponding to typical skin imaging applications, using a backing/piezoelectric multilayer structure. Onto a porous Pb(Zr0.53Ti0.47)O3 (PZT) substrate, a piezoelectric PZT-based layer with a thickness of ~ 20 μm was deposited and directly used to excite an acoustic signal into water. Here, the measured signal corresponds to the wave that is first reflected on a target in water, then propagates back to the multilayer structure, is transmitted through the thick film and further to the rear face of the porous backing, where it is again reflected and returns to the piezoelectric thick film, thus avoiding overlap with the electrical excitation signal. Two types of PZT backings with similar porosity of ~ 20 % and spherical pores with size of 1.5 μm and 10 μm were processed. The ultrasound group velocities were measured at ~ 3500 m/s for both samples. The acoustic attenuation of the backings with pore size of 1.5 μm and 10 μm were 12 dB/mm and 33 dB/mm, respectively, measured at 19 MHz. This advanced measuring technique demonstrated potential for simple measurements of acoustic properties of backing at high frequencies in operating conditions. Importantly, this method also enables rapid determination of the minimum required thickness of the backing in order to act as a semi-infinite medium, for high-frequency transducer applications.

Research activities on lead-free piezoelectric materials have been ongoing for over twenty years. Generally, the applicability of the main material families is less universal than that of lead-based compositions such as lead zirconate titanate, but in some cases the corresponding applications have already been identified. Thanks to the extensive research it is now possible to manufacture demonstrators and prototypes for different applications and the authors propose in this article to take stock of these advances. For this, we have chosen to first recall briefly the main new material systems using a simplistic ”soft” and ”hard” classification for approaching the various resonant transducer applications. Medical imaging applications that represent one of the most important fields are presented in a second step together with other low-power transducers. Then, a variety of applications are merged under the heading of high-power transducers. In addition, we mention two points that are important to consider when manufacturing at a larger scale. For the design of the transducers, complete data sets must be available, especially if modeling tools are used. Finally, the commercialization of these lead-free materials imposes essential secondary requirements in terms of availability, reproducibility, sample size etc.

The standard fabrication method for 1-3 piezocomposites for ultrasound transducers is the “dice and fill” method (DFM) in which lateral periodicity is introduced. This contributes to the appearance of spurious modes that can drastically affect the performance of the device if they appear near its thickness mode frequency, thus limiting the effective frequency range. A new 1-3 piezocomposite fabricated with a super-cell structure [1-3 super cell (13SC)] was designed in order to overcome these limitations. It consists of the merging of several periodic cells with 47% PZT volume fraction and epoxy resin as the matrix. Two lateral periodicities in one direction are defined as well as two different kerfs. The chosen cell shape is composed of five nonaligned square section rods (1 × 1 mm ² ). For comparison of performance, two regular 1-3 piezocomposites (the same materials and equivalent periodicities) were fabricated by DFM. Electroacoustic responses in water were measured for the three composites being considered as transducers. Successive regular thinnings (from 2.8 to 1.1 mm) were carried out for each sample to increase the operating frequency (from around 0.4 to 1.3 MHz) and study the evolution of the characteristics (bandwidth and sensitivity). The experimental results confirmed the behavior of those obtained with numerical simulations, showing that the 13SC composite can be used in this entire frequency range, unlike regular composites.