Imaging Transducer Research Articles

Evaluation of High Frequency Piezoelectric Micromachined Ultrasound Transducers for Photoacoustic Imaging.

In this work, the design, fabrication, and characterization of piezoelectric micromachined ultrasound transducer (PMUT) arrays for photoacoustic imaging applications are reported. An 80-element linear PMUT array with each element having 53 PMUT cells of 125 μm cell diameter were fabricated using 650 nm thick lead zirconate titanate (PZT) as the active piezoelectric layer. The PMUTs are designed to operate at ~10 MHz resonant frequency. The PMUT elements are validated for photoacoustic imaging using an agar gel phantom with embedded pencil leads as the imaging target. Photoacoustic A-line response of the targets captured by single PMUT element shows ~7 MHz center frequency with ~4.8 MHz bandwidth. B-mode images reconstructed from A-lines recorded during the linear scanning of a single element clearly imaged all the targets, thus validating the potential of the fabricated PMUTs for photoacoustic imaging.

Ultrasound Transducers for Biomedical Imaging and Therapy

Ultrasound Transducers for Biomedical Imaging and Therapy

Directed acoustic shearography for crack detection around fastener holes in aluminum plates

The detection of fatigue cracks around fastener holes in riveted structures is important to ensure the safety of the structures and to prevent major structural failure. However, due to the complex structure around the fastener holes, the existing non-destructive methods are not effective to address this challenge. Here, we demonstrate a directed acoustic shearography system using a ring-shaped phased array transducer for crack imaging around fastener holes in thick aluminum plates. Supported by numerical simulations, a 16-element ring-shaped phased array transducer was designed, fabricated and tested. The 16-element phased array transducer was excited by an in-house developed phased array drive circuit to control the directions of acoustic waves. Directed acoustic shearography testing with the ring-shaped phased array transducer showed that by controlling the direction of acoustic waves, the imaging contrast of the defects could be significantly improved. Directed acoustic shearography was able to detect subsurface crack which was not detectable by conventional acoustic shearography excited at the same voltage and frequency. This work demonstrates a promising practical NDT tool for fast and full-field detection of surface and subsurface defects with significantly enhanced sensitivity.

Development of a KNN Ceramic-Based Lead-Free Linear Array Ultrasonic Transducer.

High-frequency array transducers can provide higher imaging resolution than traditional transducers, thus resolving smaller features and producing finer images. Commercially available ultrasonic transducers are mostly made with lead-based piezoelectric materials, which are harmful to the environment and public health. This paper presents the development of the 64-elements high-frequency (18.3 MHz) lead-free linear array ultrasonic transducer based on (K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3 (KNLNTS) piezoceramic. Array elements were spaced at a 75- pitch, and interconnected via a custom flexible circuit. The two matching layers and a light backing material were used to improve the performance of the array. The developed KNLNTS ceramic-based lead-free linear array exhibited a center frequency of 18.3 MHz, an average -6-dB bandwidth of 42%, an average two-way insertion loss of 41.8 dB, and a crosstalk between the adjacent elements of less than -53 dB near the center frequency. An image of a tungsten wire phantom was acquired using a Verasonics Vantage research ultrasound system. Results from imaging tests demonstrated a good imaging capability with a spatial resolution of axially and laterally, indicating that the lead-free linear array ultrasonic transducer based on KNLNTS ceramics is a promising alternative to lead-based transducers for ultrasound medical imaging.

Fabrication and testing of polymer-based capacitive micromachined ultrasound transducers for medical imaging

The ultrasonic transducer industry is dominated by piezoelectric materials. As an emerging alternative, capacitive micromachined ultrasound transducers (CMUTs) offer wider bandwidth, better integration with electronics, and ease of fabricating large arrays. CMUTs have a sealed cavity between a fixed electrode and a suspended metalized membrane. Manufacturing cost and sensitivity are limiting factors in current CMUTs that depend on the fabrication equipment and, especially, on the materials used. For widespread use of CMUTs, a much lower fabrication cost that uses inexpensive materials, which maintain or improve upon existing sensitivity, is needed. Herein, a new fabrication process is described for polymer-based CMUTs (polyCMUTs) using the photopolymer SU-8 and Omnicoat. The first ultrasound B-mode image of a wire phantom created with a 64-element linear array using synthetic aperture beamforming techniques is presented. A 12 VAC signal superimposed on a 10 VDC signal was used on the transmission side, and only a bias-tee, with no amplifiers, was used on the receiving side. The low operational voltage and high sensitivity of this device can be partially attributed to a pre-biasing condition on the membrane. By using a novel sacrificial layer combined with a top electrode embedded inside the membrane, we demonstrated that SU-8 can be used to manufacture CMUTs inexpensively. Moreover, the fabrication used relatively simple equipment, and the number of fabrication steps was reduced compared to traditional CMUT fabrication. This new fabrication process has the potential to increase the use of CMUTs in the ultrasound market, including the market for wearable transducers.

Full-waveform inversion and least-squares reverse-time migration imaging of collimated ultrasonic-beam data for high-resolution wellbore integrity monitoring

Los Alamos National Laboratory (LANL) has recently developed an ultrasonic side-lobe suppressed Bessel collimated beam transducer for low-frequency imaging through highly attenuating materials. We study the effectiveness of using such a collimated ultrasonic-beam transducer for imaging concrete heterogeneities outside borehole casing for high-resolution borehole imaging and wellbore integrity monitoring. We obtain detailed concrete heterogeneities including a low-velocity zone using ultrasound data acquired with a 3D imaging prototype and our recently developed full-waveform inversion and least-squares reverse-time migration methods. Full-waveform inversion fits synthetic ultrasonic waveforms with recorded waveforms to build a high-resolution velocity model. Least-squares reverse-time migration produces a high-resolution structural image showing material discontinuities. Our results demonstrate that LANL's 3D imaging system with a collimated ultrasonic-beam transducer together with full-waveform inversion and least-squares reverse-time migration imaging has great potential for high-resolution single-well borehole imaging and non-destructive evaluation.

Fast, Low-Frequency Plane-Wave Imaging for Ultrasound Contrast Imaging

Plane-wave ultrasound contrast imaging offers a faster, less destructive means for imaging microbubbles compared with traditional ultrasound imaging. Even though many of the most acoustically responsive microbubbles have resonant frequencies in the lower-megahertz range, higher frequencies (>3 MHz) have typically been employed to achieve high spatial resolution. In this work we implement and optimize low-frequency (1.5-4 MHz) plane-wave pulse inversion imaging on a commercial, phased-array imaging transducer in vitro and illustrate its use in vivo by imaging a mouse xenograft model. We found that the 1.8-MHz contrast signal was about four times that acquired at 3.1 MHz on matched probes and nine times greater than echoes received on a higher-frequency probe. Low-frequency imaging was also much more resilient to motion. In vivo, we could identify sub-millimeter vasculature inside a xenograft tumor model and easily assess microbubble half-life. Our results indicate that low-frequency imaging can provide better signal-to-noise because it generates stronger non-linear responses. Combined with high-speed plane-wave imaging, this method could open the door to super-resolution imaging at depth, while high power pulses could be used for image-guided therapeutics.

A 2-D Ultrasound Transducer With Front-End ASIC and Low Cable Count for 3-D Forward-Looking Intravascular Imaging: Performance and Characterization

Intravascular ultrasound (IVUS) is an imaging modality used to visualize atherosclerosis from within the inner lumen of human arteries. Complex lesions like chronic total occlusions require forward-looking IVUS (FL-IVUS), instead of the conventional side-looking geometry. Volumetric imaging can be achieved with 2-D array transducers, which present major challenges in reducing cable count and device integration. In this work, we present an 80-element lead zirconium titanate matrix ultrasound transducer for FL-IVUS imaging with a front-end application-specific integrated circuit (ASIC) requiring only four cables. After investigating optimal transducer designs, we fabricated the matrix transducer consisting of 16 transmit (TX) and 64 receive (RX) elements arranged on top of an ASIC having an outer diameter of 1.5 mm and a central hole of 0.5 mm for a guidewire. We modeled the transducer using finite-element analysis and compared the simulation results to the values obtained through acoustic measurements. The TX elements showed uniform behavior with a center frequency of 14 MHz, a -3-dB bandwidth of 44%, and a transmit sensitivity of 0.4 kPa/V at 6 mm. The RX elements showed center frequency and bandwidth similar to the TX elements, with an estimated receive sensitivity of /Pa. We successfully acquired a 3-D FL image of three spherical reflectors in water using delay-and-sum beamforming and the coherence factor method. Full synthetic-aperture acquisition can be achieved with frame rates on the order of 100 Hz. The acoustic characterization and the initial imaging results show the potential of the proposed transducer to achieve 3-D FL-IVUS imaging.

Forward-looking 30-MHz phased-array transducer for peripheral intravascular imaging

This paper describes the design, fabrication, and testing of a forward-looking 30-MHz phased-array transducer for peripheral intravascular imaging. The array features 32 piezoelectric elements arranged in a linear 2-2 composite configuration dry etched into a piece of PMN-30%PT single crystal and separated by non-conductive epoxy kerfs at a 25-μm pitch, yielding a total active aperture of 0.8 mm in the azimuth direction and 1.0 mm in the elevation direction, with an elevation natural focal depth of 5.0 mm. The array includes non-conductive epoxy backing and two front matching layers. A custom flexible circuit connects the array piezoelectric elements to a bundle of 32 individual 48-AWG micro-coaxial cables enclosed within a 0.6-m long 8 F catheter. Performance characterization was evaluated via finite element analysis simulations and afterwards compared to measured results, which showed an average center frequency of 28.9 MHz, an average bandwidth of 36.4% at -6 dB, and crosstalk less than -26.5 dB. Imaging of a tungsten fine-wire phantom resulted in axial and lateral spatial resolutions of approximately 65 μm and 215 μm, respectively. The imaging capability was further evaluated with a porcine carotid artery phantom, demonstrating the suitability of the proposed phased-array transducer for peripheral intravascular imaging.

A 35 MHz/105 MHz Dual-Element Focused Transducer for Intravascular Ultrasound Tissue Imaging Using the Third Harmonic.

The superharmonic imaging of tissue has the potential for high spatial and contrast resolutions, compared to the fundamental and second harmonic imaging. For this technique, the spectral bandwidth of an ultrasound transducer is divided for transmission of ultrasound and reception of its superharmonics (i.e., higher than the second harmonic). Due to the spectral division for the transmission and reception, transmitted ultrasound energy is not sufficient to induce superharmonics in media without using contrast agents, and it is difficult that a transducer has a −6 dB fractional bandwidth of higher than 100%. For the superharmonic imaging of tissue, thus, multi-frequency array transducers are the best choice if available; transmit and receive elements are separate and have different center frequencies. However, the construction of a multi-frequency transducer for intravascular ultrasound (IVUS) imaging is particularly demanding because of its small size of less than 1 mm. Here, we report a recently developed dual-element focused IVUS transducer for the third harmonic imaging of tissue, which consists of a 35-MHz element for ultrasound transmission and a 105-MHz element for third harmonic reception. For high quality third harmonic imaging, both elements were fabricated to have the same focus at 2.5 mm. The results of tissue mimicking phantom tests demonstrated that the third harmonic images produced by the developed transducer had higher spatial resolution and deeper imaging depth than the fundamental images.

Development of Dual-Frequency Oblong-Shaped-Focused Transducers for Intravascular Ultrasound Tissue Harmonic Imaging.

Tissue harmonic imaging (THI), an essential mode of commercial ultrasound imaging scanners, can provide images with high spatial and contrast resolutions. For THI, the frequency spectrum of a transducer is generally divided for the transmission of fundamental signal and the reception of its second harmonic. Therefore, it is difficult to use the THI mode for intravascular ultrasound (IVUS) imaging because typical IVUS transducers have a narrow -6-dB fractional bandwidth of about 50%. Due to its small aperture (about 0.5 mm) and the strength of IVUS being too weak, it is difficult to construct a high-quality tissue harmonic image. In this paper, we report a recently developed dual-frequency oblong-shaped-focused IVUS transducer for high-quality intravascular THI; the transducer consists of three elements arranged side by side in the horizontal (i.e., elevation) direction. The two outer elements with a center frequency of 35 MHz are responsible for ultrasound transmission and the center element has a center frequency of 70 MHz for the reception of the second-harmonic signals. All three elements have a spherical shape with a radius of 3 mm to efficiently generate harmonics in the region of interest. This configuration of the developed IVUS transducer was determined to facilitate high-quality THI, which was based on the results of Field II simulation and finite-element analysis. The images of wires and a tissue-mimicking phantom indicated that the tissue harmonic images produced by the developed transducer have not only a high spatial resolution but also a deep imaging depth, compared to the 35- and 70-MHz fundamental images.

Co-registered photoacoustic and ultrasound imaging for tongue cancer detection

Tongue cancer is an increasingly common disease with high morbidity. Besides clinical observation, biomedical imaging techniques have been investigated for early detection of tongue cancer. In this paper, we proposed a co-registered dual-modality photoacoustic (PA) and ultrasound imaging technique to simultaneously map the functional and structural information of human tongue, which has the potential to detect and diagnose tongue cancer in early stage. The imaging probe comprises a 20-MHz side-view focused transducer for ultrasound imaging and PA detection, a light path constructed by a multimode optical fiber, and a prism for PA illumination. Phantom experiments were conducted to evaluate the performance of the system including penetration depth, spatial resolution and signal-to-noise ratio. In vivo imaging of animal tumor and human tongue was carried out to show the feasibility of the proposed technique to detect tumor lesions in human tongue. The results of phantom and in vivo experiments suggest that the proposed technique has the potential to detect the early-stage cancer lesions in human tongue.

A new compact analytical model of nanoelectromechanical systems-based capacitive micromachined ultrasonic transducers for pulse echo imaging

A compact analytical model of a capacitive micromachined ultrasonic transducer (CMUT) exhibiting a damped oscillatory output for pulse echo imaging is proposed. The analysis is improved herein by taking into account the acoustic damping coefficient, proportional to the velocity of vibration of the CMUT membrane equation of motion. Solutions for both radial and temporal variables of the modeled equation are obtained. The result is found to be a comprehensively damped oscillation that would enable an enhanced time interval for a pulse echo imaging sensor system. Finite element method modeling of the CMUT was also performed using PZFlex, a finite element analysis-based simulation tool. The results obtained from the analytical model are in good agreement with those from PZFlex.

Strain, temperature, and electric-field effects on the phase transition and piezoelectric responses of K0.5Na0.5NbO3 thin films

A KNbO3-based solid solution system is environmentally friendly with good electromechanical performance. This work established the misfit strain–strain and temperature–strain phase diagrams for K0.5Na0.5NbO3 thin films and calculated the polarization switching, phase transition, and piezoelectric responses of K0.5Na0.5NbO3 thin films under various strains, temperatures, and electric fields. The results show that the piezoelectric coefficient d33 can be enhanced near the phase boundaries. For the ferroelectric phase with a nonzero out-of-plane polarization component, an optimal electric field is identified for maximizing d33, which is desired in applications such as thin-film piezoelectric micro-electromechanical systems, transducers for ultrasound medical imaging, and energy harvesting. The present results are expected to provide guidance for the future experimental study of KxNa1−xNbO3 thin films and the optimization of ferroelectric thin film-based devices.

Clinical photoacoustic imaging platforms

Photoacoustic imaging (PAI) is a new promising medical imaging technology available for diagnosing and assessing various pathologies. PAI complements existing imaging modalities by providing information not currently available for diagnosing, e.g., oxygenation level of the underlying tissue. Currently, researchers are translating PAI from benchside to bedside to make unique clinical advantages of PAI available for patient care. The requirements for a successful clinical PAI system are; deeper imaging depth, wider field of view, and faster scan time than the laboratory-level PAI systems. Currently, many research groups and companies are developing novel technologies for data acquisition/signal processing systems, detector geometry, and an acoustic sensor. In this review, we summarize state-of-the-art clinical PAI systems with three types of the imaging transducers: linear array transducer, curved linear array transducer, and volumetric array transducer. We will also discuss the limitations of the current PAI systems and describe latest techniques being developed to address these for further enhancing the image quality of PAI for successful clinical translation.

Increasing the field-of-view of row–column-addressed ultrasound transducers: implementation of a diverging compound lens

The purpose of this work is to investigate compound lenses for row-column-addressed (RCA) ultrasound transducers for increasing the field-of-view (FOV) to a curvilinear volume region, while retaining a flat sole to avoid trapping air between the transducer sole and the patient, which would otherwise lead to unwanted reflections. The primary motivation behind this research is to develop a RCA ultrasound transducer for abdominal or cardiac imaging, where a curvilinear volume region is a necessity. RCA transducers provide 3-D ultrasound imaging with fewer channels than fully-addressed 2-D arrays (2N instead of N2), but they have inherently limited FOV. By increasing the RCA FOV, these transducers can be used for the same applications as fully-addressed transducers while retaining the same price range as conventional 2-D imaging due to the lower channel count. Analytical and finite element method (FEM) models were employed to evaluate design options. Composite materials were developed by loading polymers with inorganic powders to satisfy the corresponding speed of sound and specific acoustical impedance requirements. A Bi2O3 powder with a density of 8.9g/cm3 was used to decrease the speed of sound of a room temperature vulcanizing (RTV) silicone, RTV615, from 1.03mm/μs to 0.792mm/μs. Using micro-balloons in RTV615 and a urethane, Hapflex 541, their speeds of sound were increased from 1.03mm/μs to 1.50mm/μs and from 1.52mm/μs to 1.93mm/μs, respectively. A diverging add-on lens was fabricated of a Bi2O3 loaded RTV615 and an unloaded Hapflex 541. The lens was tested using a RCA probe, and a FOV of 32.2° was measured from water tank tests, while the FEM model yielded 33.4°. A wire phantom with 0.15mm diameter wires was imaged at 3MHz down to a depth of 14cm using a synthetic aperture imaging sequence with single element transmissions. The beamformed image showed that wires outside the array footprint were visible, demonstrating the increased FOV.

Two dimensional photoacoustic imaging using microfiber interferometric acoustic transducers

Photoacoustic imaging transducer with a pair of wavelength-matched Bragg gratings (forming a Fabry–Perot cavity) inscribed on a short section of microfiber has been developed. A tunable laser with wavelength that matched to one of selected fringe slopes was used to transmit the acoustic induced wavelength. Interferometric fringes with high finesse in transmission significantly enhanced the sensitivity of the transducer even under very small acoustic perturbations. The performance of this novel transducer was evaluated through the imaging studies of human hairs (∼98 μm in diameter). The spatial resolution is 300 μm. We have demonstrated that the novel transducer developed in this study is a versatile tool for photoacoustic imaging study.

Performance Evaluations on Using Entropy of Ultrasound Log-Compressed Envelope Images for Hepatic Steatosis Assessment: An In Vivo Animal Study.

Ultrasound B-mode imaging based on log-compressed envelope data has been widely applied to examine hepatic steatosis. Modeling raw backscattered signals returned from the liver parenchyma by using statistical distributions can provide additional information to assist in hepatic steatosis diagnosis. Since raw data are not always available in modern ultrasound systems, information entropy, which is a widely known nonmodel-based approach, may allow ultrasound backscattering analysis using B-scan for assessing hepatic steatosis. In this study, we explored the feasibility of using ultrasound entropy imaging constructed using log-compressed backscattered envelopes for assessing hepatic steatosis. Different stages of hepatic steatosis were induced in male Wistar rats fed with a methionine- and choline-deficient diet for 0 (i.e., normal control) and 1, 1.5, and 2 weeks (n = 48; 12 rats in each group). In vivo scanning of rat livers was performed using a commercial ultrasound machine (Model 3000, Terason, Burlington, MA, USA) equipped with a 7-MHz linear array transducer (Model 10L5, Terason) for ultrasound B-mode and entropy imaging based on uncompressed (HE image) and log-compressed envelopes (HB image), which were subsequently compared with histopathological examinations. Receiver operating characteristic (ROC) curve analysis and areas under the ROC curves (AUC) were used to assess diagnostic performance levels. The results showed that ultrasound entropy imaging can be used to assess hepatic steatosis. The AUCs obtained from HE imaging for diagnosing different steatosis stages were 0.93 (≥mild), 0.89 (≥moderate), and 0.89 (≥severe), respectively. HB imaging produced AUCs ranging from 0.74 (≥mild) to 0.84 (≥severe) as long as a higher number of bins was used to reconstruct the signal histogram for estimating entropy. The results indicated that entropy use enables ultrasound parametric imaging based on log-compressed envelope signals with great potential for diagnosing hepatic steatosis.

Temperature and frequency dependence of the coercive field of 0.71PbMb1/3Nb2/3O3–0.29PbTiO3 relaxor-based ferroelectric single crystal

The hysteresis loop of ferroelectric materials becomes narrower with the increase in temperature due to energy barrier reduction, while the coercive field level increases with frequency due to the inertia of polarization reversal. These two competing effects determine the limiting operation field of medical imaging transducers at high frequencies. We have measured the coercive field of the 0.71PbMb1/3Nb2/3O3–0.29PbTiO3 single crystal as functions of both temperature and frequency. It was found that the coercive field linearly decreases with temperature at all frequencies. Related theoretical analysis was also performed to understand the physics behind the observed phenomena.

High-frequency counter-flow plasma synthetic jet actuator and its application in suppression of supersonic flow separation

We come up with a control strategy for suppression of supersonic flow separation based on high-frequency Counter-flow Plasma Synthetic Jet Actuator (CPSJA). The main purpose of this investigation is to verify if its control authority can be enhanced by the jet/shock interaction. We use a blunt nose to generate a bow shock, a step on a flat plate to introduce a massive separation in a Mach 2 wind tunnel, and the CPSJA to generate Plasma Synthetic Jet (PSJ). In this study, pulsed capacitive discharge is provided for an array of CPSJAs, which makes the actuation (discharge) frequency f1 = 1 kHz, f2 = 2 kHz and f3 = 3 kHz. We use the high-speed schlieren imaging and fast response pressure transducers as well as a numerical simulation to investigate the quiescent PSJ properties, the interaction between the jet and bow shock, and its disturbance effect on the downstream separated region. The schlieren images show that PSJ is characterized by a succession of vortex rings; the jet strength weakens with the increase of frequency. A 4.5 mN jet thrust is found for all the frequencies. The simulation results show that jet/shock interaction produces vorticity in the vortex ring of the jet, enhancing turbulent mixing in PSJ so that a great deal of momentum is produced into the flow. We found the downstream flow is significantly disturbed by the enhanced actuation. Actuation with frequency of f2, f3 which is close to the natural frequency fn of the separation bubble suppresses the separation with the upstream laminar boundary layer being periodically attenuated, which has a better control effect than f1. The control effect is sensitive to the position where PSJ interacts with the shear layer, but the amount of energy deposited in one pulse is not crucial in a separation reduction in the experiment.