Transducer Bandwidth Research Articles

Optoacoustic imaging using fiber-optic interferometric sensors

An interferometric sensor based on nonmetallic silica optical fiber is presented as an ultrasonic wideband transducer for optoacoustic imaging applications. We have characterized the sensitivity of the optical fiber sensor by detecting optoacoustic signals from an optically absorbing object embedded in a tissue-mimicking phantom and have compared the signals recorded with those detected from the same phantom using an array of piezoelectric transducers. The optical fiber sensor was also scanned along the phantom surface in order to reconstruct two-dimensional optoacoustic images of the phantom. These images have been compared with images obtained using the Laser Optoacoustic Imaging System, LOIS-64B, demonstrating the feasibility of our fiber-optic sensor as a wideband ultrasonic transducer.

Finite-Element Method Analysis of Low-Frequency Wideband Array Composed of Disk Bender Transducers with Differential Connections

In recent ocean investigations using underwater sonar transducers, low-frequency and wideband long-range sonar systems have been demanded for strong acoustic radiation and improved detective resolution capability in shallow-sea regions. We developed a disk bender transducer with a dual radiation surface as a miniaturized, light weight, low-frequency, and high-power transducer. However, there were problems in that the fractional bandwidth was small because the radiation surface was far smaller than the radiated wavelength, and the acoustic load per unit radiation area was small. Therefore, we suggest a technique to enable a wideband sonar array using differential connections of multiple disk bender transducers with different resonance frequencies to solve these problems. In this paper, we report results that endorse the above-mentioned technique obtained by finite-element method (FEM) analysis. The results confirm that this technique produces a wideband transducer array with low-frequency and high-power characteristics. We found that a wideband characteristic of more than 100% could be achieved with as a 6 dB fractional bandwidth by differential connection of disk bender transducers with three different resonance frequencies. In addition, we found that a superior horizontally oriented directivity was provided by locating the transducers in a plane symmetrical to the horizontal plane.

Optoacoustic technique for thickness measurement of submicron metal coatings

The new nondestructive method for thickness measurement of submicron metal coatings on transparent substrate is developed. The method is based on the optoacoustic (OA) transformation in the system, where the coating is covered by an optically transparent liquid. Theoretical treatment of the problem consists of two steps. At the first step laser-induced thermal field in the system is calculated, taking into account the large thermal conductivity of the metal film and partial heat diffusion into the liquid. At the second step the system of wave equations for scalar potential of vibration velocities is solved. Heat sources, determined at the first step, are free form of wave equations. Three chrome coatings of different thickness (approximately 0.2, 0.3, and 0.6 μm) deposited on the quartz substrate are tested experimentally. Two different organic liquids (acetone and ethanol) are used to cover chrome coatings. Nanosecond diode-pumped Nd:YAG laser operated at the main harmonic is used to perform OA transformation (laser pulse duration is τ L = 12 ns, the laser energy is about 0.2 mJ). Two detection modes are used. In forward mode laser pulse irradiates the film from the side of the substrate and in backward mode—from the side of the liquid. Detection of induced ultrasonic pulses is performed by the wide-band piezoelectric transducer in the liquid in both cases. The thickness of the coatings is determined by the least squares fitting of the theoretical dependencies of spectral transfer functions of OA transformation to experimental data. It is demonstrated, that the developed technique can be used for measurement of metal coatings thickness within the range from 50 nm to 5 μm with the error about 50 nm.

WE‐D‐210A‐03: Systems and Probes for Ultrasound Molecular Imaging

Specialized ultrasound systems and targeted microbubbles optimized for vascular molecular imaging continue to advance. The development of methods for sensitive and selective imaging of adherent, targeted contrast agents requires improvements in both the physical systems and probes. We have developed transducers and signal processing techniques to maximize transducer bandwidth and integrated the components with a clinical ultrasound scanner. The transducer integrates low and high‐frequency arrays; the center‐row high‐frequency array is surrounded on each side with confocally‐focused low‐frequency arrays. By transmitting from the low‐frequency (1.6 MHz) arrays and receiving through the high‐frequency (7 MHz) array, harmonic microbubble echoes can be detected and distinguished from the surrounding tissue. Further, by combining echoes over a pulse train, a targeted agent to free agent signal ratio of 15∼22 dB can be obtained without waiting for agent clearance. Further, we report on radiolabeling methods to fully assess microbubble biodistribution with and without molecular targeting or insonation. We find that microbubbles are rapidly accumulated within the liver and spleen, although a small (but significant) mass of the microbubble shell accumulates as a result of local insonation.We acknowledge the support of NIH CA 103828 and CA 112356.

Multiple matching scheme for broadband 0.72Pb(Mg(13)Nb(23))O(3)-0.28PbTiO(3) single crystal phased-array transducer.

Lead magnesium niobate-lead titanate single crystal 0.72Pb(Mg(13)Nb(23))O(3)-0.28PbTiO(3) (abbreviated as PMN-PT) was used to fabricate high performance ultrasonic phased-array transducer as it exhibited excellent piezoelectric properties. In this paper, we focus on the design and fabrication of a low-loss and wide-band transducer for medical imaging applications. A KLM model based simulation software PiezoCAD was used for acoustic design of the transducer including the front-face matching and backing. The calculated results show that the -6 dB transducer bandwidth can be improved significantly by using double lambda8 matching layers and hard backing. A 4.0 MHz PMN-PT transducer array (with 16 elements) was fabricated and tested in a pulse-echo arrangement. A -6 dB bandwidth of 110% and two-way insertion loss of -46.5 dB were achieved.

A single-element transducer with nonuniform thickness for high-frequency broadband applications

The design, fabrication, and evaluation of a high-frequency single-element transducer are described. The transducer has an annular geometry, with the thickness of the piezoelectric material increasing from the center to the outside. This single-element annular transducer (SEAT) can provide a broader frequency range than a conventional single-element transducer with a uniform thickness (single-element uniform transducer, or SEUT). We compared the characteristics of a SEAT and a SEUT. Both transducers used 36 degrees-rotated, Y-cut lithium niobate (LiNbO(3)) material. The SEAT had a diameter of 6 mm and comprised 6 subelements of equal area (electrically connected by a single electrode on each side) whose thickness ranged from 60 microm (center) to 110 microm (outside), which resulted in the center frequency of the subelements varying from 59.8 MHz to 25 MHz. The overall center frequency was 42.4 MHz. The annular pattern was constructed using an ultrasonic sculpturing machine that reduced the root-mean-square value of the surface roughness to 454.47 nm. The bandwidth of the SEAT was 19% larger than that of the SEUT. However, compared with the SEUT, the 2-way insertion loss of the SEAT was increased by 3.1 dB. The acoustic beam pattern of the SEAT was also evaluated numerically by finite-element simulations and experimentally by an ultrasound beam analyzer. At the focus (10.5 mm from the transducer surface), the -6 dB beam width was 108 microm. There was reasonable agreement between the data from simulations and experiments. The SEAT can be used for imaging applications that require a wider transducer bandwidth, such as harmonic imaging, and can be manufactured using the same techniques used to produce transducers with multiple frequency bands.

Monitoring current waveform of the SeaTEM system

As a check on direct current measurements of the transmitted waveforms as measured with a wideband transducer, we monitored the EM field of the SeaTEM system during several flights over a ground loop. The ground loop was laid out over very resistive granite in Western Australia. Ground measurements were made of the current and voltage induced in the ground loop. For the first time, we also collected point-sampled total field and component magnetic fields using commercial sensor systems to investigate the effectiveness of direct B field rather than dB/dt measurements.

Tangential resolution improvement in thermoacoustic and photoacoustic tomography using a negative acoustic lens

We developed a novel concept of using a negative acoustic lens to increase the acceptance angle of an unfocused large-area ultrasonic transducer (detector), leading to more than twofold improvement of the tangential resolution in both thermoacoustic and photoacoustic tomography. In both thermoacoustic and photoacoustic tomography, for a given transducer bandwidth, the aperture size of the detector affects the tangential resolution greatly when the object of interest is near the detector surface. We were able to overcome such tangential resolution deterioration by attaching an acoustic concave lens, made of acrylic in front of the flat detector surface. We then quantified the tangential resolution improvement using phantom images. We also showed that the use of the negative lens preserves the shape of an object after the image is reconstructed.

2P5-5 Development of Wide-Band Ultrasonic Transducer for Imaging of Inside Bone(Poster Session)

2P5-5 Development of Wide-Band Ultrasonic Transducer for Imaging of Inside Bone(Poster Session)

Study on the broadband tonpilz transducer with a single hole

To get a wide-band transducer, the piezoelectric sandwiched transducer with a frustum hole in its head piece is presented in this paper. The equivalent circuit is derived, and the expressions of the equivalent mass and the equivalent impedance of the transducer are obtained by using one-dimensional (1D) design theory. Moreover, the expressions of the mechanical quality factor and the frequency bandwidth are obtained and the transmitting voltage response of the transducer is calculated by using finite element method. The theoretical results show that the frequency bandwidth of the transducer with a hole is wider than that without a hole when their resonant frequencies are almost equal. The tested results are in good agreement with the theoretical calculations.

Characterization of Mn-modified Pb(Mg1∕3Nb2∕3)O3–PbZrO3–PbTiO3 single crystals for high power broad bandwidth transducers

The effect of MnO(2) addition on the dielectric and piezoelectric properties of 0.4Pb(Mg(13)Nb(23))O(3)-0.25PbZrO(3)-0.35PbTiO(3) single crystals was investigated. Analogous to acceptor doping in "hard" Pb(Zr,Ti)O(3) based polycrystalline materials, the Mn doped crystals exhibited enhanced mechanical Q ( approximately 1050) with low dielectric loss ( approximately 0.2%), while maintaining ultrahigh electromechanical coupling k(33)>90%, inherent in domain engineered single crystals. The effect of acceptor doping was also evident in the build-up of an internal bias (E(i) approximately 1.6 kVcm), shown by a horizontal offset in the polarization-field behavior. Together with the relatively high usage temperature (T(R-T) approximately 140 degrees C), the Mn doped crystals are promising candidates for high power and broad bandwidth transducers.

Imaging with concave large-aperture therapeutic ultrasound arrays using conventional synthetic-aperture beamforming

Several dual-mode ultrasound array (DMUA) systems are being investigated for potential use in image- guided surgery. In therapeutic mode, DMUAs generate pulsed or continuous-wave (CW) high-intensity focused ultrasound (HIFU) beams capable of generating localized therapeutic effects within the focal volume. In imaging mode, pulse-echo data can be collected from the DMUA elements to obtain B-mode images or other forms of feedback on the state of the target tissue before, during, and after the application of the therapeutic HIFU beam. Therapeutic and technological constraints give rise to special characteristics of therapeutic arrays. Specifically, DMUAs have concave apertures with low f-number values and are typically coarsely sampled using directive elements. These characteristics necessitate pre- and post-beamforming signal processing of echo data to improve the spatial and contrast resolution and maximize the image uniformity within the imaging field of view (IxFOV). We have recently developed and experimentally validated beamforming algorithms for concave large-aperture DMUAs with directive elements. Experimental validation was performed using a 1 MHz, 64-element, concave spherical aperture with 100 mm radius of curvature. The aperture was sampled in the lateral direction using elongated elements 1-lambda x 33.3-lambda with 1.333-lambda center-to-center spacing (lambda is the wavelength). This resulted in f-number values of 0.8 and 2 in the azimuth and elevation directions, respectively. In this paper, we present a new DMUA design approach based on different sampling of the shared concave aperture to improve image quality while maintaining therapeutic performance. A pulse-wave (PW) simulation model using a modified version of the Field II program is used in this study. The model is used in generating pulse-echo data for synthetic-aperture (SA) beamforming for forming images of a variety of targets, e.g., wire arrays and speckle-generating cyst phantoms. To provide validation for the simulation model and illustrate the improvements in image quality, we show SA images of similar targets using pulse-echo data acquired experimentally using our existing 64-element prototype. The PW simulation model is used to investigate the effect of transducer bandwidth as well as finer sampling of the concave DMUA aperture on the image quality. The results show that modest increases in the sampling density and transducer bandwidth result in significant improvement in spatial and contrast resolutions in addition to extending the DMUA IxFOV.

The trapped fluid transducer: Modeling and optimization

Exact and approximate formulas for calculating the sensitivity and bandwidth of an electroacoustic transducer with an enclosed or trapped fluid volume are developed. The transducer is composed of a fluid-filled rectangular duct with a tapered-width plate on one wall emulating the biological basilar membrane in the cochlea. A three-dimensional coupled fluid-structure model is developed to calculate the transducer sensitivity by using a boundary integral method. The model is used as the basis of an optimization methodology seeking to enhance the transducer performance. Simplified formulas are derived from the model to estimate the transducer sensitivity and the fundamental resonant frequency with good accuracy and much less computational cost. By using the simplified formulas, one can easily design the geometry of the transducer to achieve the optimal performance. As an example design, the transducer achieves a sensitivity of around -200 dB (1 VmuPa) at 10 kHz frequency range with piezoelectric sensing. In analogy to the cochlea, a tapered-width plate design is considered and shown to have a more uniform frequency response than a similar plate with no taper.

Analysis of porous plate/water layered structures by means of the transition terms method

This paper is devoted to the study of water-saturated porous plate/water layered structures by means of the transition terms defined from the reflection and transmission coefficients R and T as follows: TTsym=(1-(R+T))/(2i) , TTasym=(1-(R-T))/(2i). They are the eigenvalues of the scattering matrix of the water-immersed structure and are directly connected with its symmetric or antisymmetric vibrations. The N porous plates associated in our structures obey Biot's theory which is first used to calculate the reflection and transmission coefficients of a unique water-saturated plate. An induction on N process allows to find the reflection and transmission coefficients of a given N plate/water-layer structure. The plates used in the experiments at normal incidence are 5mm thick. The reflection and transmission coefficients of sets of 1, 2, 3, and 4 water immersed plates, separated from each other by a 1cm water gap, are measured thanks to two wideband transducers with central frequency 0.5MHz. There are good agreements between the calculated and experimental transition terms which exhibit the symmetric and antisymmetric resonances of the structures. They obey the Breit-Wigner resonant form which characteristics can be obtained

The design of a wideband and widebeam piston transducer in a finite closed circular baffle

The design of a high power piezoelectric underwater transducer operating at frequency range 45kHz‐85kHz with acoustic power capability in excess of 150W is described. The transducer consists of two back‐to‐back elements. Each element is formed by stacked PZT‐4 ceramic rings, a matching and a steel backing layer, and placed in a finite rigid circular baffle. We investigate the dependence of bandwidth and beamwidth to the combination of piston and baffle radii, a and b, respectively. With a ka of 2.45 (k:wave number) and b/a ratio of 2, the transducer resonates at 65kHz with 70% bandwidth and has a beamwidth of 60° at each half space. We show that when two transducers are placed at right angles spatially and driven in parallel, we can obtain an omnidirectional beam pattern horizontally in the lower frequency band. The beam pattern exhibits two dips in each quadrant at the higher end of the frequency band, which are within 7 dB. We also investigated power handling capability of the transducer from thermal point of view using finite element analysis. The input impedance measurements agree well with the numerical results within the pass band.

Backscattering from an elastic target near a water-sediment interface at oblique incidence: First results of tank experiments

Sound scattering from a target situated near a water-sediment interface was studied in laboratory conditions in order to control separately all the parameters involved in the scattering process. Targets of different sizes were ensonified with wide band transducers covering the frequency range 200 kHz to 1 MHz. First, the target scattering strength was measured in the free space conditions, and the scattering strength of the water-sediment interface was measured at oblique incidence. These characteristics were used to provide a rough estimate for the signal-to-noise ratio for the second set of experiments where the target was situated near the interface to study effects of target-boundary interactions. The intensity of the total scattered field was measured as a function of the beamwidth, transducer/object and object/interface distances, frequency, grazing angle, target size and the interface roughness parameters. The interface considered here is a flattened sand surface which was studied earlier [Ivakin and Sessarego, High frequency scattering from flattened sand sediments: effects of granular structure, J. Acoust. Soc. Am., 122, (5) 2007]. The targets were spherical glass beads of different size. Side scan sonar images are presented and possibilities of their qualitative interpretation are discussed.

Ultrasonic pulse propagation in a bonded three‐layered structure

Bonded layers are used in the assembly of many critical functional parts of industrial equipment. In this work, ultrasonic pulse propagation in a steel‐rubber‐rubber bonded composite structure is investigated by means of computer simulation and pulse echo experimental evaluation. Ultrasonic pulse propagation is modelled using a 2D time domain finite‐difference software. For the experimental measurements, two test samples were fabricated by bonding a thin layer of steel and two thin layers of rubber, including debonded areas at marked regions of each interface. Several ultrasonic traces were acquired by contact pulse‐echo testing, using a 5 MHz wideband transducer, from the external steel surface. The large acoustic impedance mismatch existing between steel and rubber layers makes that only a very small part of the ultrasonic energy is transmitted through the first (steel‐rubber)interface. The high attenuation in rubber materials and the possible overlapping of multiple echoes are additional characteristics of the complex ultrasonic pulse propagation in this flat structure. Some differences in time and frequency domains, between the received signals from normal bonded areas and completely debonded areas are discussed, looking for defect detection at the first (steel‐rubber) and second (rubber‐rubber) interfaces.

Preliminary study of contrast-enhanced harmonic endosonography with second-generation contrast agents.

We developed a novel echoendoscope that enables contrast harmonic imaging using ultrasound contrast agents and performed contrast-enhanced harmonic endosonography (EUS) both in vitro and in vivo. An experimental convex-array echoendoscope equipped with a wideband transducer and a specific mode for contrast harmonic imaging was used. A Doppler phantom model was employed in in vitro experiments to determine the optimal mechanical indices for contrast harmonic imaging by the echoendoscope. In the in vivo experiments, the echoendoscope was inserted into the stomachs of dogs. The digestive organs were observed after intravenous infusion of a contrast agent, Definity, using contrast-enhanced harmonic EUS. Two patients, one with pancreatic carcinoma and one with a gastrointestinal stromal tumor (GIST), underwent contrast-enhanced harmonic EUS. In vitro experiments revealed that the optimal mechanical indices were 0.35-0.40 for intermittent imaging and 0.30 or less for real-time imaging. In the in vivo experiments, branching vessels and subsequent homogeneous distribution of the signal in the pancreatic tissue were observed. During clinical application, typical vascular patterns were observed in pancreatic carcinoma and a GIST. Contrast-enhanced harmonic EUS visualized parenchymal perfusion and the fine vascular structure in digestive organs and should be a useful and powerful method for clinical investigations.

2A1-F23 A Simulation of CDMA Based Ultrasonic Localization System in a Large Scale Environment

This paper discusses a simulation platform for the faster localization method which enables simultaneous 3D multi-targets localization by using CDMA for the TOF measurement of ultrasonic signals. Signal from each transmitter is assigned a unique pseudorandom "Gold codes" so that receivers can recognize them even the signals are mixed. The system works in the Tran-MD mode in which mobile devices are designed to be transmitters and base stations are designed to be receivers. Ultrasonic transducer beam pattern and bandwidth, Multi-path problem and near-far effect, etc. are taken into the consideration in this simulation platform. Simulation results proved the accuracy and robustness of such localization method if the proper triangulation algorithm is applied.

Acoustic characterization of a new trisacryl contrast agent. Part I: In vitro study

The objective of the study was to acoustically characterize trisacryl polymeric microparticles (TMP), which are derived from biocompatible embolic agents. With significant acoustic properties, these polymeric particles could be potentially used as targeted ultrasound contrast agents, directed towards a specific site, with ligands conjugation on the polymeric network surface. In the in vitro study, a pulser/receiver (PRF of 1 kHz), associated to different transducers (5, 10 and 15 MHz), was used to measure the acoustic properties of the TMP inserted in a Couette flow device. Acoustic characterization according to TMP concentration (0.12–15.63 mg/ml), frequency (4.5–17 MHz, defined by each transducer bandwidth), ultrasound pressure (137–378 kPa) and exposure time (0–30 min) was conducted. Particle attenuation was also evaluated according to TMP concentration and emission frequency. Backscattering increased non linearly with concentration and maximum enhancement was of 16.4 dB ± 0.89 dB above 7.8 mg/ml. This parameter was found non-linear with increasing applied pressure and no harmonic oscillation could be noticed. Attenuation reached approximately 1.4 dB/cm at 15 MHz and for the 15.6 mg/ml suspension. The TMP have revealed in vitro ultrasound properties comparable to those observed with known contrast agents, studied in similar in vitro systems. However, such set-ups combined with a rather aqueous suspending medium, have some limitations and further investigations need now to be conducted to approach in vivo conditions in terms of flow and blood environment.