Showing papers in "IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control in 2003"

Adaptive imaging using the generalized coherence factor

Abstract: Sound-velocity inhomogeneities degrade both spatial and contrast resolutions. This paper proposes a new adaptive imaging technique that uses the generalized coherence factor (GCF) to reduce the focusing errors resulting from the sound-velocity inhomogeneities. The GCF is derived from the spatial spectrum of the received aperture data after proper receive delays have been applied. It is defined as the ratio of the spectral energy within a prespecified low-frequency range to the total energy. It is demonstrated that the low-frequency component of the spectrum corresponds to the coherent portion of the received data, and that the high-frequency component corresponds to the incoherent portion. Hence, the GCF reduces to the coherence factor defined in the literature if the prespecified low-frequency range is restricted to DC only. In addition, the GCF is also an index of the focusing quality and can be used as a weighting factor for the reconstructed image. The efficacy of the GCF technique is demonstrated for focusing errors resulting from the sound-velocity inhomogeneities. Simulations and real ultrasound data are used to evaluate the efficacy of the proposed GCF technique. The characteristics of the GCF, including the effects of the signal-to-noise ratio and the number of channels, are also discussed. The GCF technique also is compared with the correlation-based technique and the parallel adaptive receive compensation algorithm; the improvement in image quality obtained with the proposed technique rivals that of the latter technique. In the presence of a displaced phase screen, this proposed technique also outperforms the correlation-based technique. Computational complexity and implementation issues also are addressed.

A rapid signal processing technique to remove the effect of dispersion from guided wave signals

Abstract: Guided acoustic and ultrasonic waves have been utilized in various manners for non-destructive evaluation and testing. If a guided wave mode is dispersive, a pulse of energy will spread out in space and time as it propagates. For a long-range guided wave inspection application, this constrains the choice of operating point to regions on the dispersion curves where dispersion effects are small. A signal processing technique is presented that enables this constraint on operating point to be relaxed. The technique makes use of a priori knowledge of the dispersion characteristics of a guided wave mode to map signals from the time to distance domains. In the mapping process, dispersed signals are compressed to their original shape. The theoretical basis of the technique is described and an efficient numerical implementation is presented. The robustness of the technique to inaccuracies in the dispersion data is also addressed. The application of the technique to experimental data is shown and the resulting improvement in spatial resolution is demonstrated. The implications of using dispersion compensation in practical systems are briefly discussed.

Design of efficient, broadband single-element (20-80 MHz) ultrasonic transducers for medical imaging applications

Abstract: This paper discusses the design, fabrication, and testing of sensitive broadband lithium niobate (LiNbO/sub 3/) single-element ultrasonic transducers in the 20-80 MHz frequency range. Transducers of varying dimensions were built for an f# range of 2.0-3.1. The desired focal depths were achieved by either casting an acoustic lens on the transducer face or press-focusing the piezoelectric into a spherical curvature. For designs that required electrical impedance matching, a low impedance transmission line coaxial cable was used. All transducers were tested in a pulse-echo arrangement, whereby the center frequency, bandwidth, insertion loss, and focal depth were measured. Several transducers were fabricated with center frequencies in the 20-80 MHz range with the measured -6 dB bandwidths and two-way insertion loss values ranging from 57 to 74% and 9.6 to 21.3 dB, respectively. Both transducer focusing techniques proved successful in producing highly sensitive, high-frequency, single-element, ultrasonic-imaging transducers. In vivo and in vitro ultrasonic backscatter microscope (UBM) images of human eyes were obtained with the 50 MHz transducers. The high sensitivity of these devices could possibly allow for an increase in depth of penetration, higher image signal-to-noise ratio (SNR), and improved image contrast at high frequencies when compared to previously reported results.

Longitudinal and transverse magnetoelectric voltage coefficients of magnetostrictive/ piezoelectric laminate composite: experiments

Abstract: This paper presents a novel, long-type of magnetostrictive and piezoelectric laminate composite design in which the layers are, respectively, magnetized/poled along their length axes, and a theory for modeling its behavior. Using piezoelectric and magnetostrictive constitutive equations, and an equation of motion, a magneto-elasto-electric bieffect equivalent circuit is developed. The circuit is used to predict the longitudinal and transverse magnetoelectric (ME) voltage coefficients of our Terfenol-D/Pb(Zr/sub 1-x/Ti/sub x/)O/sub 3/ laminate design. It is found that the longitudinal ME voltage coefficient is significantly higher (/spl sim/5x) than the transverse one, and that our new laminate design has significantly higher ME voltage coefficients under small applied direct current (DC) magnetic bias fields than designs reported previously by other groups. Experimental values were found to be coincidental with predicted ones.

Omni-directional guided wave transducer arrays for the rapid inspection of large areas of plate structures

Abstract: Omni-directional guided wave array transducers contain a circular pattern of elements that individually behave as omni-directional point transmitters or receivers. The data set acquired from such an array contains time-domain signals from each permutation of transmitter and receiver. A phased addition algorithm is developed that allows an omni-directional, B-scan image of the surrounding plate to be synthesized from any geometry of array. Numerically simulated data from a single reflector is used to test the performance of the algorithm. The results from an array containing a fully populated circular area of elements (Type I array) are found to be good, but those from an array containing a single ring of elements (Type II array) contain many large side-lobes. An enhancement to the basic phased addition algorithm is presented that uses deconvolution to suppress these side-lobes. The deconvolution algorithm enables a Type II array to equal the performance of a Type I array of the same overall diameter. The effect of diameter on angular resolution is investigated. Experimental data obtained from a guided wave array containing electromagnetic acoustic transducers (EMAT) elements for exciting and detecting the S/sub 0/ Lamb wave mode in a 5-mm thick aluminium plate are processed with both algorithms and the results are discussed.

A comparison of the performance of time-delay estimators in medical ultrasound

Abstract: Time-delay estimation (TDE) is a common operation in ultrasound signal processing. In applications such as blood flow estimation, elastography, phase aberration correction, and many more, the quality of final results is heavily dependent upon the performance of the time-delay estimator implemented. In the past years, several algorithms have been developed and applied in medical ultrasound, sonar, radar, and other fields. In this paper we analyze the performances of the widely used normalized and non-normalized correlations, along with normalized covariance, sum absolute differences (SAD), sum squared differences (SSD), hybrid-sign correlation, polarity-coincidence correlation, and the Meyr-Spies method. These techniques have been applied to simulated ultrasound radio frequency (RF) data under a variety of conditions. We show how parameters, which include center frequency, fractional bandwidth, kernel window size, signal decorrelation, and signal-to-noise ratio (SNR) affect the quality of the delay estimate. Simulation results also are compared with a theoretical performance limit set by the Cramer-Rao lower bound (CRLB). Results show that, for high SNR, high signal correlation, and large kernel size, all of the algorithms closely match the theoretical bound, with relative performances that vary by as much as 20%. As conditions degrade, the performances of various algorithms differ more significantly. For signals with a correlation level of 0.98, SNR of 30 dB, center frequency of 5 MHz with a fractional bandwidth of 0.5, and kernel size of 2 /spl mu/s, the standard deviation of the jitter error is on the order of few nanoseconds. Normalized correlation, normalized covariance, and SSD have an approximately equal jitter error of 2.23 ns (the value predicted by the CRLB is 2.073 ns), whereas the polarity-coincidence correlation performs less well with a jitter error of 2.74 ns.

Calculation and measurement of electromechanical coupling coefficient of capacitive micromachined ultrasonic transducers

Abstract: The electromechanical coupling coefficient is an important figure of merit of ultrasonic transducers. The transducer bandwidth is determined by the electromechanical coupling efficiency. The coupling coefficient is, by definition, the ratio of delivered mechanical energy to the stored total energy in the transducer. In this paper, we present the calculation and measurement of coupling coefficient for capacitive micromachined ultrasonic transducers (CMUTs). The finite element method (FEM) is used for our calculations, and the FEM results are compared with the analytical results obtained with parallel plate approximation. The effect of series and parallel capacitances in the CMUT also is investigated. The FEM calculations of the CMUT indicate that the electromechanical coupling coefficient is independent of any series capacitance that may exist in the structure. The series capacitance, however, alters the collapse voltage of the membrane. The parallel parasitic capacitance that may exist in a CMUT or is external to the transducer reduces the coupling coefficient at a given bias voltage. At the collapse, regardless of the parasitics, the coupling coefficient reaches unity. Our experimental measurements confirm a coupling coefficient of 0.85 before collapse, and measurements are in agreement with theory.

Volumetric ultrasound imaging using 2-D CMUT arrays

Abstract: Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as a candidate to overcome the difficulties in the realization of 2-D arrays for real-time 3-D imaging. In this paper, we present the first volumetric images obtained using a 2-D CMUT array. We have fabricated a 128/spl times/128-element 2-D CMUT array with through-wafer via interconnects and a 420-/spl mu/m element pitch. As an experimental prototype, a 32/spl times/64-element portion of the 128/spl times/128-element array was diced and flip-chip bonded onto a glass fanout chip. This chip provides individual leads from a central 16/spl times/16-element portion of the array to surrounding bondpads. An 8/spl times/16-element portion of the array was used in the experiments along with a 128-channel data acquisition system. For imaging phantoms, we used a 2.37-mm diameter steel sphere located 10 mm from the array center and two 12-mm-thick Plexiglas plates located 20 mm and 60 mm from the array. A 4/spl times/4 group of elements in the middle of the 8/spl times/16-element array was used in transmit, and the remaining elements were used to receive the echo signals. The echo signal obtained from the spherical target presented a frequency spectrum centered at 4.37 MHz with a 100% fractional bandwidth, whereas the frequency spectrum for the echo signal from the parallel plate phantom was centered at 3.44 MHz with a 91% fractional bandwidth. The images were reconstructed by using RF beamforming and synthetic phased array approaches and visualized by surface rendering and multiplanar slicing techniques. The image of the spherical target has been used to approximate the point spread function of the system and is compared with theoretical expectations. This study experimentally demonstrates that 2-D CMUT arrays can be fabricated with high yield using silicon IC-fabrication processes, individual electrical connections can be provided using through-wafer vias, and flip-chip bonding can be used to integrate these dense 2-D arrays with electronic circuits for practical 3-D imaging applications.

Characterization of PVA cryogel for intravascular ultrasound elasticity imaging

Abstract: The present study characterizes the mechanical properties of polyvinyl alcohol (PVA) cryogel in order to show its utility for intravascular elastography. PVA cryogel becomes harder with an increasing number of freeze-thaw cycles, and Young's modulus and Poisson's ratio are measured for seven samples. Mechanical tests were performed on cylindrical samples with a pressure column and on a hollow cylinder with the calculation of an intravascular elastogram. An image of the Young's modulus was obtained from the elastogram using cylinder geometry properties. Results show the mechanical similitude of PVA cryogel with the biological tissues present in arteries. A good agreement between Young's modulus obtained from pressure column and from elastogram was also observed.

Microbubble-enhanced cavitation for noninvasive ultrasound surgery

Abstract: Experiments were conducted to explore the potential of stabilized microbubbles for aiding tissue ablation during ultrasound therapy. Surgically exteriorized canine kidneys were irradiated in situ using single exposures of focused ultrasound. In each experiment, tip to eight separate exposures were placed in the left kidney. The right kidney was then similarly exposed, but while an ultrasound contrast agent was continually infused. Kidneys were sectioned and examined for gross observable tissue damage. Tissue damage was produced more frequently, by lower intensity and shorter duration exposures, in kidneys irradiated with the contrast agent present. Using 250-ms exposures, the minimum intensity that produced damage was lower in kidneys with microbubbles than those without (controls) in 10 of 11 (91%) animals. In a separate study using /spl sim/3200 W/cm/sup 2/ exposures, the minimum duration that produced damage was shorter after microbubbles were introduced in 11 of 12 (92%) animals. With microbubbles, gross observable tissue damage was produced with exposure intensity /spl ges//spl sim/800 W/cm/sup 2/ and exposure duration /spl ges/10 /spl mu/s. The overall intensity and duration tissue damage thresholds were reduced by /spl sim/2/spl times/ and /spl sim/100/spl times/, respectively. Results indicate that acoustic cavitation is a primary damage mechanism. Lowering in vivo tissue damage thresholds with stabilized microbubbles acting as cavitation nuclei may make acoustic cavitation a more predictable, and thus practical, mechanism for noninvasive ultrasound surgery.

Effects of acoustic heterogeneity in breast thermoacoustic tomography

Abstract: The effects of wavefront distortions induced by acoustic heterogeneities in breast thermoacoustic tomography (TAT) are studied. Amplitude distortions are shown to be insignificant for different scales of acoustic heterogeneities. For wavelength-scale, or smaller, heterogeneities, amplitude distortion of the wavefront is minor as a result of diffraction when the detectors are placed in the far field of the heterogeneities. For larger-scale heterogeneities at the parenchyma wall, by using a ray approach (geometric optics), we show that no refraction-induced multipath interference occurs and, consequently, that no severe amplitude distortion, such as is found in ultrasound tomography, exists. Next, we consider the effects of phase distortions (errors in time-of-flight) in our numerical studies. The numerical results on the spreads of point sources and boundaries caused by the phase distortions are in good agreement with the proposed formula. After that, we demonstrate that the blurring of images can be compensated for by using the distribution of acoustic velocity in the tissues in the reconstructions. The effects of the errors in the acoustical velocities on this compensation also are investigated. An approach to implement the compensation using only TAT data is proposed. Lastly, the differences in the effects of acoustic heterogeneity and the generation of speckles in breast TAT and breast ultrasound imaging are discussed.

Transmitting electric energy through a metal wall by acoustic waves using piezoelectric transducers

Abstract: The feasibility of transmitting electric energy through a metal wall by propagating acoustic waves using piezoelectric transducers is examined by studying the efficiency of power transmission and its dependence upon the relevant system parameters for a simplified system consisting of an elastic plate sandwiched by two piezoelectric layers. One of these layers models the driving transducer for generating acoustic wave, and the other layer models the receiving transducer for converting the acoustic energy into electric energy to power a load circuit. The output voltage, the output power, and the efficiency of this system are expressed as explicit functions of the system parameters. A numerical example is included to illustrate the dependence of the system performance upon the physical and geometrical parameters.

Modeling and computer simulation of ultrasonic/sonic driller/corer (USDC)

Abstract: Simulation and analytical models for the ultrasonic/sonic drill/corer (USDC) are described in this paper. The USDC was developed as a tool for in-situ rock sampling and analysis in support of the NASA planetary exploration program. The USDC uses a novel drive mechanism, which transfers ultrasonic vibrations of a piezoelectric actuator into larger oscillations of a free-flying mass (free-mass). The free-mass impact on the drill bit creates a stress pulse at the drill tip/rock interface causing fracture in the rock. The main parts of the device (transducer, free-mass, bit, and rock) and the interactions between them were analyzed and numerically modeled to explore the drive mechanism. Each of these interactions is normally described by a time-dependent 2- or 3-D model involving slowly converging solutions, which makes the conventional approach unsuitable for USDC optimization studies. A simplified integrated model using tabulated data was developed to simulate the operation of the USDC on desktop PC and successfully predicted the characteristics of the device under a variety of conditions. The simulated results of the model and the experimental data used to verify the model are presented.

In-vivo synthetic aperture flow imaging in medical ultrasound

Abstract: A new method for acquiring flow images using synthetic aperture techniques in medical ultrasound is presented. The new approach makes it possible to have a continuous acquisition of flow data throughout the whole image simultaneously, and this can significantly improve blood velocity estimation. Any type of filter can be used for discrimination between tissue and blood flow without initialization, and the number of lines used for velocity estimation is limited only by the nonstationarity of the flow. The new approach is investigated through both simulations and measurements. A flow rig is used for generating a parabolic laminar flow, and a research scanner is used for acquiring RF data from individual transducer elements. A reference profile is calculated from a mass flow meter. The parabolic velocity profile is estimated using the new approach with a relative standard deviation of 2.2% and a mean relative bias of 3.4% using 24 pulse emissions at a flow angle of 45 degrees. The 24 emissions can be used for making a full-color flow map image. An in-vivo image of flow in the carotid artery for a 29-year-old male also is presented. The full image is acquired using 24 emissions.

Piezoelectric ultrasonic micromotor with 1.5 mm diameter

Abstract: A piezoelectric ultrasonic micromotor has been developed using a lead zirconate titanate (PZT) ceramic/metal composite tube stator that was 1.5 mm in diameter and 7 mm in length. The micromotor was operated in its first bending vibration mode (/spl sim/70 kHz), producing speeds from hundreds to over 2000 rpm in both rotational directions. The maximum torque-output was 45 /spl mu/N-m, which is far superior to previous PZT thin film-based micromotors. This micromotor showed good reliability and stability for more than 300 hours of continued operation.

Measurement of longitudinal piezoelectric coefficient of thin films by a laser-scanning vibrometer

Abstract: A laser scanning vibrometer (LSV) was used for the first time to measure the piezoelectric coefficient of ferroelectric thin films based on the converse piezoelectric effect. The significant advantages of the use of the LSV or this purpose were demonstrated. Several key points were discussed in order to achieve reliable and accurate results.

Detection of ultrawide-band ultrasound pulses in optoacoustic tomography

Abstract: A laser optoacoustic imaging system (LOIS) uses time-resolved detection of laser-induced pressure profiles in tissue in order to reconstruct images of the tissue based on distribution of acoustic sources. Laser illumination with short pulses generates distribution of acoustic sources that accurately replicates the distribution of absorbed optical energy. The complex spatial profile of heterogeneous distribution of acoustic sources can be represented in the frequency domain by a wide spectrum of ultrasound ranging from tens of kilohertz to tens of megahertz. Therefore, LOIS requires a unique acoustic detector operating simultaneously within a wide range of ultrasonic frequencies. Physical principles of an array of ultrawide-band ultrasonic transducers used in LOIS designed for imaging tumors in the depth of tissue are described. The performance characteristics of the transducer array were modeled and compared with experiments performed in gel phantoms resembling optical and acoustic properties of human tissue with small tumors. The amplitude and the spectrum of laser-induced ultrasound pulses were measured in order to determine the transducer sensitivity and the level of thermal noises within the entire ultrasonic band of detection. Spatial resolution of optoacoustic images obtained with an array of piezoelectric transducers and its transient directivity pattern within the field of view are described. The detector design considerations essential for obtaining high-quality optoacoustic images are presented.

Efficient charge recovery method for driving piezoelectric actuators with quasi-square waves

Abstract: In this paper, an efficient charge recovery method for driving piezoelectric actuators with low frequency square waves in low-power applications such as mobile microrobots is investigated. Efficiency issues related to periodic mechanical work of the actuators and the relationship among the driving electronics efficiency, the piezoelectric coupling factor, and the actuator energy transmission coefficient are discussed. The proposed charge recovery method exploiting the energy transfer between an inductor and a general capacitive load is compared with existing techniques that lead to inherent inefficiencies. A charge recovery method is then applied to piezoelectric actuators, especially to bimorph ones. Unitary efficiency can be obtained theoretically for purely capacitive loads while intrinsic losses such as hysteresis necessarily lower the efficiency. In order to show the validity of the method, a prototype driving electronics consisting of an extended H-bridge is constructed and tested by experiments and simulations. Preliminary results show that 75% of charge (i.e., more than 56% of energy) can be recovered for bending actuators such as bimorphs without any component optimization at low fields.

Single crystal PMN-0.33PT/epoxy 1-3 composites for ultrasonic transducer applications

Abstract: Lead magnesium niobate-lead titanate 0.67Pb (Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-0.33PbTiO/sub 3/ (PMN-0.33PT, abbreviated as PMN-PT) single crystals were used to fabricate PMN-PT/epoxy 1-3 composites with different volume fractions of PMN-PT ranging from 0.4 to 0.8. The electromechanical properties of the 1-3 composites were determined by the resonance technique. Theoretical modeling of the 1-3 composites matched quite well with the measured material properties. It was demonstrated that the thickness electromechanical coupling coefficients of the composites could reach as high as 0.8. A 2.4 MHz plane ultrasonic transducer was fabricated using a PMN-PT/epoxy 1-3 composite with 0.37 volume fraction of PMN-PT. It shows a -6 dB bandwidth of /spl sim/61% and an insertion loss of -14 dB.

A new regime for operating capacitive micromachined ultrasonic transducers

Abstract: We report on a new operation regime for capacitive micromachined ultrasonic transducers (cMUTs). Traditionally, cMUTs are operated at a bias voltage lower than the collapse voltage of their membranes. In the new proposed operation regime, first the cMUT is biased past the collapse voltage. Second, the bias voltage applied to the collapsed membrane is reduced without releasing the membrane. Third, the cMUT is excited with an ac signal at the bias point, keeping the total applied voltage between the collapse and snapback voltages. In this operation regime, the center of the membrane is always in contact with the substrate. Our finite element methods (FEM) calculations reveal that a cMUT operating in this new regime, between collapse and snapback voltages, possesses a coupling efficiency (k/sub T//sup 2/) higher than a cMUT operating in the conventional regime below its collapse voltage. This paper compares the simulation results of the coupling efficiencies of cMUTs operating in conventional and new operation regimes.

Porous PZT ceramics for receiving transducers

Abstract: PZT-air (porous PZT) and PZT-polymer (polymer impregnated porous PZT) piezocomposites with varying porosity/polymer volume fractions have been manufactured. The composites were characterized in terms of hydrostatic charge (d/sub h/) and voltage (g/sub h/) coefficients, permittivity, hydrostatic figure of merit (d/sub h/.g/sub h/), and absolute sensitivity (M). With decreasing PZT ceramic volume, g/sub h/ increased, and d/sub h/.g/sub h/ had a broad maximum around 80 to 90% porosity/polymer content. The absolute sensitivity was also increased. In each case, PZT-air piezocomposites performed better than PZT-polymer piezocomposites. Hydrophones constructed from piezocomposites showed slightly lower measured receiving sensitivities than calculated values for piezocomposite materials, which was due to the loading effect of the cable and the low permittivity associated with the piezocomposites.

A 1.6-mm, metal tube ultrasonic motor

Abstract: A miniaturized metal tube ultrasonic motor, the dimensions of which are 1.6 mm in diameter and 6 mm in length, was developed. Two flattened surfaces with 90 degrees were ground on the outer surface of the stator. Two PZT-based piezoelectric ceramics were bonded onto these flat surfaces. The asymmetrical surface of the stator developed the split of the two degenerated orthogonal bending modes, resulting in a wobble motion. The working frequency of the 1.6-mm motor with 6 mm in length was 130 kHz. A torque of 0.5 mNm was reached at a maximum power of 45 mW with a speed of 45 rad/sec. The maximum efficiency was 16%.

Native tissue imaging at superharmonic frequencies

Abstract: The second harmonic imaging mode has been adapted to image tissue and shown considerable improvements in image quality in several applications compared to the fundamental mode. The improvements were attributed to the effects of wave distortion due to nonlinear propagation in tissue. However, imaging tissue at the second harmonic frequency only has various drawbacks. Because the energy in the second harmonic frequency band is much lower than that in the fundamental frequency band, there must be excellent sensitivity and dynamic range in the receiving system to achieve an acceptable amount of signal-to-noise ratio. To increase the sensitivity, the spectral overlap between the fundamental and the second harmonic has to be diminished, which in return deteriorates the imaging resolution. Consequently, a trade-off is mandatory between resolution and sensitivity. Using simulations and measurements, we show that, at appropriate scanning acoustic settings, higher harmonics are generated in tissue. The higher harmonics represent additional, relevant information for tissue imaging and characterization. An elegant way to take advantage of the higher harmonics and to bring all the information together is to combine and incorporate all the multiple higher harmonics into a single component that we call the superharmonic component. Using a newly developed array transducer having a wide frequency band, B-mode images of a phantom were made in the superharmonic mode transmitting at 1.2 MHz. These images have exceptionally improved clarity and yield a dramatically cleaner and sharper contrast between the different structures being imaged. In addition to increased signal-to-noise ratio, superharmonic imaging shows better contrast and axial resolution as well as acceptable penetration depth.

Directional velocity estimation using focusing along the flow direction. I: theory and simulation

Abstract: A new method for directional velocity estimation is presented. The method uses beam formation along the flow direction to generate data in which the correct velocity magnitude can be directly estimated from the shift in position of the received consecutive signals. The shift is found by cross-correlating the beamformed lines. The approach can find the velocity in any direction, including transverse to the traditionally emitted ultrasound beam. The velocity estimation is studied through extensive simulations using Field II. A 128-element, 7-MHz linear array is used. A parabolic velocity profile with a peak velocity of 0.5 m/s is simulated for different beam-to-flow angles and for different emit foci. At 45/spl deg/ the relative standard deviation over the profile is 1.6% for a transmit focus at 40 mm. At 90/spl deg/ the approach gave a relative standard deviation of 6.6% with a transmit focus of 80 mm, when using 8 pulse-echo lines and stationary echo canceling. Pulsatile flow in the femoral artery was also simulated using Womersley's flow model. A purely transverse flow profile could be obtained with a relative standard deviation of less than 10% over the whole cardiac cycle using 8 pulse emissions for each imaging direction, which is sufficient to show clinically relevant transverse color flow images.

Radiation force imaging of viscoelastic properties with reduced artifacts

Abstract: It is well-known that changes in the mechanical properties of tissues are correlated with the presence of disease. In the eye, for example, the vitreous body undergoes dramatic changes in mechanical properties during age-related degradation. These changes may play a significant role in the formation of retinal detachment or other vitreoretinal diseases. We previously presented a noninvasive method called kinetic acoustic vitreoretinal examination (KAVE), which may be used to detect these mechanical changes. KAVE uses acoustic radiation force as a means to produce small, localized displacements within the tissues. Returning echoes are processed using ultrasonic motion tracking so that the response of the tissue to the induced force can be evaluated. By repeating this process at a number of locations, images depicting viscoelastic properties of tissues can be formed. Through the combination of appropriate mechanical modeling and signal processing, we are able to generate images of parameters such as relative mass, relative elasticity, and relative viscosity. These parameters are called relative because they depend on the force applied, which is typically unknown. In this paper, we present new force-free images depicting the time constant /spl tau/, the damping ratio /spl xi/, and the natural frequency /spl omega/ of the phantom material. These images are significant in that they lack the artifacts common in the relative property images.

Estimates of echo correlation and measurement bias in acoustic radiation force impulse imaging

Abstract: Acoustic radiation force impulse (ARFI) imaging is a novel imaging modality in which pulses from a diagnostic ultrasound scanner are used to displace tissue and track its motion. The region displaced has lateral and elevational dimensions of similar scale to the ultrasound beams used to track the motion. Therefore, there is a range of tissue displacements present within the tracking beam, leading to decorrelation of the echo signal. Expressions are derived for the expected value of the displacement estimate and the cross-correlation at the expected displacement. Numerical simulations confirm the analytical model.

Elastic, anelastic, and piezoelectric coefficients of langasite: resonance ultrasound spectroscopy with laser-Doppler interferometry

Abstract: This paper presents advanced techniques to determine all independent elastic-stiffness coefficients C/sub ij/, the associated internal friction Q/sub ij//sup -1/, and piezoelectric coefficients e/sub ij/ of monocrystal langasite (La/sub 3/Ga/sub 5/SiO/sub 14/) using a single rectangular parallelepiped specimen. Langasite's crystal structure belongs to the trigonal system with point group 32, and thus possesses six independent C/sub ij/, two e/sub ij/, and two dielectric coefficients /spl epsiv//sub ij/. All of the elastic and piezoelectric coefficients affect the mechanical resonance frequencies of the solid specimen, and measuring them very accurately permits one to determine the C/sub ij/ and e/sub ij/ with known density, dimensions, and e/sub ij/. We developed a piezoelectric tripod to support the specimen upward and measured the free-vibration resonance frequencies with minimum load from its own weight. This weak and stable acoustic coupling ensures accuracy of the frequency measurement better than 10/sup -5/, enough to determine the coefficients reliably. Our C/sub ij/ fall in the range of results measured with previous (conventional) methods. Our e/sub 11/ is smaller than the reported values by 1.2-13%, and e/sub 14/ is larger by 44-97%. For the internal friction measurement, we used a solenoid coil to vibrate the specimen without any contact. The longitudinal-wave internal friction considerably exceeds the shear-wave internal friction, which can be explained by phonon-phonon interactions.

Linear system models for ultrasonic imaging: application to signal statistics

Abstract: Linear equations for modeling echo signals from shift-variant systems forming ultrasonic B-mode, Doppler, and strain images are analyzed and extended. The approach is based on a solution to the homogeneous wave equation for random inhomogeneous media. When the system is shift-variant, the spatial sensitivity function-defined as a spatial weighting function that determines the scattering volume for a fixed point of time-has advantages over the point-spread function traditionally used to analyze ultrasound systems. Spatial sensitivity functions are necessary for determining statistical moments in the context of rigorous image quality assessment, and they are time-reversed copies of point-spread functions for shift variant systems. A criterion is proposed to assess the validity of a local shift-invariance assumption. The analysis reveals realistic situations in which in-phase signals are correlated to the corresponding quadrature signals, which has strong implications for assessing lesion detectability. Also revealed is an opportunity to enhance near- and far-field spatial resolution by matched filtering unfocused beams. The analysis connects several well-known approaches to modeling ultrasonic echo signals.

Developing an IGS time scale

Abstract: Currently, the International GPS Service (IGS) provides a set of clock products for both satellites and tracking receivers, tabulated at 5-min intervals. These products allow users to determine consistent coordinates and clock values for an isolated GPS receiver with an internal accuracy at the few-cm level. However, because the underlying time scale for the IGS combined clocks is based on a linear alignment to broadcast GPS Time for each day separately, the day-to-day stability of this reference is poor. We show the results of a new filter package written to automate the production of an integrated IGS frequency scale based on a dynamically weighted ensemble of the included frequency standards. The new scale is loosely steered to GPS Time.

Resonant spectrum method to characterize piezoelectric films in composite resonators

Abstract: In this paper, we present a direct method to characterize a piezoelectric film that is sandwiched with two electrodes and deposited on a substrate to form a four-layer thickness extension mode composite resonator (also known as over-moded resonator). Based on the parallel and series resonant frequency spectra of a composite resonator, the electromechanical coupling factor, the density and the elastic constant of the piezoelectric film can be evaluated directly. Experimental results on samples consisting of ZnO films on fused quartz substrates with different thickness are presented. They show good agreement with theoretical prediction. The mechanical effect of the electrode on the method is investigated, and numerical simulation shows that the effect of the electrodes can be properly corrected by the modified formulae presented in this paper. The effect of mechanical loss in piezoelectric film and in substrate on this method also has been investigated. It is proven that the method is insensitive to the losses.