Showing papers in "IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control in 2002"

Capacitive micromachined ultrasonic transducers: next-generation arrays for acoustic imaging?

Abstract: Piezoelectric materials have dominated the ultrasonic transducer technology. Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as an alternative technology offering advantages such as wide bandwidth, ease of fabricating large arrays, and potential for integration with electronics. The aim of this paper is to demonstrate the viability of CMUTs for ultrasound imaging. We present the first pulse-echo phased array B-scan sector images using a 128-element, one-dimensional (1-D) linear CMUT array. We fabricated 64- and 128-element 1-D CMUT arrays with 100% yield and uniform element response across the arrays. These arrays have been operated in immersion with no failure or degradation in performance over the time. For imaging experiments, we built a resolution test phantom roughly mimicking the attenuation properties of soft tissue. We used a PC-based experimental system, including custom-designed electronic circuits to acquire the complete set of 128/spl times/128 RF A-scans from all transmit-receive element combinations. We obtained the pulse-echo frequency response by analyzing the echo signals from wire targets. These echo signals presented an 80% fractional bandwidth around 3 MHz, including the effect of attenuation in the propagating medium. We reconstructed the B-scan images with a sector angle of 90 degrees and an image depth of 210 mm through offline processing by using RF beamforming and synthetic phased array approaches. The measured 6-dB lateral and axial resolutions at 135 mm depth were 0.0144 radians and 0.3 mm, respectively. The electronic noise floor of the image was more than 50 dB below the maximum mainlobe magnitude. We also performed preliminary investigations on the effects of crosstalk among array elements on the image quality. In the near field, some artifacts were observable extending out from the array to a depth of 2 cm. A tail also was observed in the point spread function (PSF) in the axial direction, indicating the existence of crosstalk. The relative amplitude of this tail with respect to the mainlobe was less than -20 dB.

Ultrafast compound imaging for 2-D motion vector estimation: application to transient elastography

Abstract: This paper describes a new technique for two-dimensional (2-D) imaging of the motion vector at a very high frame rate with ultrasound. Its potential is experimentally demonstrated for transient elastography. But, beyond this application, it also could be promising for color flow and reflectivity imaging. To date, only axial displacements induced in human tissues by low-frequency vibrators were measured during transient elastography. The proposed technique allows us to follow both axial and lateral displacements during the shear wave propagation and thus should improve Young's modulus image reconstruction. The process is a combination of several ideas well-known in ultrasonic imaging: ultra-fast imaging, multisynthetic aperture beamforming, 1-D speckle tracking, and compound imaging. Classical beamforming in the transmit mode is replaced here by a single plane wave insonification increasing the frame rate by at least a factor of 128. The beamforming is achieved only in the receive mode on two independent subapertures. Comparison of successive frames by a classical 1-D speckle tracking algorithm allows estimation of displacements along two different directions linked to the subapertures beams. The variance of the estimates is finally improved by tilting the emitting plane wave at each insonification, thus allowing reception of successive decorrelated speckle patterns.

Shear elasticity probe for soft tissues with 1-D transient elastography

Abstract: Important tissue parameters such as elasticity can be deduced from the study of the propagation of low frequency shear waves. A new method for measuring the shear velocity in soft tissues is presented in this paper. Unlike conventional transient elastography in which the ultrasonic transducer and the low frequency vibrator are two separated parts, the new method relies on a probe that associates the vibrator and the transducer, which is built on the axis of the vibrator. This setup is easy to use. The low frequency shear wave is driven by the transducer itself that acts as a piston while it is used in pulse echo mode to acquire ultrasonic lines. The results obtained with the new method are in good agreement with those obtained with the conventional one.

Shear modulus imaging with 2-D transient elastography

Abstract: In previous work, we have shown that time-resolved 2-D transient elastography is a promising technique for characterizing the elasticity of soft tissues. It involves the measurement of the displacements induced by the propagation of low frequency (LF) pulsed shear waves in biological tissues. In this paper, we present a novel apparatus that contains a LF vibrating device surrounding a linear array of 128 ultrasonic transducers that performs ultrafast ultrasonic imaging (up to 10,000 frames/s) and that is able to follow in real time the propagation of a LF shear wave in the human body. The vibrating device is made of two rods, fixed to electromagnetic vibrators, that produce in the ultrasonic image area a large amplitude shear wave. The geometry has been chosen both to enhance the sensitivity and to create a quasi linear shear wave front in the imaging plane. An inversion algorithm is used to recover the shear modulus map from the spatio-temporal data, and the first experimental results obtained from tissue-equivalent materials are presented.

A 30-MHz piezo-composite ultrasound array for medical imaging applications

Abstract: Ultrasound imaging at frequencies above 20 MHz is capable of achieving improved resolution in clinical applications requiring limited penetration depth. High frequency arrays that allow real-time imaging are desired for these applications but are not yet currently available. In this work, a method for fabricating fine-scale 2-2 composites suitable for 30-MHz linear array transducers was successfully demonstrated. High thickness coupling, low mechanical loss, and moderate electrical loss were achieved. This piezo-composite was incorporated into a 30-MHz array that included acoustic matching, an elevation focusing lens, electrical matching, and an air-filled kerf between elements. Bandwidths near 60%, 15-dB insertion loss, and crosstalk less than -30 dB were measured. Images of both a phantom and an ex vivo human eye were acquired using a synthetic aperture reconstruction method, resulting in measured lateral and axial resolutions of approximately 100 /spl mu/m.

Clutter filter design for ultrasound color flow imaging

Abstract: For ultrasound color flow images with high quality, it is important to suppress the clutter signals originating from stationary and slowly moving tissue sufficiently. Without sufficient clutter rejection, low velocity blood flow cannot be measured, and estimates of higher velocities will have a large bias. The small number of samples available (8 to 16) makes clutter filtering in color flow imaging a challenging problem. In this paper, we review and analyze three classes of filters: finite impulse response (FIR), infinite impulse response (IIR), and regression filters. The quality of the filters was assessed based on the frequency response, as well as on the bias and variance of a mean blood velocity estimator using an autocorrelation technique. For FIR filters, the frequency response was improved by allowing a non-linear phase response. By estimating the mean blood flow velocity from two vectors filtered in the forward and backward direction, respectively, the standard deviation was significantly lower with a minimum phase filter than with a linear phase filter. For IIR filters applied to short signals, the transient part of the output signal is important. We analyzed zero, step, and projection initialization, and found that projection initialization gave the best filters. For regression filters, polynomial basis functions provide effective clutter suppression. The best filters from each of the three classes gave comparable bias and variance of the mean blood velocity estimates. However, polynomial regression filters and projection-initialized IIR filters had a slightly better frequency response than could be obtained with FIR filters.

Sparse 2-D arrays for 3-D phased array imaging - design methods

Abstract: One of the most promising techniques for limiting complexity for real-time 3-D ultrasound systems is to use sparse 2-D layouts. For a given number of channels, optimization of performance is desirable to ensure high quality volume images. To find optimal layouts, several approaches have been followed with varying success. The most promising designs proposed are Vernier arrays, but also these suffer from high peaks in the sidelobe region compared with a dense array. In this work, we propose new methods based on the principles of suppression of grating lobes to form symmetric and non-symmetric regular sparse periodic and radially periodic designs. The proposed methods extend the concept of sparse periodic layouts by exploiting either an increased number of symmetry axes or radial symmetry. We also introduce two new strategies to form designs with nonoverlapping elements. The performance of the new layouts range from the performance of Vernier arrays to almost that of dense arrays. Our designs have simplicity in construction, flexibility in the number of active elements, and the possibility of trade off sidelobe peaks against sidelobe energy.

Two-dimensional ultrasonic strain rate measurement of the human heart in vivo

Abstract: A study is presented in which the feasibility of two-dimensional strain rate estimation of the human heart in vivo has been demonstrated. To do this, ultrasonic B-mode data were captured at a high temporal resolution of 3.8 ms and processed off-line. The motion of the RF signal patterns within the two-dimensional sector image was tracked and used as the basis for strain rate estimation. Both axial and lateral motion and strain rate estimates showed a good agreement with the results obtained by more established, one-dimensional techniques.

Nondestructive subharmonic imaging

Abstract: Ultrasound contrast agent microbubbles are intravascular agents that can be used to estimate blood perfusion. Blood perfusion may be estimated by destroying the bubbles in a vascular bed and observing the refresh of contrast agents back into the vascular bed. Contrast agents can be readily destroyed by traditional imaging techniques. The design of a nondestructive imaging technique is necessary for the accurate quantification of contrast agent refresh. In this work, subharmonic imaging is investigated as a method for nondestructive imaging with the contrast agent microbubble MP1950 (Mallinckrodt, Inc., St. Louis, MO). Optical observation during insonation, in conjunction with a modified Rayleigh-Plesset (R-P) analysis, provides insight into the mechanisms of and parameters required for subharmonic frequency generation. Subharmonic imaging with a transmission frequency that is the same as the resonant frequency of the bubble is shown to require a minimum pressure of insonation that is greater than the experimentally-observed bubble destruction threshold. Subharmonic imaging with a transmission frequency that is twice the resonant frequency of the bubble produces a subharmonic frequency response while minimizing bubble instability. Optimization is performed using optical experimental analysis and R-P analysis.

A piezoelectric motor using two orthogonal bending modes of a hollow cylinder

Abstract: This paper proposes a compact ultrasonic motor with low manufacturing costs, a simpler driving circuit, and scalability. The stator of the motor presented in this paper consists of a hollow metal cylinder, whose outside surface was flattened on two sides at 90 degrees to each other, on which two rectangular piezoelectric plates were bonded. Because the cylinder has a partially square/partially circular outside surface, the stator has two degenerated bending modes that are orthogonal to each other. A wobbling motion is generated on the cylinder when only one piezoelectric plate is excited at a frequency between the two orthogonal bending modes. A rod through a pair of ferrules was used as the rotor of this motor. The prototype motor, whose stator was 2.4 mm in diameter and 10 mm in length, operated at 69.5 kHz, was experimentally characterized, and a maximum torque of 1.8 mNm was obtained.

In vivo droplet vaporization for occlusion therapy and phase aberration correction

Abstract: The objective was to determine whether a transpulmonary droplet emulsion (90%, 30 /spl mu/m) temporarily in vivo. Such bubbles could occlude a targeted capillary bed when used in a large number density. Alternatively, for a very sparse population of droplets, the resulting gas bubbles could serve as point beacons for phase aberration corrections in ultrasonic imaging. Gas bubbles can be made in vivo by acoustic droplet vaporization (ADV) of injected, superheated, dodecafluoropentane droplets. Droplets vaporize in an acoustic field whose peak rarefactional pressure exceeds a well-defined threshold. In this new work, it has been found that intraarterial and intravenous injections can be used to introduce the emulsion into the blood stream for subsequent ADV (Band M-mode on a clinical scanner) in situ. Intravenous administration results in a lower gas bubble yield, possibly because of filtering in the lung, dilution in the blood volume, or other circulatory effects. Results show that for occlusion purposes, a reduction in regional blood flow of 34% can be achieved. Individual point beacons with a +24 dB backscatter amplitude relative to white matter were created by intravenous injection and ADV.

Clutter filters adapted to tissue motion in ultrasound color flow imaging

Abstract: The quality of ultrasound color flow images is highly dependent on sufficient attenuation of the clutter signals originating from stationary and slowly moving tissue. Without sufficient clutter rejection, the detection of low velocity blood flow will be poor, and the velocity estimates will have a large bias. In some situations, e.g., when imaging the coronary arteries or when the operator moves the probe in search for small vessels, there is considerable movement of tissue. It has been suggested that clutter rejection can be improved by mixing down the signal with an estimate of the mean frequency prior to high pass filtering. In this paper, we compare this algorithm with several other adaptive clutter filtering algorithms using both experimental data and simulations. We found that realistic accelerations of the tissue have a large effect on the clutter rejection. The best results were obtained by mixing down the signal with non-constant phase increments estimated from the signal. This adapted the filter to a possibly accelerated tissue motion and produced a significant improvement in clutter rejection.

Design, fabrication, and evaluation of high frequency, single-element transducers incorporating different materials

Abstract: The performance of high frequency, single-element transducers depends greatly on the mechanical and electrical properties of the piezoelectric materials used. This study compares the design and performance of transducers incorporating different materials. The materials investigated include 1-3 lead zirconate titanate (PZT) fiber composite, lead titanate (PbTiO/sub 3/) ceramic, poly(vinylidene fluoride) (PVDF) film, and lithium niobate (LiNbO/sub 3/) single crystal. All transducers were constructed with a 3-mm aperture size and an f-number between 2 and 3. Backing and matching materials were selected based on design goals and fabrication limitations. A simplified coaxial cable tuning method was employed to match the transducer impedance to 50 /spl Omega/ for the PZT fiber composite and PbTiO/sub 3/ ceramic transducers. Transducers were tested for two-way loss and -6 dB bandwidth using the pulse/echo response from a flat quartz target. Two-way loss varied from 21 to 46 dB, and bandwidths measured were in the range from 47 to 118%. In vitro ultrasonic backscatter microscope (UBM) images of an excised human eye were obtained for each device and used to compare imaging performance. Both press-focusing and application of a lens proved to be useful beam focusing methods for high frequency. Under equal gain schemes, the LiNbO/sub 3/ and PbTiO/sub 3/ transducers provided better image contrast than the other materials.

Statistics of envelope of high-frequency ultrasonic backscatter from human skin in vivo

Abstract: The statistics of envelope of high-frequency ultrasonic backscatter signals from in vivo normal human dermis and subcutaneous fat were studied. The capability of six probability distributions (Rayleigh, Rician, K, Nakagami, Weibull, and Generalized Gamma) to model empirical envelope data was studied using the Kolmogorov-Smirnov (KS) goodness of fit statistic. The parameters of all the distributions were obtained using the maximum likelihood method. It was found that the Generalized Gamma distribution with two shape parameters provided the best fit among all the distributions in terms of the KS goodness of fit. The K and Weibull distributions also modeled the envelope statistics well. The Rayleigh and Rician distributions provided poorer fits. The parameters of the Generalized Gamma distribution, however, showed a larger variability than those of the other distributions. The intersubject variability in the estimated parameters of all the distributions was found to be comparable to the intrasubject variability. Fat was seen to exhibit significantly more pre-Rayleigh behavior compared to the dermis. The parameters of the Generalized Gamma distribution also showed significant differences between the dermis at the forearm and fingertip regions.

Dynamics and fragmentation of thick-shelled microbubbles

Abstract: Localized delivery could decrease the systemic side effects of toxic chemotherapy drugs. The unique delivery agents we examine consist of microbubbles with an outer lipid coating, an oil layer, and a perfluorobutane gas core. These structures are 0.5-12 /spl mu/m in radius at rest. Oil layers of these acoustically active lipospheres (AALs) range from 0.3-1.5 /spl mu/m in thickness and thus the agents can carry a large payload compared to nano-scale drug delivery systems. We show that triacetin-based drug-delivery vehicles can be fragmented using ultrasound. Compared with a lipid-shelled contrast agent, the expansion of the drug-delivery vehicle within the first cycle is similar, and a subharmonic component is demonstrated at an equivalent radius, frequency, and driving pressure. For the experimental conditions explored here, the pulse length required for destruction of the drug-delivery vehicle is significantly greater, with at least five cycles required, compared with one cycle for the contrast agent. For the drug-delivery vehicle, the observed destruction mechanism varies with the initial radius, with microbubbles smaller than resonance size undergoing a symmetric collapse and producing a set of small, equal-sized fragments. Between resonance size and twice resonance size, surface waves become visible, and the oscillations become asymmetrical. For agents larger than twice the resonance radius, the destruction mechanism changes to a pinch-off, with one fragment containing a large fraction of the original volume.

An accurate model for capacitive micromachined ultrasonic transducers

Abstract: Modeling of capacitive micromachined ultrasonic transducers (cMUTs) is based on a two-port network with an electrical and a mechanical side. To obtain a distributed model, a solution of the differential equation of motion of the diaphragm for each element of the transducer has to be found. Previous works omit the mechanical load of the cavity behind the diaphragm, i.e., the effect of the gas inside. In this paper, we propose a distributed model for cMUTs that takes this effect into account. A closed-form solution of the mechanical impedance of the membranes has been obtained, including the effect of the restoring forces because of the stiffness of the membrane and because of the compression of the air in the cavity. Simulation results based on the presented model are compared with the experimental data for two types of cMUTs reported in the recent literature. It is demonstrated that the compression of the air has a significant effect on the fundamental frequency of the air transducer, with a deviation of about 22% from the prediction of a model that does not consider the interaction between the vibrating diaphragm and the air cushion.

A new high resolution color flow system using an eigendecomposition-based adaptive filter for clutter rejection

Abstract: We present a new signal processing strategy for high frequency color flow mapping in moving tissue environments. A new application of an eigendecomposition-based clutter rejection filter is presented with modifications to deal with high blood-to-clutter ratios (BCR). Additionally, a new method for correcting blood velocity estimates with an estimated tissue motion profile is detailed. The performance of the clutter filter and velocity estimation strategies is quantified using a new swept-scan signal model. In vivo color flow images are presented to illustrate the potential of the system for mapping blood flow in the microcirculation with external tissue motion.

Angular-profile tuning of guided waves in hollow cylinders using a circumferential phased array

Abstract: Angular-profile tuning of guided waves in hollow cylinders is implemented by using partial loading of the elements in a circumferentially placed phased array. Each partial loading element generates nonaxisymmetric guided waves in a pipe. In earlier work, numerical calculations and experiments have shown that, for nonaxisymmetric guided waves, circumferential distribution of particle displacements (i.e., the angular profile) changes with propagation distance, frequency, and mode. To change the angular profile at a certain distance, either frequency or mode has to be changed for a single partial loading element. This is not the case, however, for a circumferential phased array. The total angular profile of a circumferential array is the superposition of contributions from all elements. If given the knowledge of the angular profile for a single element, the total guided wave angular profile can be controlled and thus focused at any specific circumferential location by a circumferentially placed phased array with adjustable voltage level and phase inputs. This angular profile tuning technique can be used for implementing a circumferential scan with focused, guided wave beams, which leads to the detection of smaller defects as a result of stronger focused beams. Algorithms and specific nondestructive evaluation (NDE) applications for pipe inspection using this technique are discussed.

Optimizing the beam pattern of a forward-viewing ring-annular ultrasound array for intravascular imaging

Abstract: Intravascular ultrasound (IVUS) imaging systems using circumferential arrays mounted on cardiac catheter tips fire beams orthogonal to the principal axis of the catheter. The system produces high resolution cross-sectional images but must be guided by conventional angioscopy. A real-time forward-viewing array, integrated into the same catheter, could greatly reduce radiation exposure by decreasing angiographic guidance. Unfortunately, the mounting requirement of a catheter guide wire prohibits a full-disk imaging aperture. Given only an annulus of array elements, prior theoretical investigations have only considered a circular ring of point transceivers and focusing strategies using all elements in the highly dense array, both impractical assumptions. In this paper, we consider a practical array geometry and signal processing architecture for a forward-viewing IVUS system. Our specific design uses a total of 210 transceiver firings with synthetic reconstruction for a given 3-D image frame. Simulation results demonstrate this design can achieve side-lobes under -40 dB for on-axis situations and under -30 dB for steering to the edge of a 60/spl deg/ cone.

A novel multi-degree-of-freedom thick-film ultrasonic motor

Abstract: This paper describes a new multi-degree-of-freedom (MDOF) ultrasonic motor that comprises few parts and is based on low-cost thick-film technology. Conventional ultrasonic motors using bulk lead zirconate titanate (PZT) or thin-film PZT layers are relatively expensive at the present time. Thick-film printed PZT technology provides the opportunity to reduce the costs of ultrasonic motors. To demonstrate the feasibility of this approach, an ultrasonic motor was fabricated from alumina using thick-film printed PZT actuators. The thick-film PZT and electrode layers were printed on a thin alumina plate, and a tiny cylinder was mounted at its center. This cylinder magnifies the lateral displacement of the stator, holds the spherical rotor, and transmits the driving force to the sphere. Three bending vibrations, B/sub 22/, B/sub 30/, B/sub 03/, of the plate were applied to rotate the sphere. Sufficient displacements for rotating the sphere were obtained near the resonance of B/sub 22/ by applying an excitation voltage of 200 V peak-to-peak via a three-phase drive circuit. Rotations in three orthogonal directions have been observed by controlling the phase of the driving signal to the PZT electrodes, and a MDOF ultrasonic motor was successfully realized.

Development of the line-focus-beam ultrasonic material characterization system

Abstract: A line-focus-beam ultrasonic material characterization (LFB-UMC) system has been developed to evaluate large diameter crystals and wafers currently used in electronic devices. The system enables highly accurate detection of slight changes in the physical and chemical properties in and among specimens. Material characterization proceeds by measuring the propagation characteristics, viz., phase velocity and attenuation, of Rayleigh-type leaky surface acoustic waves (LSAWs) excited on the water-loaded specimen surface. The measurement accuracy depends mainly upon the translation accuracy of the mechanical stages used in the system and the stability of the temperature environment. New precision mechanical translation stages have been developed, and the mechanical system, including the ultrasonic device and the specimen, has been installed in a temperature-controlled chamber to reduce thermal convection and conduction at the specimen. A method for precisely measuring temperature and longitudinal velocity in the water couplant has been developed, and a measurement procedure for precisely measuring the LSAW velocities has been completed, achieving greater relative accuracy to better than /spl plusmn/0.002% at any single chosen point and /spl plusmn/0.004% for two-dimensional measurements over a scanning area of a 200-mm diameter silicon single-crystal substrate. The system was developed to address various problems arising in science and industry associated with the development of materials and device fabrication processes.

Characterization of all the elastic, dielectric, and piezoelectric constants of uniaxially oriented poled PVDF films

Abstract: Polyvinylidene fluoride (PVDF), a piezoelectric material, has many useful applications, for example, as sensors, transducers, and surface acoustic wave (SAW) devices. Models of performance of these devices would be useful engineering tools. However, the benefit of the model is only as accurate as the material properties used in the model. The purpose of this investigation is to measure the elastic, dielectric and piezoelectric properties over a frequency range, including the imaginary part (loss) of these properties. Measurements are difficult because poled material is available as thin films, and not all quantities can be measured in that form. All components of the elastic stiffness, dielectric tensor, and electromechanical coupling tensor are needed in the models. The material studied here is uniaxially oriented poled PVDF that has orthorhombic mm2 symmetry. Presented are the frequency dependence of all nine complex elastic constants, three complex dielectric constants, and five complex piezoelectric constants. The PVDF was produced at Raytheon Research Division, Lexington, MA. Measurements were made on thin films and on stacked, cubical samples. The elastic constants c/sub 44//sup D/ and c/sub 55//sup D/, the dielectric constants e/sub 11//sup T/ and e/sub 22//sup T/, as well as the piezoelectric constants g/sub 15/ and g/sub 24/ reported here have not been published before. The values were determined by ultrasonic measurements using an impedance analyzer and a least square data-fitting technique.

Low-cost, high-performance pulse generator for ultrasound imaging

Abstract: The design of a low-cost pulse generator for ultrasound imaging is presented. The pulser can provide 10 kW pulses with less than 0.7 Vpp ringing. Pulses ranging from 10 to 500 ns have been successfully generated.

Exact analysis of dispersive SAW devices on ZnO/diamond/Si-layered structures

Abstract: In this paper, a formulation for calculating the effective permittivity of a piezoelectric layered SAW structure is given, and the exact frequency response of ZnO/diamond/Si-layered SAW is calculated. The effective permittivity and phase velocity dispersion of a ZnO/diamond/Si-layered half space are calculated and discussed. The frequency response of an unapodized SAW transducer is calculated, and the center frequency shift caused by the velocity dispersion is explained. In addition, the electromechanical coupling coefficients of the ZnO/diamond/Si-layered half space based on two different formulas are calculated and discussed. Finally, based on the results of the study, we propose an exact analysis for modeling the layered SAW device. The advantage of using the effective permittivity method is that, not only the null frequency bandwidth, but also the center frequency shift and insertion loss can be evaluated.

Improvements in the ultrasonic contrast of targeted perfluorocarbon nanoparticles using an acoustic transmission line model

Abstract: Targeted acoustic contrast agents offer the potential for sensitive ultrasonic detection of pathologic tissues. We have previously reported the development of a ligand-targeted, lipid-encapsulated, liquid perfluorodichlorooctane ultrasonic contrast system with a small nominal particle size (approximately 250-nm diameter)Perfluorocarbon nanoparticles substantially increase reflectivity when bound to targeted surfaces, and we propose that this system can be approximated physically as a simple, thin layer, acoustic transmission line. In this study, we evaluate this model and compare the ultrasonic reflectivity of different perfluorocarbon formulations with widely varying acoustic impedances targeted to either nitrocellulose membranes or plasma thrombi in vitro. Five perfluorocarbons were investigated: perfluorohexane (PFH), perfluorooctane (PFO), perfluorooctyl bromide (PFOB), perfluorodichlorooctane (PFDCO), and perfluorodecalin (PFD). Ultrasonic reflection was measured by acoustic microscopy (17 to 35 MHz). Acoustic reflectivity was increased (P < 0.05) by all targeted perfluorocarbon formulations, and the magnitude of the contrast effect was inversely correlated with the perfluorocarbon acoustic impedance. PFH nanoparticles exhibited the greatest enhancement, and PFD nanoparticles showed the least. The acoustic transmission line model predicted well the relative differences in acoustic reflectivity and frequency dependence among the perfluorocarbon formulations. For future clinical applications, PFO nanoparticles may provide the best combination of acoustic enhancement, in vivo physical stability, and safety.

Thin film bulk acoustic wave filter

Abstract: Thin film bulk acoustic wave (BAW) resonators (FBAR) are fabricated on a silicon nitride bridge using a ZnO piezolayer on a glass substrate and surface micromachining by standard thin film technology. These resonators exhibit a coupling constant k/sub t//sup 2/=7.8% at the first thickness extensional wave mode and are used as impedance elements in a ladder filter in the 1-GHz frequency band of mobile telecommunications. An electrical equivalent circuit is used to characterize the properties of the resonators and to show how the performance of the filter depends on the parameters of the resonators. 2.5% bandwidth, 2.8-dB insertion loss, and 35-dB selectivity are obtained in a filter with six resonators. The technology can be used to manufacture miniature microwave filters without any additional inductances.

Ultrasonic nondestructive testing of scattering media using the decomposition of the time-reversal operator

Abstract: In ultrasonic nondestructive testing, the iterative time-reversal process is an adaptive technique that can be used to detect flaws in complex samples with a large array of transducers. The decomposition of the time-reversal operator (DORT) method is a detection technique that is derived from the mathematical analysis of the iterative time-reversal process. Contrary to time-reversal techniques, the DORT method does not require programmable generators, and it allows the simultaneous detection and separation of several defects. In this paper, the method is applied to a Ti6-4 titanium cylindrical sample to separate the echo of a defect from the speckle due to microstructure contribution. The grain structure of this titanium alloy makes detection very difficult and, for large depths, conventional techniques do not allow the detection of small flaws with a satisfactory signal-to-noise ratio. The efficiency of the DORT method to detect a flat bottom hole with a diameter of 0.4 mm located at a depth of 140 mm in a titanium alloy sample is shown.

Piezoelectrically actuated flextensional micromachined ultrasound transducers. I. Theory

Abstract: This series of two papers considers piezoelectrically actuated flextensional micromachined ultrasound transducers (PAFMUTs) and consists of theory, fabrication, and experimental parts. The theory presented in this paper is developed for an ultrasound transducer application presented in the second part. In the absence of analytical expressions for the equivalent circuit parameters of a flextensional transducer, it is difficult to calculate its optimal parameters and dimensions and difficult to choose suitable materials. The influence of coupling between flexural and extensional deformation and that of coupling between the structure and the acoustic volume on the dynamic response of piezoelectrically actuated flextensional transducer are analyzed using two analytical methods: classical thin (Kirchhoff) plate theory and Mindlin plate theory. Classical thin plate theory and Mindlin plate theory are applied to derive two-dimensional plate equations for the transducer and to calculate the coupled electromechanical field variables such as mechanical displacement and electrical input impedance. In these methods, the variations across the thickness direction vanish by using the bending moments per unit length or stress resultants. Thus, two-dimensional plate equations for a step-wise laminated circular plate are obtained as well as two different solutions to the corresponding systems. An equivalent circuit of the transducer is also obtained from these solutions.

Finite element analysis of underwater capacitor micromachined ultrasonic transducers

Abstract: A simple electromechanical equivalent circuit model is used to predict the behavior of capacitive micromachined ultrasonic transducers (cMUT). The equivalent circuit model of the cMUT lacks important features such as coupling to the substrate and the ability to predict crosstalk between elements of an array of transducers. To overcome these deficiencies, a finite element model of the cMUT is constructed using the commercial code ANSYS(R). Calculation results of the complex load impedance seen by single capacitor cells are presented, then followed by a calculation of the plane wave real load impedance seen by a parallel combination of many cells that are used to make a transducer. Crosstalk between 1-D array elements is found to be due to two main sources: coupling through a Stoneley wave propagating at the transducer-water interface and coupling through Lamb waves propagating in the substrate. To reduce the crosstalk level, the effect of structural variations of the substrate are investigated, which includes a change of its thickness and etched trenches or polymer walls between array elements.

1.5-D high intensity focused ultrasound array for non-invasive prostate cancer surgery

Abstract: The aim of this study is to demonstrate the feasibility of a new spherically curved 1.5-D phased array for the treatment of localized prostatic cancer. The device is designed to conform to the Ablatherm(R) machine (EDAP-Technomed. France), a commercially available machine in which high intensity focused ultrasound (HIFU) treatment for prostate cancer is administered transrectally. It uses high intensity electronically focused ultrasound to steer a beam along two axes, allowing enough depth to be reached to treat large prostates and eliminating two degrees of mechanical movement. Through computer simulation, it was determined that a curved 1.5-D configuration offered the optimal design. Two configurations were then proposed, and their ability to steer a beam within a target volume centered on the geometric focus of the transducer was simulated. An eight-element prototype was constructed to test the piezo-composite material and its electro-acoustical efficiency. Then, an array was constructed, and a multichannel amplifier and control system were added, to permit remote operation. Acoustical and electrical measurements were made to verify performance. Finally, the 1.5-D array was tested in vitro on samples of pig liver to confirm the ability to induce lesions.