Broadband Ultrasound Transducers Research Articles

Enhancement of Ultrasonic Transducer Bandwidth by Acoustic Impedance Gradient Matching Layer.

High-performance broadband ultrasound transducers provide superior imaging quality in biomedical ultrasound imaging. However, a matching design that perfectly transmits the acoustic energy between the active piezoelectric element and the target medium over the operating spectrum is still lacking. In this work, an anisotropic gradient acoustic impedance composite material as the matching layer of an ultrasonic transducer was designed and fabricated; it is a non-uniform material with the continuous decline of acoustic impedance along the direction of ultrasonic propagation in a sub-wavelength range. This material provides a broadband window for ultrasonic propagation in a wide frequency range and achieves almost perfect sound energy transfer efficiency from the piezoelectric material to the target medium. Nano tungsten particles and epoxy resin were selected as filling and basic materials, respectively. Along the direction of ultrasonic propagation, the proportion of tungsten powder was carefully controlled to decrease gradually, following the natural exponential form in a very narrow thickness range. Using this new material as a matching layer with high-performance single crystals, the −6 dB bandwidth of the PMN-PT ultrasonic transducer could reach over 170%, and the insertion loss was only −20.3 dB. The transducer achieved a temporal signal close to a single wavelength, thus there is the potential to dramatically improve the resolution and imaging quality of the biomedical ultrasound imaging system.

Ultrasonic properties of human scalp

The goal of this study was to characterize the ultrasonic properties of human scalp. Thirty-two specimens were prepared from formalin-fixed scalp tissue from four human donors (age 35–65, 2 male, 2 female). Tissue specimens were mounted in acrylic frames with a 30 × 30-mm acoustic window. The specimens were scanned in a water tank with a broadband ultrasound transducer with center frequency 7.5 MHz using a motion-controlled system. The ultrasonic region of interest (ROI) was a 20 × 20-mm region with a step size of 410 μm. The signals acquired were analyzed to determine the speed of sound (SOS) and frequency slope of attenuation (FSA) at all scan locations (2500 locations per specimen). The mean ± standard deviation (SD) of the SOS and FSA over all the specimens respectively was 1536 ± 9 m/s and 1.88 ± 0.51 dBcm-1MHz-1. The SD within an ROI was 3–15 m/s for SOS and 0.15–0.85 dBcm-1MHz-1 for FSA, depending on specimen, indicating some heterogeneity in the ultrasonic properties of the tissue.

Acoustic holography combined with radiation force measurements to characterize broadband ultrasound transducers and calibrate hydrophone sensitivity

Accurate characterization of broadband ultrasound (US) transducers operating either in pulsed or CW modes is necessary for their use in biomedical applications. The acoustic holography method allows reconstruction of the pattern of vibrations at the transducer surface for use as a model boundary condition to simulate ultrasound fields in water or tissue. A calibrated hydrophone is often not available to perform such measurements. Here, it is proposed to combine transient acoustic holography with radiation force balance (RFB) measurements at multiple frequencies to quantitatively determine 3D distributions of the acoustic field at different source frequencies, the corresponding acoustic power of the source, and the hydrophone sensitivity within the frequency bandwidth of the transducer. First, a transient acoustic hologram is measured using a short-pulse excitation of the source by raster scanning the hydrophone along a surface in front of the source and recording the waveform at a large number of points (typically 10-40 thousand). Then, RFB measurements are conducted for different frequencies within the pulse bandwidth to determine axial component of the acoustic radiation force. These data are related to the corresponding values calculated from the holograms at each frequency component to determine the corresponding hydrophone sensitivities. [Work supported by RSF-14-12-00974.]

Broadband gradient impedance matching using an acoustic metamaterial for ultrasonic transducers

High-quality broadband ultrasound transducers yield superior imaging performance in biomedical ultrasonography. However, proper design to perfectly bridge the energy between the active piezoelectric material and the target medium over the operating spectrum is still lacking. Here, we demonstrate a new anisotropic cone-structured acoustic metamaterial matching layer that acts as an inhomogeneous material with gradient acoustic impedance along the ultrasound propagation direction. When sandwiched between the piezoelectric material unit and the target medium, the acoustic metamaterial matching layer provides a broadband window to support extraordinary transmission of ultrasound over a wide frequency range. We fabricated the matching layer by etching the peeled silica optical fibre bundles with hydrofluoric acid solution. The experimental measurement of an ultrasound transducer equipped with this acoustic metamaterial matching layer shows that the corresponding −6 dB bandwidth is able to reach over 100%. This new material fully enables new high-end piezoelectric materials in the construction of high-performance ultrasound transducers and probes, leading to considerably improved resolutions in biomedical ultrasonography and compact harmonic imaging systems.

Cavitation-enhanced delivery of insulin in agar and porcine models of human skin

Ultrasound-assisted transdermal insulin delivery offers a less painful and less invasive alternative to subcutaneous insulin injections. However, ultrasound-based drug delivery, otherwise known as sonophoresis, is a highly variable phenomenon, in part dependent on cavitation. The aim of the current work is to investigate the role of cavitation in transdermal insulin delivery. Fluorescently stained, soluble Actrapid insulin was placed on the surface of human skin-mimicking materials subjected to 265 kHz, 10% duty cycle focused ultrasound. A confocally and coaxially aligned 5 MHz broadband ultrasound transducer was used to detect cavitation. Two different skin models were used. The first model, 3% agar hydrogel, was insonated with a range of pressures (0.25–1.40 MPa peak rarefactional focal pressure—PRFP), with and without cavitation nuclei embedded within the agar at a concentration of 0.05% w/v. The second, porcine skin was insonated at 1.00 and 1.40 MPa PRFP. In both models, fluorescence measurements were used to determine penetration depth and concentration of delivered insulin. Results show that in agar gel, both insulin penetration depth and concentration only increased significantly in the presence of inertial cavitation, with up to a 40% enhancement. In porcine skin the amount of fluorescent insulin was higher in the epidermis of those samples that were exposed to ultrasound compared to the control samples, but there was no significant increase in penetration distance. The results underline the importance of instigating and monitoring inertial cavitation during transdermal insulin delivery.

Multidisciplinary approach to the persistent double distal tendon of the biceps brachii

The aim of this study is to correlate the ultrasound (US) appearance of the persistent double or bifid distal tendon of the biceps brachii muscle with anatomical and histological data. This will provide a new model to study the pathological distal biceps brachii tendon (DBBT). The DBBT of 20 cadaveric elbows were examined with linear array broadband US transducers (frequency band 14-6MHz) using an anterior approach. Trypan blue dye was injected underneath the paratenon under US guidance in 16 specimens. After they were dissected, five of them were processed to obtain histological slices stained with hematoxylin-eosin and antiserum to protein S100. At US, the DBBT is a tendon in which the fascicles are organized in two different hyperechoic components separated by a hyperechoic septum related to the endotenon. The endotenon is lax, flexible, and makes folding and gliding of the two portions feasible. The DBBT is surrounded by a hyperechoic paratenon adjacent to the tendon surface, which is only differentiable by US when dye is interposed between such structures. The connective septum of endotenon located between the two main components of the DBBT is responsible for the US image of two separate tendons and functionally enables it to work as two separate entities, thus allowing respective folding and gliding. The paratenon surrounding the lacertus fibrosus and the DBBT plays an important stabilization role, enabling them to change shape and arrangement during joint motion. It is also an important conduit for nerves and blood vessels.

Video-rate functional photoacoustic microscopy at depths

We report the development of functional photoacoustic microscopy capable of video-rate high-resolution in vivo imaging in deep tissue. A lightweight photoacoustic probe is made of a single-element broadband ultrasound transducer, a compact photoacoustic beam combiner, and a bright-field light delivery system. Focused broadband ultrasound detection provides a 44-μm lateral resolution and a 28-μm axial resolution based on the envelope (a 15-μm axial resolution based on the raw RF signal). Due to the efficient bright-field light delivery, the system can image as deep as 4.8 mm in vivo using low excitation pulse energy (28 μJ per pulse, 0.35 mJ/cm² on the skin surface). The photoacoustic probe is mounted on a fast-scanning voice-coil scanner to acquire 40 two-dimensional (2-D) B-scan images per second over a 9-mm range. High-resolution anatomical imaging is demonstrated in the mouse ear and brain. Via fast dual-wavelength switching, oxygen dynamics of mouse cardio-vasculature is imaged in realtime as well.

Adaptive optics photoacoustic spectroscopic imaging

For decreasing wavefront error and obtaining higher resolution image of biological tissues, an adaptive optics photoacoustic spectroscopic imaging (AOPSI) system is proposed in this paper. In this system, an adaptive optics (AO) sub-system consisting of a Shack–Hartmann wavefront sensor (SHWS) and a liquid crystal on silicon (LCOS) is designed to correct the wavefront aberration of the illuminating light for getting high resolution image. The photoacoustic (PA) signals of AOPSI generated by irradiating the sample with a tunable optical parametric oscillator (OPO) lasers emitting light at 680–950nm are received by a broadband ultrasound transducer. The received data are rearranged according to the wavelength sequences and decoded for image reconstruction. Moreover, the PA spectroscopy is used to obtain different wavelength PA images, with which one can accurately distinguish organization structures and identify organizational components, etc. The final simulation results demonstrated that when the wavefront errors were corrected by the AO system, the AOPSI images showed significant quality improvement which will be helpful to enhance the ability and application of PA imaging.

Sonoanatomy Of The Brachial Plexus With Single Broad Band-High Frequency (L17-5 Mhz) Linear Transducer

Sonoanatomy Of The Brachial Plexus With Single Broad Band-High Frequency (L17-5 Mhz) Linear Transducer

Ultrasonic model and system for measurement of corneal biomechanical properties and validation on phantoms

Non-invasive measurement of biomechanical properties of corneas may provide important information for ocular disease management and therapeutic procedures. An ultrasonic non-destructive evaluation method with a wave propagation model was developed to determine corneal biomechanical properties in vivo. In this study, we tested the feasibility of the approach in differentiating the mechanical properties of soft contact lenses as corneal phantoms. Three material types of soft contact lenses (six samples in each group) were measured using a broadband ultrasound transducer. The ultrasonic reflections from the contact lenses were recorded by a 500 MHz/8-bit digitizer, and displayed and processed by a PC. A reference signal was recorded to compute the normalized power spectra using Fast Fourier Transformation. An inverse algorithm based on least-squares minimization was used to reconstruct three parameters of the contact lenses: density, thickness, and elastic constants λ+2 μ. The thickness of each sample was verified using an electronic thickness gauge, and the averaged density for each type of lenses was verified using Archimedes’ principle and manufacturer's report. Our results demonstrated that the ultrasonic system was able to differentiate the elastic properties of the three types of the soft contact lenses with statistical significance ( P-value<0.001). The reconstructed thicknesses and densities agreed well with the independent measurements. Our studies on corneal phantoms indicated that the ultrasonic system was sensitive and accurate in measuring the material properties of cornea-like structures. It is important to optimize the system for in vivo measurements.

Intraoral Ultrasound in the Diagnosis and Treatment of Suspected Peritonsillar Abscess in the Emergency Department

Peritonsillar abscess (PTA) can be a life-threatening disease and may lead to significant complications without drainage. To describe the utility of ultrasound (US) in the evaluation of potential PTA and US-guided PTA drainage. The authors performed a retrospective US quality assurance review of all patients over 18 years of age scanned by emergency physicians for possible PTA. All patients presenting with PTA signs and symptoms including erythema and swelling of the tonsillar pillar and uvular deviation were eligible to be scanned. Patients confirmed to have an abscess on US had US-guided drainage followed by intravenous antibiotics in the emergency department and discharge home on oral antibiotics. Scans were performed by credentialed attending emergency physicians and residents. Researchers recorded patient symptoms, US findings, results of abscess drainage if performed, and any complications of drainage. All US examinations were performed with sheathed endocavity broadband US transducers on minimum depth and maximum resolution settings. Statistical analysis included descriptive statistics. Forty-three patients received intraoral US examinations for suspected PTA. Thirty-five (81%) were diagnosed as having abscess on US. All abscesses were drained with an 18- or 14-gauge needle under US guidance. There were no drainage complications. There was one false positive with a focal area of edema yielding no pus on needle aspiration under direct US visualization. No patient returned unexpectedly after drainage. These data suggest that intraoral US of suspected PTA allows for reliable diagnosis and safe and accurate abscess drainage.

Intraoral Ultrasound in the Diagnosis and Treatment of Suspected Peritonsillar Abscess in the Emergency Department

Peritonsillar abscess (PTA) can be a life-threatening disease and may lead to significant complications without drainage. Objectives: To describe the utility of ultrasound (US) in the evaluation of potential PTA and US-guided PTA drainage. Methods: The authors performed a retrospective US quality assurance review of all patients over 18 years of age scanned by emergency physicians for possible PTA. All patients presenting with PTA signs and symptoms including erythema and swelling of the tonsillar pillar and uvular deviation were eligible to be scanned. Patients confirmed to have an abscess on US had US-guided drainage followed by intravenous antibiotics in the emergency department and discharge home on oral antibiotics. Scans were performed by credentialed attending emergency physicians and residents. Researchers recorded patient symptoms, US findings, results of abscess drainage if performed, and any complications of drainage. All US examinations were performed with sheathed endocavity broadband US transducers on minimum depth and maximum resolution settings. Statistical analysis included descriptive statistics. Results: Forty-three patients received intraoral US examinations for suspected PTA. Thirty-five (81%) were diagnosed as having abscess on US. All abscesses were drained with an 18- or 14-gauge needle under US guidance. There were no drainage complications. There was one false positive with a focal area of edema yielding no pus on needle aspiration under direct US visualization. No patient returned unexpectedly after drainage. Conclusions: These data suggest that intraoral US of suspected PTA allows for reliable diagnosis and safe and accurate abscess drainage.

Functionally Graded Broadband Ultrasound Transducer Created by Forming an Internal Temperature Gradient

A broadband ultrasound transducer composed of a piezoelectric plate with a graded piezoelectric parameter is presented. Grading of the piezoelectric constant is performed by forming an appropriate temperature gradient in the thickness direction of a fully polarized piezoelectric ceramic plate that has a low Curie temperature. It is shown experimentally that the functionally graded transducer is capable of emitting an almost single-pulse ultrasonic wave.

Broadband Ultrasound Transducers Using Piezoelectrically Graded Materials

Broadband ultrasound transducers are presented that utilize piezoelectric materials having an effectively graded piezoelectric parameter and/or applied electric field. These transducers are constructed by forming a number of fine V-grooves on one surface of a piezoelectric plate. Broadband frequency characteristics of the proposed transducer is demonstrated through experiments of wave generation in water. The finite element method analysis has proved that even order resonant modes are also excited in the plate with a grooved configuration.

Broadband Transducers Using Effectively Graded Piezoelectric Plates for Generation of Short-Pulse Ultrasound

Broadband ultrasound transducers that utilize piezoelectric materials having an effectively graded piezoelectric parameter and/or an excitation electric field applied across the piezoelectric material are investigated. These transducers are constructed by forming a number of fine V-grooves on one surface of a piezoelectric plate. The broadband frequency characteristic of the proposed transducer is demonstrated by experiments of wave generation in water. Analysis by the finite-element method proves that even-order resonance modes are also excited in the plate with a grooved configuration.

Influence of data processing on cyclic variation of integrated backscatter and wall thickness in stunned porcine myocardium

This study was performed to investigate the relationship between the cyclic variation of integrated backscatter and myocardial wall thickening in stunned myocardium. Different definitions of cyclic variation were evaluated to be able to compare with other studies. Ultrasound data were acquired from 10 openchested Yorkshire pigs (25–33 kg) at baseline, during regional ischemia and during 30 min of stunning, using a broadband ultrasound transducer (3–7 MHz) sutured directly upon the left ventricular myocardial wall. Cyclic variation of integrated backscatter and myocardial wall thickening were calculated using three definitions obtained from the literature. Independent of the definition, cyclic variation of wall thickness and integrated backscatter were blunted during acute ischemia and returned transiently to or above baseline during the first minute of reperfusion, followed by a gradual decrease to a level under baseline during stunning. An early return of the cyclic variation of the integrated backscatter was not observed in pigs, independent of the data processing used. The relationship between integrated backscatter and wall thickness was maintained.