Transducer Configuration Research Articles

Multi-frequency ultrasound transducers for medical applications: a survey

Nowadays, ultrasound imaging (US) has made extensive applications in the field of the medical imaging due to its particular advantages, in which the transducer plays an important role. The traditional single-frequency transducers can not meet the requirements of various medical applications, such as harmonic imaging (HI), second-order ultrasound field imaging (SURF) and acoustic radiation force imaging (ARFI). Therefore, multi-frequency, especially dual-frequency (DF) transducers are proposed, each of which transmits and/or receives the ultrasound in different frequencies. The multi-frequency transducer configurations mainly contain five types which named vertical stack, horizontal stack, transducers with polyvinylidene fluoride (PVDF) layer, interleaved arrays and annular array. In this paper, the design, fabrication and recent developments of the multi-frequency transducers are presented and compared by defragmenting more than a hundred papers.

Transurethral high-intensity ultrasound for treatment of stress urinary incontinence (SUI): simulation studies with patient-specific models

Background: Stress urinary incontinence (SUI) is prevalent in adult women, attributed to weakened endopelvic supporting tissues, and typically treated using drugs and invasive surgical procedures. The objective of this in silico study is to explore transurethral high-intensity ultrasound for delivery of precise thermal therapy to the endopelvic tissues adjacent to the mid-urethra, to induce thermal remodeling as a potential minimally invasive treatment alternative.Methods: 3D acoustic (Rayleigh–Sommerfeld) and biothermal (Pennes bioheat) models of the ultrasound applicator and surrounding tissues were devised. Parametric studies over transducer configuration [frequency, radius-of-curvature (ROC)] and treatment settings (power, duration) were performed, and select cases on patient-specific models were used for further evaluation. Transient temperature and thermal dose distributions were calculated, and temperature and dose metrics reported.Results: Configurations using a 5-MHz curvilinear transducer (3.5 × 10 mm, 28 mm ROC) with single 90 s sonication can create heated zones with 11 mm penetration (>50 °C) while sparing the inner 1.8 mm (<45 °C) radial depth of the urethral mucosa. Sequential and discrete applicator rotations can sweep out bilateral coagulation volumes (1.4 W power, 15° rotations, 600 s total time), produce large volumetric (1124 mm³ above 60 EM43 °C) and wide angular (∼50.5° per lateral sweep) coverage, with up to 15.6 mm thermal penetration and at least 1.6 mm radial urethral protection (<5 EM43 °C).Conclusion: Transurethral applicators with curvilinear ultrasound transducers can deliver spatially selective temperature elevations to lateral mid-urethral targets as a possible means to tighten the endopelvic fascia and adjacent tissues.

Design and Simulation of a Ring-Shaped Linear Array for Microultrasound Capsule Endoscopy.

Video capsule endoscopy (VCE) has significantly advanced visualization of the gastrointestinal tract since its introduction in the last 20 years. Work is now under way to combine VCE with microultrasound imaging. However, small maximum capsule dimensions, coupled with the electronics required to integrate ultrasound imaging capabilities, pose significant design challenges. This paper describes a simulation process for testing transducer geometries and imaging methodologies to achieve satisfactory imaging performance within the physical limitations of the capsule size and outlines many of the tradeoffs needed in the design of this new class of ultrasound capsule endoscopy (USCE) device. A hybrid MATLAB model is described, incorporating Krimholtz-Leedom-Matthaei circuit elements and digitizing and beamforming elements to render a gray-scale B-mode. This model is combined with a model of acoustic propagation to generate images of point scatterers. The models are used to demonstrate the performance of a USCE transducer configuration comprising a single, unfocused transmit ring of radius 5 mm separated into eight segments for electrical impedance control and a 512-element receive linear array, also formed into a ring. The MATLAB model includes an ultrasonic pulser circuit connected to a piezocrystal composite transmit transducer with a center frequency of 25 MHz. B-scan images are simulated for wire target phantoms, multilayered phantoms, and a gut wall model. To demonstrate the USCE system's ability to image tissue, a digital phantom was created from single-element ultrasonic transducer scans of porcine small bowel ex vivo obtained at a frequency of 45 MHz.

Development of a dual temporal-spatial chirp method for the generation of broadband surface acoustic waves

This study deals with the optimization of transducers for Rayleigh-type Surface Acoustic Waves (SAW) generation. These transducers are based on Interdigital Transducers (IDT) and are specifically developed to characterize properties of thin layers, coatings, and functional surfaces. In order to characterize these coatings and structures, it is necessary to work with wide bandwidths IDT operating in the high frequencies. Therefore, in this study a spatial chirp-based on an IDT are realized for wideband SAW generation. The use of impulse temporal excitation (Dirac-type negative pulse) leads to a wide band emitter excitation but with significantly limited SAW output amplitudes due to the piezoelectric crystal breakdown voltage. This limitation can be circumvented by applying a temporal chirp excitation corresponding in terms of frequency band and duration to the spatial chirp transducer configuration. This dual temporal-spatial chirp method was studied in the 20 to 125 MHz frequency range and allowed to obtain higher SAW displacement amplitudes with an excitation voltage lower than that of the impulse excitation.

Automated geometric optimization for robotic HIFU treatment of liver tumors

BackgroundHigh intensity focused ultrasound (HIFU) represents a non-invasive method for the destruction of cancerous tissue within the body. Heating of targeted tissue by focused ultrasound transducers results in the creation of ellipsoidal lesions at the target site, the locations of which can have a significant impact on treatment outcomes. Towards this end, this work describes a method for the optimization of lesion positions within arbitrary tumors, with specific anatomical constraints. Materials & methodsA force-based optimization framework was extended to the case of arbitrary tumor position and constrained orientation. Analysis of the approximate reachable treatment volume for the specific case of treatment of liver tumors was performed based on four transducer configurations and constraint conditions derived. Evaluation was completed utilizing simplified spherical and ellipsoidal tumor models and randomly generated tumor volumes. The total volume treated, lesion overlap and healthy tissue ablated was evaluated. Two evaluation scenarios were defined and optimized treatment plans assessed. ResultsThe optimization framework resulted in improvements of up to 10% in tumor volume treated, and reductions of up to 20% in healthy tissue ablated as compared to the standard lesion rastering approach. Generation of optimized plans proved feasible for both sub- and intercostally located tumors. ConclusionsThis work describes an optimized method for the planning of lesion positions during HIFU treatment of liver tumors. The approach allows the determination of optimal lesion locations and orientations, and can be applied to arbitrary tumor shapes and sizes.

Measurement of a 3D Ultrasonic Wavefield Using Pulsed Laser Holographic Microscopy for Ultrasonic Nondestructive Evaluation.

In ultrasonic array imaging, 3D ultrasonic wavefields are normally recorded by an ultrasonic piezo array transducer. Its performance is limited by the configuration and size of the array transducer. In this paper, a method based on digital holographic interferometry is proposed to record the 3D ultrasonic wavefields instead of the array transducer, and the measurement system consisting of a pulsed laser, ultrasonic excitation, and synchronization and control circuit is designed. A consecutive sequence of holograms of ultrasonic wavefields are recorded by the system. The interferograms are calculated from the recorded holograms at different time sequence. The amplitudes and phases of the transient ultrasonic wavefields are recovered from the interferograms by phase unwrapping. The consecutive sequence of transient ultrasonic wavefields are stacked together to generate 3D ultrasonic wavefields. Simulation and experiments are carried out to verify the proposed technique, and preliminary results are presented.

Excitation of plane Lamb wave in plate-like structures under applied surface loading

Lamb waves play an important role in structure health monitoring (SHM) systems. The excitation of Lamb waves has been discussed for a long time with absorbing results. However, little effort has been made towards the precise characterization of Lamb wave excitation by various transducer models with mathematical foundation. In this paper, the excitation of plane Lamb waves with plane strain assumption in isotropic plate structures under applied surface loading is solved with the Hamiltonian system. The response of the Lamb modes excited by applied loading is expressed analytically. The effect of applied loading is divided into the product of two parts as the effect of direction and the effect of distribution, which can be changed by selecting different types of transducer and the corresponding transducer configurations. The direction of loading determines the corresponding displacement of each mode. The effect of applied loading on the in-plane and normal directions depends on the in-plane and normal displacements at the surface respectively. The effect of the surface loading distribution on the Lamb mode amplitudes is mainly reflected by amplitude versus frequency or wavenumber. The frequencies at which the maxima and minima of the S0 or A0 mode response occur depend on the distribution of surface loading. The numerical results of simulations conducted on an infinite aluminum plate verify the theoretical prediction of not only the direction but also the distribution of applied loading. A pure S0 or A0 mode can be excited by selecting the appropriate direction and distribution at the corresponding frequency.

Phantom study evaluating detection of simulated upper airway occlusion using piezoelectric ultrasound transducers

Obstructive sleep apnea/hypopnea syndrome (OSAHS) is characterized by repetitive narrowing or full collapse of the upper airway concomitant with continued respiratory effort during sleep lasting 10s or more. OSAHS is the most prevalent form of sleep-disordered breathing, affecting more than 17% of the middle-aged U.S. population. Hence, many individuals need to be tested for having OSAHS. Currently, detection of airway occlusion due to OSAHS is achieved by indirect measurements, often requiring multiple sensor types, such as a flow transducer combined with chest and abdomen plethysmography. The need for the use of multiple sensors in the current OSAHS detection systems adds to the cost and complexity of the current systems and associated procedures. Development of a simple sensor system that allows direct detection of airway occlusion is advantageous, as it simplifies detection of OSAHS and paves the way for home diagnosis of OSAHS. The utilization of ultrasonic transducers is attractive, as it is non-invasive and non-ionizing. We present a new ultrasound sensing system for direct detection of the occlusion in the upper airway in OSAHS patients during sleep. The system takes into consideration the constraints arising from the location of probing and the acoustic requirements for transducers. The physiological and theoretical backgrounds are presented for using ultrasonic pulses to detect the presence and degree of occlusion in the airway. The proposed methodology for creating an anthropomorphically-correct neck and airway phantoms to test the hypothesis and the results of the tests are presented. HypothesisAn ultrasonic signal transmitted through or reflected from an open airway will have different features compared to those associated with a partially or fully occluded airway. MethodologyA system, comprising a phantom model of the airway and neck with the approximate anatomical-correct dimensions and acoustic properties of the airway, is designed and built. It allows simulating fully open airway as well as hypopnea and apnea events. Further, it facilitates probing using multiple ultrasonic frequencies and transducer configurations for use with different neck sizes. Ultrasound waves are generated using a piezoelectric source to the model of the airway and received by piezoelectric receivers on the opposite side. Energy, the area under the curve, and the peak value of the received signal, are used to detect the airway occlusion. ResultsThe amount of reflected ultrasonic energy from the phantom model of the airway back to the transmitting transducer reduces as the airway model occlusion increases. Also, transmitted signal through the airway model increases as the amount occlusion of the airway model increases. ConclusionsThe results of this study support the hypothesis that it is feasible to use ultrasonic pulses to detect partial and full upper airway occlusion.

Equivalent-circuit models for electret-based vibration energy harvesters

This paper presents a complete analysis to build a tool for modelling electret-based vibration energy harvesters. The calculational approach includes all possible effects of fringing fields that may have significant impact on output power. The transducer configuration consists of two sets of metal strip electrodes on a top substrate that faces electret strips deposited on a bottom movable substrate functioning as a proof mass. Charge distribution on each metal strip is expressed by series expansion using Chebyshev polynomials multiplied by a reciprocal square-root form. The Galerkin method is then applied to extract all charge induction coefficients. The approach is validated by finite element calculations. From the analytic tool, a variety of connection schemes for power extraction in slot-effect and cross-wafer configurations can be lumped to a standard equivalent circuit with inclusion of parasitic capacitance. Fast calculation of the coefficients is also obtained by a proposed closed-form solution based on leading terms of the series expansions. The achieved analytical result is an important step for further optimisation of the transducer geometry and maximising harvester performance.

Spin wave generation by surface acoustic waves

Surface acoustic waves (SAW) on piezoelectric substrates can excite spin wave resonance (SWR) in magnetostrictive films through magnetoelastic coupling. This acoustically driven SWR enables the excitation of a single spin wave mode with an in-plane wave vector k matched to the magnetoelastic wave vector. A 2D frequency domain finite element model is presented that fully couples elastodynamics, micromagnetics, and piezoelectricity with interface spin pumping effects taken into account. It is used to simulate SAW driven SWR on a ferromagnetic and piezoelectric heterostructure device with an interdigital transducer configuration. These results, for the first time, present the spatial distribution of magnetization components that, together with elastic wave, exponentially decays along the propagation direction due to magnetic damping. The results also show that the system transmission rate S21(dB) can be tuned by both an external bias field and the SAW wavevector. Acoustic spin pumping at magnetic film/normal metal interface leads to damping enhancement in magnetic films that decreases the energy absorption rate from elastic energy. This weakened interaction between the magnetic energy and elastic energy leads to a lower evanescence rate of the SAW that results in a longer distance propagation. With strong magnetoelastic coupling, the SAW driven spin wave is able to propagate up to 1200 μm. The results give a quantitative indication of the acoustic spin pumping contribution to linewidth broadening.

Generation of broadband surface acoustic waves using a dual temporal-spatial chirp method

Wideband surface acoustic wave (SAW) generation with a spatial chirp-based interdigital transducer was optimized for non-destructive characterization and testing of coatings and thin layers. The use of impulse temporal excitation (Dirac-type negative pulse) leads to a wide band emitter excitation but with significantly limited SAW output amplitudes due to the piezoelectric crystal breakdown voltage. This limitation can be circumvented by applying a temporal chirp excitation corresponding in terms of frequency band and duration to the spatial chirp transducer configuration. This dual temporal-spatial chirp method was studied in the 20 to 125 MHz frequency range and allowed to obtain higher SAW displacement amplitudes with an excitation voltage lower than that of the impulse excitation.

Use of acoustic and stereo camera systems for assessing demersal fish of Robinson Crusoe Island (Juan Fernández Archipelago, off central Chile)

The Juan Fernandez Archipelago, constituted by Robinson Crusoe, Santa Clara and Selkirk Islands, are peaks of a continuous submarine ridge extended in the east-west direction off the coast of central Chile. It supports a rich endemic fish community, which maintains the livelihood of hundreds of fishermen and their families. In recent years there has been a growing interest in assessing the demersal fish species composition, distribution, and abundance in this system. To accomplish this task traditional trawl surveys are inappropriate because of the complexity and fragile nature of the seafloor, therefore we used scientific echosounders combined with a stereo camera system. Two acoustic surveys (2015 and 2016) were conducted using various configurations of transducers and cameras from ROV and tow-bodies. Results indicate a strong association of demersal fish to bottom reefs and species compositions varying with depth. Using ex-situ target strength (TS) estimates combined with species composition and size d...

Transducer design for AlN Lamb wave resonators

AlN Lamb wave resonators enjoy advanced and attractive properties for enabling the next-generation single-chip radio frequency front-end, but their moderate effective electromechanical coupling coefficient (k2eff) poses a limit to their application in filters and multiplexers. Despite the fact that the reported k2eff enhancement techniques of doped AlN thin films which are expensive and trade off the quality factor (Q), the transducer topology itself extensively impacts the k2eff value. Although an AlN cross-sectional Lame mode resonator exhibiting a k2eff of 6.34% has been demonstrated without the need for changing the piezoelectric material, a detailed study of transducer design for AlN Lamb wave resonators has not been conducted. In this work, we investigate the impact of (i) transducer configurations, (ii) electrode materials, (iii) electrode thicknesses, and (iv) interdigital transducer duty factors on the k2eff dispersive characteristics of one-port AlN Lamb wave resonators by using the finite element analysis approach. By properly designing and optimizing the transducers, the k2eff of one-port AlN Lamb wave resonators can be boosted to as high as 7.7%, showing great potential for applications of cellular frequency selection.

Development of a 2D analytical model for the prediction of directivity pattern of transducers in the generation of guided wave modes

The guided waves (GW) are extensively used in the nondestructive testing (NDT) and structure health monitoring (SHM) of the large and complex structures. It enables to detect the defects up to few meters away from the transducers. However, the defects could occur at longer distances. In order to cover the test area of interest for the estimation of defects and to find the exact position of the transducers to be glued/ embedded, the directivity pattern of the transducers must be known. The aim of the presented work is to develop the 2D analytical model for the estimation of directivity patterns of the transducers at various frequencies and distances. The model was developed using the Huygens's principle of wave propagation distances with considering the known phase dispersive characteristics of the guided wave modes in the medium. The principle of modelling will work to plot the directivity pattern at any distance, at any excitation frequency and with any configuration and shape of the transducers. In order to demonstrate the model, the directivity patterns of P1-type macro-fiber composite (MFC) transducer in the generation of fundamental Lamb modes and shear horizontal mode at 300 mm from the center of MFC were obtained. The input signal used was 80 kHz, 3 period signal and propagation medium was Al plate of 2 mm thickness. The prediction about the position of where MFC transducer should be placed or glued on the object and selection of proper wave modes in order to estimate the defects in various materials is also discussed. The results were validated using finite element analysis (FEA) performed in ANSYS and further verified by the experimental analysis using low-frequency (LF) ultrasonic measurement system ULTRALAB, developed by Ultrasound Research Institute, Kaunas University of Technology. The proposed analytical model will not only facilitate to decide the position and number of transducers but also leads to choosing the configuration of transducer and wave modes suitable to be used for the inspection of defects.

(A new time of flight) Acoustic flow meter using wide band signals and adaptive beamforming techniques

In this paper we present the result of our research concerning the improvement of acoustic time of flight flow metering for water pipes. Current flow meters are based on the estimation of direct time of flight by matched filtering of the received and emitted signals by acoustic transducers. Currently, narrow band signals are used, as well as a single emitter/receptor transducer configuration. Although simple, this configuration presents a series of limitations such as energy losses due to pipe wall/water interface, pressure/flow transients, sensitivity to flow induced vibrations, acoustic beam deformations and shift due to changes in flow velocity and embedded turbulence in the flow. The errors associated with these limitations reduce the overall robustness of existing flow meters, as well as the measured flow rate range and lower accuracy. In order to overcome these limitations, two major innovations were implemented at the signal processing level. The first one concerns the use of wide band signals that optimise the power transfer throughout the acoustic path and also increase the number of velocity/flow readings per second. Using wide band signals having a high duration-bandwidth product increases the precision in terms of time of flight measurements and, in the same time, improves the system robustness. The second contribution consists in the use of a multiple emitter - multiple receivers configuration (for one path) in order to compensate the emitted acoustic beam shift, compensate the time of flight estimation errors and thus increase the flow meter's robustness in case of undesired effects such as the “flow blow” and transient/rapid flow rate/velocity changes. Using a new signal processing algorithm that take advantage of the controlled wide band content coming from multiple receivers, the new flow meters achieves a higher accuracy in terms of flow velocity over a wider velocity range than existing systems. Tests carried out on real scale experimental facility showed an increase in acoustic time of flight estimation, accuracy of 50% with respect to the existing measurements techniques based only on signal correlation.

Guided Wave Tomography of Pipe Bends.

Detection and monitoring of corrosion and erosion damage in pipe bends are open challenges due to the curvature of the elbow, the complex morphology of these defects, and their unpredictable location. Combining model-based inversion with guided ultrasonic waves propagating along the elbow and inside its walls offers the possibility of mapping wall-thickness losses over the entire bend and from a few permanently installed transducers under the realm of guided wave tomography (GWT). This paper provides the experimental demonstration of GWT of pipe bends based on a novel curved ray tomography algorithm and an optimal transducer configuration consisting of two ring arrays mounted at the ends of the elbow and a line of transducers fixed to the outer side of the elbow (extrados). Using realistic, localized corrosion defects, it is shown that detection of both the presence and progression of damage can be achieved with 100% sensitivity regardless of damage position around the bend. Importantly, this is possible for defects as shallow as 0.50% of wall thickness (WT) and for maximum depth increments of just 0.25% WT. However, due to the highly irregular profile of corrosion defects, GWT generally underestimates maximum depth relative to the values obtained from 3-D laser scans of the same defects, leading in many cases to errors between 3% WT and 8% WT.

Developments in New Fluid Rotational Seismometers: Instrument Performance and Future Directions

In this article we describe prototype designs and tests for low‐cost rotational medium‐ and strong‐motion seismometers using three types of proof mass (two liquid and one solid) and a number of transducer configurations. This article describes the third set of designs and tests in our development program. The details of our results for most of these are in the Ⓔ electronic supplement to this article, whereas here we concentrate on three of the most promising and representative design combinations. Most of our results pertain to sensors with water or silicon oil as the proof mass, though we also tested a torsion‐bar design with a solid proof mass. We find that most mass–transducer combinations lead to output proportional to rotational acceleration, with varying degrees of fidelity. Most combinations we tested can be dismissed from further development for reasons of performance or inconvenience during analysis of acceleration response (compare with Ⓔ electronic supplement). In this article, we describe three of the more promising combinations, one each for the three types of response functions we measured. Of these three, one mass–transducer combination in particular (a hinged sensing element and capacitive transduction) has output voltage closely proportional to rotational displacement (angle) over a wide frequency range; such displacement proportionality obviates two of the integration steps normally required to solve for continuum single‐point motions or correct for tilt‐induced errors in horizontal translational sensors. Thus, although we illustrate two other designs of some promise, we propose a new design that follows this displacement‐proportional path while increasing the device’s sensitivity to on‐axis rotations, improving its manufacturing ease and lowering its sensitivity to translational motions. Online Material: Results and related material for mass–transceiver combinations, drawings and photographs of liquid proof masses, amplitude and phase transfer functions, clip and linearity tests, and noise plots.

Optimal configuration and combination of piezoelectric transducer and inductor for synchronized-switch-damping-on-an-inductor technique

This article describes the optimal configuration and combination of piezoelectric transducers and inductors for the synchronized-switch-damping-on-an-inductor technique. The technique suppresses structural vibrations by inverting the polarity of the electric voltage in a piezoelectric transducer using a switched inductive shunt circuit at each displacement extremum. The energy dissipation rate of synchronized switch damping on an inductor depends on the impedances of the transducer and the inductor in the circuit, especially the resistive component, in this inversion. For this study, mathematical models of the equivalent resistances of transducers and inductors for this inversion phenomenon were formulated based on experiments with various transducers and inductors. Using these models, the optimal ratio of the thickness–area of patch-type piezoelectric transducers and that of the length–cross-sectional area of the lead of the inductors were analytically obtained. The optimization of series–parallel connections of multiple transducers and inductors was also shown to be equivalent to this one. The optimal mass budget allocation for the transducers and inductors was also formulated. Two examples of optimization, involving an increase in energy dissipation rates by a factor of 4, were presented. The examples showed that the time taken to suppress free vibrations in a clamped beam was reduced to half through the optimization.

Optimal Sensor Positioning for Damage Detection in Composite Sensorised Panels

In this work the optimal configuration of transducers for damage detection and localization has been investigated. A particular interest is given to three optimization methods: mini-max, average Probability of Non Detection (POND) and ray tracing approach, coupled with genetic algorithm. After optimal configurations have been computed for each technique, they are experimentally tested and compared on a composite panel with one or two damages by generating and receiving Lamb waves signals. Damage detection is carried out with the Probability Based Damage Index Method (PBDIM). It was found that, in most cases, the ray tracing method and the average POND technique give better results, with a good detection of damages in comparison to the minimax POND technique, even if the latter seems numerically better.

Mode-converted ultrasonic scattering in polycrystals with elongated grains.

Elastic wave scattering is used to study polycrystalline media for a wide range of applications. Received signals, which include scattering from the randomly oriented grains comprising the polycrystal, contain information from which useful microstructural parameters may often be inferred. Recently, a mode-converted diffuse ultrasonic scattering model was developed for evaluating the scattered response of a transverse wave from an incident longitudinal wave in a polycrystalline medium containing equiaxed single-phase grains with cubic elastic symmetry. In this article, that theoretical mode-converted scattering model is modified to account for grain elongation within the sample. The model shows the dependence on scattering angle relative to the grain axis orientation. Experimental measurements were performed on a sample of 7475-T7351 aluminum using a pitch-catch transducer configuration. The results show that the mode-converted scattering can be used to determine the dimensions of the elongated grains. The average grain shape determined from the experimental measurements is compared with dimensions extracted from electron backscatter diffraction, an electron imaging technique. The results suggest that mode-converted diffuse ultrasonic scattering has the potential to quantify detailed information about grain microstructure.