Broadband Ultrasonic Transducers Research Articles

Broadband ultrasonic transducer based on nested composite structure with gradient acoustic impedance

This study introduces an innovative Broadband Ultrasonic Transducer employing a Nested Composite Structure with Gradient Acoustic Impedance, significantly enhancing the performance in medical diagnostics and non-destructive testing. The developed transducer, based on a unique nested PZT/epoxy composite design, achieves an optimized acoustic impedance gradient from 36.56 MRayls to 4.53 MRayls. This strategic design effectively addresses the impedance mismatch, a long-standing challenge in ultrasonic imaging. Rigorous Finite Element Analysis (FEA) using PZFlex software indicates that the transducer offers a −6 dB bandwidth of 69.31%, a substantial improvement over the existing models. Experimental results corroborate with theoretical predictions, demonstrating enhanced bandwidth (∼71.24%) and transmission efficiency. This advancement highlights the potential of gradient impedance in fabricating high-performance ultrasonic transducers, setting a new benchmark for ultrasonic imaging applications.

Plasmon-enhanced optoacoustic transducer with Ecoflex thin film for broadband ultrasound generation using overdriven pulsed laser diode.

Ultrasonic transducers facilitate noninvasive biomedical imaging and therapeutic applications. Optoacoustic generation using nanoplasmonic structures provides a technical solution for highly efficient broadband ultrasonic transducer. However, bulky and high-cost nanosecond lasers as conventional excitation sources hinder a compact configuration of transducer. Here, we report a plasmon-enhanced optoacoustic transducer (PEAT) for broadband ultrasound generation, featuring an overdriven pulsed laser diode (LD) and an Ecoflex thin film. The PEAT module consists of an LD, a collimating lens, a focusing lens, and an Ecoflex-coated 3D nanoplasmonic substrate (NPS). The LD is overdriven above its nominal current and precisely modulated to achieve nanosecond pulsed beam with high optical peak power. The focused laser beam is injected on the NPS with high-density electromagnetic hotspots, which allows for the efficient plasmonic photothermal effect. The thermal expansion of Ecoflex finally generates broadband ultrasound. The overdriven pulsed LD achieves a maximum optical peak power of 40W, exceeding the average optical power of 3W. The thick Ecoflex-coated NPS exhibits an eightfold optoacoustic enhancement with a fractional bandwidth higher than 160% and a peak frequency of 2.5MHz. In addition, the optoacoustic amplitude is precisely controlled by the optical peak power or the laser pulse width. The PEAT-integrated microfluidic chip clearly demonstrates acoustic atomization by generating aerosol droplets at the air-liquid interface. Plasmon-enhanced optoacoustic generation using PEAT can provide an approach for compact and on-demand biomedical applications, such as ultrasound imaging and lab-on-a-chip technologies.

Broadband ultrasonic transducer based on textured lead-free NKN-based piezoceramics

The performance of ultrasonic transducers in terms of broadband and high sensitivity is crucial for achieving excellent axial resolution and a high signal-to-noise ratio in ultrasonic imaging. These qualities greatly rely on the electromechanical properties of the piezo-materials used. In this work, a potassium sodium niobate (NKN)-based lead-free piezoceramic was fabricated by template grain growth method, resulting in outstanding electromechanical properties (d33 ≈ 710 pC N−1, k33 ≈ 0.88, kt ≈ 0.61, Tc ≈ 244 °C). A 1–3 composite structure was created using this lead-free textured ceramic to fabricate an ultrasonic transducer with a center frequency of 4.18 MHz. The ultrasonic transducer exhibited a remarkable −6 dB bandwidth of up to 140% and an insertion loss of −23.63 dB. Imaging experiments confirmed that the 1–3 NKN-based textured ceramic transducer displayed excellent −6 dB axial resolution. Notably, these performance parameters exceeded those of ultrasonic transducers made with lead-based PZT-5H piezoceramics, demonstrating the significant potential of this lead-free piezoceramic as a replacement for lead-based materials.

System Matrix Reconstruction Algorithm for Thermoacoustic Imaging With Magnetic Nanoparticles Based on Acoustic Reciprocity Theorem.

According to the acoustic reciprocity theorem (ART), we propose a system matrix reconstruction algorithm of thermoacoustic imaging for magnetic nanoparticles (MNPs) by a single-pulse magnetic field. In both cases of inhomogeneous and homogeneous acoustic velocity, we respectively derive the linear equation between the sound pressure detection value and the distribution of MNPs. The image reconstruction problem is converted to an inverse matrix solution by using the truncated singular value decomposition (TSVD) method. In forward problem, the calculated forward results are consistent with the simulated thermoacoustic signal signals. In inverse problem, we build the two-dimensional breast cancer model. The TSVD method based on the ART faithfully reflects the distribution of abnormal tissue labeled by the MNPs. In the experiment, the biological sample injected with the MNPs is used as the imaging target. The reconstructed image well reflects the cross-sectional images of the MNPs area. The TSVD method based on the ART takes into account energy attenuation and inhomogeneous acoustic velocity, and use a non-focused broadband ultrasonic transducer as the receiver to obtain a larger imaging field-of-view (FOV). By comparing the image metrics, we prove that the algorithm is superior to the traditional time reversal method. The TSVD method based on the ART can better suppress noise, which is expected to reduce the cost by reducing the number of detectors. It is of great significance for future clinical applications.

Controlling the force and the position of acoustic traps with a tunable acoustofluidic chip: Application to spheroid manipulations.

A multi-node acoustofluidic chip working on a broadband spectrum and beyond the resonance is designed for cell manipulations. A simple one-dimensional (1D) multi-layer model is used to describe the stationary standing waves generated inside a cavity. The transmissions and reflections of the acoustic wave through the different layers and interfaces lead to the creation of pressure nodes away from the resonance condition. A transparent cavity and a broadband ultrasonic transducer allow the measurement of the acoustic energy over a wide frequency range using particle image velocimetry measurements and the relation between acoustic energy and the particles velocity. The automation of the setup allows the acquisition over a large spectrum with a high frequency definition. The results show a wide continuous operating range for the acoustofluidic chip, which compares well with the 1D model. The variation of the acoustic radiation force when varying the frequency can be compensated to ensure a constant amplitude for the ARF. This approach is finally applied to mesenchymal stem cell (MCS) spheroids cultured in acoustic levitation. The MSC spheroids can be moved and merged just by varying the acoustic frequency. This approach opens the path to various acoustic manipulations and to complex 3D tissue engineering in acoustic levitation.

Broadband High-Frequency Ultrasonic Transducer Based Functional Photoacoustic Mesoscopy for Psoriasis Progression.

In vivo imaging of skin is commonly used to investigate dynamic processes in the progression and treatment of psoriasis. Photoacoustic mesoscopy is a new non-invasive imaging modality widely used in bio-imaging, and has recently been applied to imaging skin in vivo. However, photoacoustic imaging has shortcomings. Although high-frequency ultrasonic transducers enable high-resolution photoacoustic imaging, the images may be bandwidth-limited. To overcome this limitation, we designed and fabricated a broadband ultrasonic transducer for photoacoustic mesoscopy. The center frequency of the transducer was 32 MHz (88% bandwidth at -6 dB). The transducer was used to visualize mouse and human skin morphology. Colocalization of high- and low-frequency components revealed information about both the skin surface and dermis. To explore dynamic structural changes in mouse back skin during psoriasis progression, we measured blood oxygen saturation and total hemoglobin in a mouse model using multiwavelength imaging without contrast agents. The results indicate that functional photoacoustic mesoscopy using a broadband high-frequency transducer has great potential for clinical imaging of skin disease.

Design and characterization of a broadband multi-node bulk acoustic wave device for acoustofluidic applications

The classic approaches to generate the Acoustic Radiation Force (ARF) in a standing wave cavity are based on the use of a specific height for the cavity and a narrow-band acoustic frequency to reach the resonance. We present a robust multi-node cavity working over a broadband range of frequency. This approach allows large acoustophoresis manipulations. The design of a transparent cavity and the use of a broadband ultrasonic transducer allowed the characterization of the acoustic energy and the comparison with a simple 1-D model. The acoustic properties of the system were estimated through the effects of the acoustic radiation forces on particles suspension. From the particle velocities induced by the ARF, measured with a Particle Image Velocimetry (PIV) approach, we deduced the acoustic energy over a large frequency range. The automation of the setup allowed the acquisition of a large amount of data to make parametric studies. The results show a wide continuous operating range for the acoustic radiation forces. Furthermore, the occurrence of the resonance peaks allows an increase by a factor ten of the force magnitude. This broadband effect also enables an accurate control of the node positions by tuning the proper acoustic frequency.

Multi-layer polymer-metal structures for acoustic impedance matching in high-frequency broadband ultrasonic transducers design

Although high-frequency (≥100 MHz) ultrasound has demonstrated its capability in a variety of applications, the fabrication of high frequency ultrasonic transducers with both high sensitivity and broad bandwidth remains challenging. One main reason is the mismatch of acoustic impedance between piezoelectric materials and the loading medium. Due to reliance on both specific acoustic impedance of matching materials and precise thickness control, the conventional quarter wavelength (1/4 λ) matching layer design is impractical for high-frequency transducers. Based on the transmission line theory and Mason model, our work interfaced polymer-metal-polymer matching layers for transducers over 100 MHz. The modeling result comparison between transducers without matching layer and those with two conventional matching layers demonstrated that the matching performance of polymer-metal-polymer matching layers could be as good as the one of conventional matching layers. Meanwhile, unlike conventional 1/4 λ matching layer design, our design is independent on materials with specific acoustic impedance, while precise thickness control of polymer and metal can be achieved by deposition. The polymer-metal-polymer matching layers scheme paves the way to high frequency ultrasonic transducers.

Photoacoustic Signal of Nanobubbles Induced in PEGylated Gold-Nanorod Colloid

We have developed a photoacoustic (PA) measurement system using a broadband ultrasonic transducer to detect the PA signals induced in a gold nanorod (GNRs) colloid irradiated by the nanosecond-pulsed laser beam. This study will investigate the PA signals of PEG-GNRs at longitudinal surface plasmon resonance. Several factors including aspect ratio of GNR, pulse energy, concentration, and wavelength are varied to investigate these effects on the PA signal. We show that at the certain condition the generated PA signal of PEG-GNRs stronger than uncoated GNRs. Moreover, the PA signals of PEGGNRs are more stable than uncoated GNRs in the same condition. Keywords

Contextual Application of Pulse-Compression and Multi-frequency Distance-Gain Size Analysis in Ultrasonic Inspection of Forging

Ultrasonic pulse-echo non-destructive testing, combined with Distance Gain Size (DGS) analysis, is still the main method used for the inspection of forgings such as shafts or discs. This method allows the inspection to be carried out, assuring in turns the necessary sensitivity and defect detection capability in most of the cases. However, when testing large or highly attenuating samples with standard pulse-echo, the maximum achievable signal-to-noise ratio is limited by both the beam energy physical attenuation during the propagation and by the inherent divergence of any ultrasound beam emitted by a finite geometrical aperture. To face this issue, the application of the pulse-compression technique to the ultrasonic inspection of forgings was proposed by some of the present authors, in combination with the use of broadband ultrasonic transducers and broadband chirp excitation signals. Here, the method is extended by applying DGS analysis to the pulse-compression output signal. Both standard single-frequency/narrowband DGS and multi-frequency/broadband DGS analyses applied on pulse-compression data acquired on a forging with known defects are tested and compared. It is shown that the DGS analysis works properly with pulse-compression data collected by using a separate transmitter and receiver transducers. Narrowband analysis and broadband analyses provide almost identical results, but the latter exhibits advantages over the traditional method: it allows the inspection frequency to be optimized by using a single pair of transducers and with a single measurement. In addition, the range resolution achieved is higher than the one achievable for the narrowband case.

Particle size distribution measurement based on ultrasonic attenuation spectra using burst superposed wave

Abstract In the characterization of particle size distribution (PSD) using ultrasonic attenuation method, the spectrum characteristics such as bandwidth or the number of frequencies is of great significance. To obtain desired attenuation spectra consisting of sufficient information, a direct digital synthesis (DDS) based excitation method has been proposed as burst superposed wave (BSW) featuring in ultrasonic frequencies to be freely defined and component waves’ energy to be artificially controlled. Corresponding experimental systems are designed, in which the traditional pulse wave (PW) and burst single frequency wave (BSFW) are implemented. A pair of broadband ultrasonic transducers with center frequency of 15 MHz is performed to obtain the attenuation spectra of three polystyrene material aqueous suspension (volume fraction 10%). The experimental results show that the traditional methods are unsatisfactory for the relatively large particles. Conversely, BSW can effectively broaden the scope of attenuation spectra and increase the number of frequencies. Afterwards, optimum regularization technique algorithm was used for inversion. The resultant comparisons suggest that the scope and the ultrasonic frequency number of measured attenuation spectra have exerted an obvious impact on the inversion and the BSW technology manifests obvious improvements from the accuracy and reliability of the inversion.

Frequency Dependence of Receiving Sensitivity of Ultrasonic Transducers and Acoustic Emission Sensors.

Receiving displacement sensitivities (Rx) of ultrasonic transducers and acoustic emission (AE) sensors are evaluated using sinewave packet excitation method and compared to the corresponding data from pulse excitation method with a particular emphasis on low frequency behavior below 20 kHz, down to 10 Hz. Both methods rely on the determination of transmitter displacement characteristics using a laser interferometric method. Results obtained by two calibration methods are in good agreement, with average spectral differences below 1 dB, indicating that the two calibration methods yield identical receiving sensitivities. At low test frequencies, effects of attenuation increase substantially due to increasing sensor impedance and Rx requires correction in order to evaluate the inherent sensitivity of a sensor, or open-circuit sensitivity. This can differ by more than 20 dB from results that used common preamplifiers with ~10 kΩ input impedance, leading to apparent velocity response below 100 kHz for typical AE sensors. Damped broadband sensors and ultrasonic transducers exhibit inherent velocity response (Type 1) below their main resonance frequency. In sensors with under-damped resonance, a steep sensitivity decrease occurs showing frequency dependence of f2~f5 (Type 2), while mass-loaded sensors exhibit flat displacement response (Type 0). Such behaviors originate from sensor characteristics that can best be described by the damped harmonic oscillator model. This model accounts for the three typical behaviors. At low frequencies, typically below 1 kHz, receiving sensitivity exhibits another Type 0 behavior of frequency independent Rx. Seven of 12 sensors showed this flat region, while three more appear to approach the Type 0 region. This appears to originate from the quasi-static piezoelectric response of a sensing element. In using impulse method, a minimum pulse duration is necessary to obtain spectral fidelity at low frequencies and an approximate rule is given. Various factors for sensitivity improvement are also discussed.

Photoacoustic signal attenuation analysis for the assessment of thin layers thickness in paintings

This study introduces a novel method for the thickness estimation of thin paint layers in works of art, based on photoacoustic signal attenuation analysis (PAcSAA). Ad hoc designed samples with acrylic paint layers (Primary Red Magenta, Cadmium Yellow, Ultramarine Blue) of various thicknesses on glass substrates were realized for the specific application. After characterization by Optical Coherence Tomography imaging, samples were irradiated at the back side using low energy nanosecond laser pulses of 532 nm wavelength. Photoacoustic waves undergo a frequency-dependent exponential attenuation through the paint layer, before being detected by a broadband ultrasonic transducer. Frequency analysis of the recorded time-domain signals allows for the estimation of the average transmitted frequency function, which shows an exponential decay with the layer thickness. Ultrasonic attenuation models were obtained for each pigment and used to fit the data acquired on an inhomogeneous painted mock-up simulating a real canvas painting. Thickness evaluation through PAcSAA resulted in excellent agreement with cross-section analysis with a conventional brightfield microscope. The results of the current study demonstrate the potential of the proposed PAcSAA method for the non-destructive stratigraphic analysis of painted artworks.

Investigation of Silicon Nitride as an Excellent Membrane Material for MEMS Ultrasonic Transducers

Silicon micromachining techniques developed over the last few decades’ aid in precise control over nanometer scale structures. Using these techniques to fabricate transducer elements with small electrode spacing efficient and broadband ultrasonic air transducers can be made. In this paper frequency response profile of a circular capacitive micromachined ultrasonic transducer (CMUT) element with silicon nitride as the membrane material is analytically modelled and compared with FEM simulation results. It is observed that the resonance frequency is highly dependent on the membrane material and structural properties. Silicon nitride serves as an excellent material for generation and detection of ultrasonic waves in air. The results are compared with published experimental values and appreciable agreement is obtained.

Erratum to: “Low-frequency broadband ultrasonic transducers for testing articles manufactured of large-structure and composite materials. Part 1. Complete and partial degeneracy of vibration modes in piezoelectric elements of different geometric shapes”

Erratum to: “Low-frequency broadband ultrasonic transducers for testing articles manufactured of large-structure and composite materials. Part 1. Complete and partial degeneracy of vibration modes in piezoelectric elements of different geometric shapes”

Erratum to: “Low-frequency broadband ultrasonic transducers for testing articles that are manufactured of large-structure and composite materials. Part 2. Excitation of low-frequency ultrasonic wide-band signals”

Erratum to: “Low-frequency broadband ultrasonic transducers for testing articles that are manufactured of large-structure and composite materials. Part 2. Excitation of low-frequency ultrasonic wide-band signals”

A noncontact method for determining surface density of nonporous materials with the limp-wall mass law

A new noncontact technique for determining the surface density or mass per unit area of nonporous, homogeneous membranes and foils of sub-wavelength thicknesses is introduced. Surface densities are determined through application of the limp-wall mass law and through-transmission ultrasonic measurements of bulk waves. The ultrasonic measurements are performed with commercially-available, broadband air-coupled ultrasonic transducers. Surface densities of aluminum foil, brass shim, and plastic sheets are typically found to be within 3% of their accepted values.

Low-frequency broadband ultrasonic transducers for testing articles that are manufactured of large-structure and composite materials. Part 2. excitation of low-frequency ultrasonic wide-band signals

A number of methods for the electric excitation of piezoelectric transducers (PETs) that are characterized by multiple frequencies of vibration modes and are virtually single-mode are considered. It was found that in order to substantially extend the spectrum of a radiated elastic pulse it is necessary that the frequency components that are equal to the resonance frequencies of PETs be absent in the spectrum of the excitation pulse. Several designs of PETs with a “carrier” frequency of up to 300 kHz that radiate elastic pulses with controlled durations of one, two, three, or more periods of the carrier frequency, were developed.

Low-frequency broadband ultrasonic transducers for testing articles manufactured of large-structure and composite materials. Part 1. complete and partial degeneracy of vibration modes in piezoelectric elements of different geometric shapes

The main statements of the theory of vibrations of piezoelectric elements (PEEs) in the form of thin bounded plates of different shapes are considered and a technique for calculating their resonance frequencies (vibration modes) is developed. A technique for calculating and constructing the elastic fields of vibrating rectangular and square anisotropic plates is proposed. Requirements are formulated and methods are demonstrated that make it possible to obtain single-mode vibration systems and systems that are characterized by partial degeneracy of vibration modes of PEEs in the form of rectangular and square piezoelectric plates, rings, and disks.

Basic study for ultrasound-based navigation for pedicle screw insertion using transmission and backscattered methods.

The purpose of this study was to understand the acoustic properties of human vertebral cancellous bone and to study the feasibility of ultrasound-based navigation for posterior pedicle screw fixation in spinal fusion surgery. Fourteen human vertebral specimens were disarticulated from seven un-embalmed cadavers (four males, three females, 73.14 ± 9.87 years, two specimens from each cadaver). Seven specimens were used to measure the transmission, including tests of attenuation and phase velocity, while the other seven specimens were used for backscattered measurements to inspect the depth of penetration and A-Mode signals. Five pairs of unfocused broadband ultrasonic transducers were used for the detection, with center frequencies of 0.5 MHz, 1 MHz, 1.5 MHz, 2.25 MHz, and 3.5 MHz. As a result, good and stable results were documented. With increased frequency, the attenuation increased (P<0.05), stability of the speed of sound improved (P<0.05), and penetration distance decreased (P>0.05). At about 0.6 cm away from the cortical bone, warning signals were easily observed from the backscattered measurements. In conclusion, the ultrasonic system proved to be an effective, moveable, and real-time imaging navigation system. However, how ultrasonic navigation will benefit pedicle screw insertion in spinal surgery needs to be determined. Therefore, ultrasound-guided pedicle screw implantation is theoretically effective and promising.