Showing papers in "IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control in 2013"
TL;DR: This review focuses on ultrasound-based elasticity imaging methods that generate an acoustic radiation force to induce tissue displacements that can be performed noninvasively during routine exams to provide either qualitative or quantitative metrics of tissue elasticity.
Abstract: The development of ultrasound-based elasticity imaging methods has been the focus of intense research activity since the mid-1990s. In characterizing the mechanical properties of soft tissues, these techniques image an entirely new subset of tissue properties that cannot be derived with conventional ultrasound techniques. Clinically, tissue elasticity is known to be associated with pathological condition and with the ability to image these features in vivo; elasticity imaging methods may prove to be invaluable tools for the diagnosis and/or monitoring of disease. This review focuses on ultrasound-based elasticity imaging methods that generate an acoustic radiation force to induce tissue displacements. These methods can be performed noninvasively during routine exams to provide either qualitative or quantitative metrics of tissue elasticity. A brief overview of soft tissue mechanics relevant to elasticity imaging is provided, including a derivation of acoustic radiation force, and an overview of the various acoustic radiation force elasticity imaging methods.
281 citations
TL;DR: This work showed that the ERM can be made suitable for PWI by a spatial transformation of the hyperbolic traces present in the RF data, and showed that multi-angle compounded f-k migrated images are of quality similar to those obtained with a state of the art dynamic focusing mode.
Abstract: Ultrafast ultrasound is an emerging modality that offers new perspectives and opportunities in medical imaging Plane wave imaging (PWI) allows one to attain very high frame rates by transmission of planar ultrasound wavefronts As a plane wave reaches a given scatterer, the latter becomes a secondary source emitting upward spherical waves and creating a diffraction hyperbola in the received RF signals To produce an image of the scatterers, all the hyperbolas must be migrated back to their apexes To perform beamforming of plane wave echo RFs and return high-quality images at high frame rates, we propose a new migration method carried out in the frequency-wavenumber (f-k) domain The f-k migration for PWI has been adapted from the Stolt migration for seismic imaging This migration technique is based on the exploding reflector model (ERM), which consists in assuming that all the scatterers explode in concert and become acoustic sources The classical ERM model, however, is not appropriate for PWI We showed that the ERM can be made suitable for PWI by a spatial transformation of the hyperbolic traces present in the RF data In vitro experiments were performed to outline the advantages of PWI with Stolt's f-k migration over the conventional delay-and-sum (DAS) approach The Stolt's f-k migration was also compared with the Fourier-based method developed by J-Y Lu Our findings show that multi-angle compounded f-k migrated images are of quality similar to those obtained with a stateof- the-art dynamic focusing mode This remained true even with a very small number of steering angles, thus ensuring a highly competitive frame rate In addition, the new FFT-based f-k migration provides comparable or better contrast-to-noise ratio and lateral resolution than the Lu's and DAS migration schemes Matlab codes for the Stolt's f-k migration for PWI are provided
229 citations
TL;DR: The Synthetic Aperture Real-time Ultrasound System (SARUS) for acquiring and processing synthetic aperture data for research purposes is described, along with its performance for SA, nonlinear, and 3-D flow estimation imaging.
Abstract: The Synthetic Aperture Real-time Ultrasound System (SARUS) for acquiring and processing synthetic aperture (SA) data for research purposes is described. The specifications and design of the system are detailed, along with its performance for SA, nonlinear, and 3-D flow estimation imaging. SARUS acquires individual channel data simultaneously for up to 1024 transducer elements for a couple of heart beats, and is capable of transmitting any kind of excitation. The 64 boards in the system house 16 transmit and 16 receive channels each, where sampled channel data can be stored in 2 GB of RAM and processed using five field-programmable gate arrays (FPGAs). The fully parametric focusing unit calculates delays and apodization values in real time in 3-D space and can produce 350 million complex samples per channel per second for full non-recursive synthetic aperture B-mode imaging at roughly 30 high-resolution images/s. Both RF element data and beamformed data can be stored in the system for later storage and processing. The stored data can be transferred in parallel using the system's sixty-four 1-Gbit Ethernet interfaces at a theoretical rate of 3.2 GB/s to a 144-core Linux cluster.
227 citations
TL;DR: A μDoppler ultrasound method able to detect and map the cerebral blood volume (CBV) over the entire brain with an important increase in sensitivity is presented and is the basis for a real-time functional ultrasound (fUS) imaging of the brain.
Abstract: Hemodynamic changes in the brain are often used as surrogates of neuronal activity to infer the loci of brain activity. A major limitation of conventional Doppler ultrasound for the imaging of these changes is that it is not sensitive enough to detect the blood flow in small vessels where the major part of the hemodynamic response occurs. Here, we present a μDoppler ultrasound method able to detect and map the cerebral blood volume (CBV) over the entire brain with an important increase in sensitivity. This method is based on imaging the brain at an ultrafast frame rate (1 kHz) using compounded plane wave emissions. A theoretical model demonstrates that the gain in sensitivity of the μDoppler method is due to the combination of 1) the high signal-to-noise ratio of the gray scale images, resulting from the synthetic compounding of backscattered echoes; and 2) the extensive signal averaging enabled by the high temporal sampling of ultrafast frame rates. This μDoppler imaging is performed in vivo on trepanned rats without the use of contrast agents. The resulting images reveal detailed maps of the rat brain vascularization with an acquisition time as short as 320 ms per slice. This new method is the basis for a real-time functional ultrasound (fUS) imaging of the brain.
209 citations
TL;DR: Beamforming techniques and signal processing are described, in conjunction with in vivo PA images of normal subcutaneous mouse tissue and selected tumor models, and the use of the system to estimate the spatial distribution of oxygen saturation in blood and co-registered with B-mode images of the surrounding anatomy is investigated.
Abstract: Photoacoustic (PA) imaging for biomedical applications has been under development for many years. Based on the many advances over the past decade, a new photoacoustic imaging system has been integrated into a micro-ultrasound platform for co-registered PA?ultrasound (US) imaging. The design and implementation of the new scanner is described and its performance quantified. Beamforming techniques and signal processing are described, in conjunction with in vivo PA images of normal subcutaneous mouse tissue and selected tumor models. In particular, the use of the system to estimate the spatial distribution of oxygen saturation (sO2) in blood and co-registered with B-mode images of the surrounding anatomy are investigated. The system was validated in vivo against a complementary technique for measuring partial pressure of oxygen in blood (pO2). The pO2 estimates were converted to sO2 values based on a standard dissociation curve found in the literature. Preliminary studies of oxygenation effects were performed in a mouse model of breast cancer (MDA-MB-231) in which control mice were compared with mice treated with a targeted antiangiogenic agent over a 3 d period. Treated mice exhibited a >90% decrease in blood volume, an 85% reduction in blood wash-in rate, and a 60% decrease in relative tissue oxygenation.
179 citations
TL;DR: Although the models have helped in understanding the responses of coated bubbles, the influence of the coating has not been fully elucidated to date and UCA models are still being improved, to indicate future directions for research.
Abstract: The merits of ultrasound contrast agents (UCAs) were already known in the 1960s. It was, however, not until the 1990s that UCAs were clinically approved and marketed. In these years, it was realized that the UCAs are not just efficient ultrasound scatterers, but that their main constituent, the coated gas microbubble, acts as a nonlinear resonator and, as such, is capable of generating harmonic energy. Subharmonic, ultraharmonic, and higher harmonic frequencies of the transmitted ultrasound frequency have been reported. This opened up new prospects for their use and several detection strategies have been developed to exploit this harmonic energy to discriminate the contrast bubbles from surrounding tissue. This insight created a need for tools to study coated bubble behavior in an ultrasound field and the first models were developed. Since then, 20 years have elapsed, in which a broad range of UCAs and UCA models have been developed. Although the models have helped in understanding the responses of coated bubbles, the influence of the coating has not been fully elucidated to date and UCA models are still being improved. The aim of this review paper is to offer an overview in these developments and indicate future directions for research.
145 citations
TL;DR: Using the high ensemble vector Doppler technique, blood flow through stenoses and secondary flow patterns were better visualized than in ordinary color doppler, and the full velocity spectrum could be obtained retrospectively for arbitrary points in the image.
Abstract: A quantitative angle-independent 2-D modality for flow and tissue imaging based on multi-angle plane wave acquisition was evaluated. Simulations of realistic flow in a carotid artery bifurcation were used to assess the accuracy of the vector Doppler (VD) technique. Reduction in root mean square deviation from 27 cm/s to 6 cm/s and 7 cm/s to 2 cm/s was found for the lateral (vx) and axial (vz) velocity components, respectively, when the ensemble size was increased from 8 to 50. Simulations of a Couette flow phantom (vmax = 2.7 cm/s) gave promising results for imaging of slowly moving tissue, with root mean square deviation of 4.4 mm/s and 1.6 mm/s for the x- and z-components, respectively. A packet acquisition scheme providing both B-mode and vector Doppler RF data was implemented on a research scanner, and beamforming and further post-processing was done offline. In vivo results of healthy volunteers were in accordance with simulations and gave promising results for flow and tissue vector velocity imaging. The technique was also tested in patients with carotid artery disease. Using the high ensemble vector Doppler technique, blood flow through stenoses and secondary flow patterns were better visualized than in ordinary color Doppler. Additionally, the full velocity spectrum could be obtained retrospectively for arbitrary points in the image.
128 citations
TL;DR: The robotic ultrasound systems that have been developed over the past two decades are reviewed and their potential impact on modern medicine is described.
Abstract: Robots ultrasound (RUS) can be defined as the combination of ultrasound imaging with a robotic system in medical interventions. With their potential for high precision, dexterity, and repeatability, robots are often uniquely suited for ultrasound integration. Although the field is relatively young, it has already generated a multitude of robotic systems for application in dozens of medical procedures. This paper reviews the robotic ultrasound systems that have been developed over the past two decades and describes their potential impact on modern medicine. The RUS projects reviewed include extracorporeal devices, needle guidance systems, and intraoperative systems.
121 citations
TL;DR: Simulation results were compared with pressure waveforms measured directly by hydrophone at both low and high power levels, demonstrating that details of the acoustic field, including shock formation, are quantitatively predicted.
Abstract: High-intensity focused ultrasound (HIFU) is a treatment modality that relies on the delivery of acoustic energy to remote tissue sites to induce thermal and/or mechanical tissue ablation. To ensure the safety and efficacy of this medical technology, standard approaches are needed for accurately characterizing the acoustic pressures generated by clinical ultrasound sources under operating conditions. Characterization of HIFU fields is complicated by nonlinear wave propagation and the complexity of phased-array transducers. Previous work has described aspects of an approach that combines measurements and modeling, and here we demonstrate this approach for a clinical phased-array transducer. First, low amplitude hydrophone measurements were performed in water over a scan plane between the array and the focus. Second, these measurements were used to holographically reconstruct the surface vibrations of the transducer and to set a boundary condition for a 3-D acoustic propagation model. Finally, nonlinear simulations of the acoustic field were carried out over a range of source power levels. Simulation results were compared with pressure waveforms measured directly by hydrophone at both low and high power levels, demonstrating that details of the acoustic field, including shock formation, are quantitatively predicted.
119 citations
TL;DR: It is shown theoretically that large SAW velocity and electromechanical coupling factor are simultaneously achievable when the ScAlN film is combined with a base substrate with extremely high acoustic wave velocities, such as diamond and SiC.
Abstract: This paper describes application of Sc-doped AlN (ScAlN) to wideband SAW devices in the 1 to 3 GHz range. First, it is shown theoretically that large SAW velocity and electromechanical coupling factor are simultaneously achievable when the ScAlN film is combined with a base substrate with extremely high acoustic wave velocities, such as diamond and SiC. Next, SAW delay lines are fabricated on the ScAlN/6H-SiC structure, and reasonable agreement between the theory and experiment is obtained. Finally, one-port SAW resonators are fabricated on the structure, and it is shown that high-performance is achievable in the 1 to 3 GHz range by use of the structure.
104 citations
TL;DR: The results obtained demonstrate that the combination of ultrasound and the doxorubicin liposome-loaded microbubbles can provide a new method of noninvasive image-guided drug delivery.
Abstract: Targeted drug delivery under image guidance is gaining more interest in the drug-delivery field. The use of microbubbles as contrast agents in diagnostic ultrasound provides new opportunities in noninvasive image-guided drug delivery. In the present study, the imaging and therapeutic properties of novel doxorubicin liposome-loaded microbubbles are evaluated. The results showed that at scanning settings (1.7 MHz and mechanical index 0.2), these microbubbles scatter sufficient signal for nonlinear ultrasound imaging and can thus be imaged in real time and be tracked in vivo. In vitro therapeutic evaluation showed that ultrasound at 1 MHz and pressures up to 600 kPa in combination with the doxorubicin liposomeloaded microbubbles induced 4-fold decrease of cell viability compared with treatment with free doxorubicin or doxorubicin liposome-loaded microbubbles alone. The therapeutic effectiveness is correlated to an ultrasound-triggered release of doxorubicin from the liposomes and an enhanced uptake of the free doxorubicin by glioblastoma cells. The results obtained demonstrate that the combination of ultrasound and the doxorubicin liposome-loaded microbubbles can provide a new method of noninvasive image-guided drug delivery.
TL;DR: The improved uptake of Dox and dextrans may be a result of both sonoporation and endocytosis, according to flow cytometry and confocal microscopy.
Abstract: The mechanism involved in the ultrasound-enhanced intracellular delivery of fluorescein-isothiocyanate (FITC)-dextran (molecular weight 4 to 2000 kDa) and liposomes containing doxorubicin (Dox) was studied using HeLa cells and an ultrasound transducer at 300 kHz, varying the acoustic power. The cellular uptake and cell viability were measured using flow cytometry and confocal microscopy. The role of endocytosis was investigated by inhibiting clathrin- and caveolae-mediated endocytosis, as well as macropinocytosis. Microbubbles were found to be required during ultrasound treatment to obtain enhanced cellular uptake. The percentage of cells internalizing Dox and dextran increased with increasing mechanical index. Confocal images and flow cytometric analysis indicated that the liposomes were disrupted extracellularly and that released Dox was taken up by the cells. The percentage of cells internalizing dextran was independent of the molecular weight of dextrans, but the amount of the small 4-kDa dextran molecules internalized per cell was higher than for the other dextrans. The inhibition of endocytosis during ultrasound exposure resulted in a significant decrease in cellular uptake of dextrans. Therefore, the improved uptake of Dox and dextrans may be a result of both sonoporation and endocytosis.
TL;DR: The different techniques and limitations of a range of airborne ultrasonic ranging approaches are reviewed, with an emphasis on the accuracy and repeatability of the measurements.
Abstract: Airborne ultrasonic ranging is used in a variety of different engineering applications for which other positional metrology techniques cannot be used, for example in closed-cell locations, when optical line of sight is limited, and when multipath effects preclude electromagnetic-based wireless systems. Although subject to fundamental physical limitations, e.g., because of the temperature dependence of acoustic velocity in air, these acoustic techniques often provide a cost-effective solution for applications in mobile robotics, structural inspection, and biomedical imaging. In this article, the different techniques and limitations of a range of airborne ultrasonic ranging approaches are reviewed, with an emphasis on the accuracy and repeatability of the measurements. Simple time-domain approaches are compared with their frequency-domain equivalents, and the use of hybrid models and biologically inspired approaches are discussed.
TL;DR: This paper presents a system capable of simultaneous high-power and high-data-rate transmission through solid metal barriers using ultrasound which could have a tremendous impact on improving safety and preserving structural integrity in many military applications as well as in a wide range of commercial, industrial, and nuclear systems.
Abstract: This paper presents a system capable of simultaneous high-power and high-data-rate transmission through solid metal barriers using ultrasound. By coaxially aligning a pair of piezoelectric transducers on opposite sides of a metal wall and acoustically coupling them to the barrier, an acoustic-electric transmission channel is formed which prevents the need for physical penetration. Independent data and power channels are utilized, but they are only separated by 25.4 mm to reduce the system's form factor. Commercial off-the-shelf components and evaluation boards are used to create realtime prototype hardware and the full system is capable of transmitting data at 17.37 Mbps and delivering 50 W of power through a 63.5-mm thick steel wall. A synchronous multi-carrier communication scheme (OFDM) is used to achieve a very high spectral efficiency and to ensure that there is only minor interference between the power and data channels. Also presented is a discussion of potential enhancements that could be made to greatly improve the power and data-rate capabilities of the system. This system could have a tremendous impact on improving safety and preserving structural integrity in many military applications (submarines, surface ships, unmanned undersea vehicles, armored vehicles, planes, etc.) as well as in a wide range of commercial, industrial, and nuclear systems.
TL;DR: Results suggest a beneficial complementary relationship between the elongation of the PL and the decrease of the peak negative acoustic pressures, and vice versa and suggest the closing timeline could be predicted from the initial volume of opening.
Abstract: The most challenging aspect of intravenously-administered drugs currently developed to treat central nervous system (CNS) diseases is their impermeability through the blood-brain barrier (BBB), a specialized vasculature system protecting the brain microenvironment. Focused ultrasound (FUS) in conjunction with systemically administered microbubbles has been shown to open the BBB locally, noninvasively, and reversibly. The objective of this study was to investigate the effect of FUS (center frequency: 1.5 MHz) pulse length (PL), ranging here from 67 μs to 6.7 ms, on the physiology of the FUS-induced BBB opening. Dynamic contrastenhanced (DCE) and T1-weighted magnetic resonance imaging (MRI) were used to quantify the permeability changes using transfer rate (Ktrans) mapping, the volume of BBB opening (VBBB) and the reversibility timeline of the FUS-induced BBB opening, with the systemic administration of microbubbles at different acoustic pressures, ranging from 0.30 to 0.60 MPa. Permeability and volume of opening were both found to increase with the acoustic pressure and pulse length. At 67-μs PL, the opening pressure threshold was 0.45 MPa, with BBB opening characteristics similar to those induced with 0.60 MPa at the same PL, as well as with 0.67-ms PL/0.30 MPa. On average, these cases had Ktrans = 0.0049 ± 0.0014 min-1 and VBBB = 3.7 ± 4.3 mm3, and closing occurred within 8 h. The 6.7-ms PL/0.30 MPa induced similar opening with 0.67-ms PL/0.45 MPa, and a closing timeline of 24 to 48 h. On average, Ktrans was 0.0091 ± 0.0029 min-1 and VBBB was 14.13 ± 7.7 mm3 in these cases. Also, there were no significant differences between the 6.7-ms PL/0.45 MPa, 0.67-ms PL/0.60 MPa and 6.7-ms PL/0.60 MPa cases, yielding on average a Ktrans of 0.0100 ± 0.0023 min-1 and VBBB equal to 20.1 ± 5.7 mm3. Closing occurred within 48 to 72 h in these cases. Stacked histograms of the Ktrans provided further insight to the nonuniform spatial distribution of permeability changes and revealed a correlation with the closing timeline. These results also suggest a beneficial complementary relationship between the elongation of the PL and the decrease of the peak negative acoustic pressures, and vice versa. Linear regression between Ktrans and VBBB showed a good correlation fit. Also, the time required for closing linearly increased with VBBB. The volume rate of decrease was measured to be 11.4 ± 4.0 mm3 per day, suggesting that the closing timeline could be predicted from the initial volume of opening. Finally, no histological damage was detected in any of the cases 7 d post-FUS, indicating the safety of the methodology and parameters used.
TL;DR: The simulation results showed the cross-talk varied inversely with the MLT beam opening angle and apodization could significantly reduce these artifacts at distinct opening angles, which were dependent on the transducer configuration.
Abstract: Imaging at high temporal resolution is critical for a better understanding of transient cardiac phases with potential diagnostic value. Typically, parallel receive beam forming is used to achieve this. As an alternative, transmitting multiple lines simultaneously [i.e., multi-line transmit (MLT)] has been proposed. However, this approach has received less attention, most likely because of potential cross-talk artifacts between beams. In this study, based on different transducer configurations, the cross-talk level of different MLT systems was investigated and their point spread functions (PSFs) were compared with that of conventional beam forming (single-line transmit, SLT) by computer simulation. To reduce cross-talk artifacts, 7 different windowing functions were tested on transmit and receive: rectangular, Tukey (α = 0.5), Hann, cosine, Hamming, Gaussian (α = 0.4), and Nuttall. The simulation results showed the cross-talk varied inversely with the MLT beam opening angle and apodization could significantly reduce these artifacts at distinct opening angles, which were dependent on the transducer configuration. The optimal settings for an MLT system were highly dependent on the exact transducer configuration and must be deduced based on a given transducer. In particular, for a typical cardiac transducer configuration, a 4MLT imaging system with an opening angle of 22.73° and a Tukey (α = 0.5)-Tukey (α = 0.5) windowing scheme provided very similar image quality to SLT but with a 4 times higher frame rate. In addition, the MLT approach can be combined with (multiple) parallel receive beamforming to increase frame rate further. With these methods, a frame rate of approximately 300 Hz can be achieved to generate a 90° sector image without significant loss in image quality.
TL;DR: Experimental results in a phantom and in vivo are presented and compared with dynamic receive focused SLSC images, demonstrating improved SNR and CNR away from the transmit focus and an axial depth of field four to five times longer.
Abstract: It has been demonstrated that short-lag spatial coherence (SLSC) ultrasound imaging can provide improved speckle SNR and lesion CNR compared with conventional B-mode images, especially in the presence of noise and clutter. Application of the van Cittert-Zernike theorem predicts that coherence among the ultrasound echoes received across an array is reduced significantly away from the transmit focal depth, leading to a limited axial depth of field in SLSC images. Transmit focus throughout the field of view can be achieved using synthetic aperture methods to combine multiple transmit events into a single final image. A synthetic aperture can be formed with either focused or diverging transmit beams. We explore the application of these methods to form synthetically focused channel data to create SLSC images with an extended axial depth of field. An analytical expression of SLSC image brightness through depth is derived for the dynamic receive focus case. Experimental results in a phantom and in vivo are presented and compared with dynamic receive focused SLSC images, demonstrating improved SNR and CNR away from the transmit focus and an axial depth of field four to five times longer.
TL;DR: A unidirectional EMAT with two meander-line coils is proposed, and a finite element model is used to simulate the directivity of the Rayleigh and shear vertical waves generated by these EMATs.
Abstract: The elastic waves generated by traditional meander- line coil electromagnetic acoustic transducers (EMATs) propagate in two directions, overlapping the echo signals from defects with the same distances, and the defect echo signal is hard to distinguish from the edge-reflected signal when the EMATs are near the edge of a specimen. In this paper, a unidirectional EMAT with two meander-line coils is proposed. A finite element model is used to simulate the directivity of the Rayleigh and shear vertical waves generated by these EMATs. Six transducers are fabricated using the printed circuit technique. The unidirectional Rayleigh wave and shear vertical wave are tested, and the results agree well with the simulation.
TL;DR: To suppress signals arising from undesirable movements, an algorithm based on quadrature detection and phase gating at the precise frequency of nanoparticle displacement is developed, which finds that displacements outside nanoparticle-laden regions can be similar in magnitude to those in regions containing nanoparticles.
Abstract: It has recently been demonstrated that superparamagnetic iron oxide nanoparticles can be used as magnetomotive ultrasound contrast agents. A time-varying external magnetic field acts to move the particles and, thus, the nanoparticle-laden tissue. However, the difficulty of distinguishing this magnetomotive motion from undesired movement induced in regions without nanoparticles or other motion artifacts has not been well reported. Using a high-frequency linear-array system, we found that displacements outside nanoparticle-laden regions can be similar in magnitude to those in regions containing nanoparticles. We also found that the displacement outside the nanoparticle regions had a phase shift of approximately π radians relative to that in the nanoparticle regions. To suppress signals arising from undesirable movements, we developed an algorithm based on quadrature detection and phase gating at the precise frequency of nanoparticle displacement. Thus, clutter at other frequencies can be filtered out, and the processed signal can be color-coded and superimposed on the B-mode image. The median signal-to-clutter ratio improvement using the proposed algorithm was 36 dB compared with simply summing the movement energy at all frequencies. This clutter rejection is a crucial step to move magnetomotive ultrasound imaging of nanoparticles toward in vivo investigations.
TL;DR: A new hydrophone with additional protective layers covering the electrodes was developed and tested, and the influences of the limited bandwidth and the spatial averaging effect of the hydrophone on the accurate measurements of the acoustic field parameters were investigated.
Abstract: For the characterization of high-intensity focused ultrasound (HIFU) fields, hydrophone measurements should be performed in water in the whole range of the radiated power; however, cavitation occurs at high output, leading to the destruction of the hydrophone. To avoid this problem, a new hydrophone with additional protective layers covering the electrodes was developed and tested in the experiments. A single-element 1-MHz focusing ultrasound source was used for detailed measurements of the acoustic field in the axial and lateral planes. Measurements were performed with a new membrane hydrophone up to the focal peak compressional and rarefactional pressures of 55.6 and of 12.8 MPa, respectively, when the shock front had already formed in the waveform. Numerical modeling for the beams of periodic waves with an initially uniform amplitude distribution was performed, based on the Khokhlov-Zabolotskaya-Kuznetsov equation. Numerical solutions were compared with the experimental data and found to be in good agreement (within 10%). The influences of the limited bandwidth and the spatial averaging effect of the hydrophone on the accurate measurements of the acoustic field parameters were also investigated.
TL;DR: This work uses an equivalent circuit model to analyze CMUT arrays with multiple cells and shows that one can very rapidly obtain the linear frequency and nonlinear transient responses of arrays with an arbitrary number of CMUT cells.
Abstract: Capacitive micromachined ultrasonic transducers (CMUTs) are usually composed of large arrays of closely packed cells. In this work, we use an equivalent circuit model to analyze CMUT arrays with multiple cells. We study the effects of mutual acoustic interactions through the immersion medium caused by the pressure field generated by each cell acting upon the others. To do this, all the cells in the array are coupled through a radiation impedance matrix at their acoustic terminals. An accurate approximation for the mutual radiation impedance is defined between two circular cells, which can be used in large arrays to reduce computational complexity. Hence, a performance analysis of CMUT arrays can be accurately done with a circuit simulator. By using the proposed model, one can very rapidly obtain the linear frequency and nonlinear transient responses of arrays with an arbitrary number of CMUT cells. We performed several finite element method (FEM) simulations for arrays with small numbers of cells and showed that the results are very similar to those obtained by the equivalent circuit model.
TL;DR: A new spatiotemporal correlation analysis to perform CUDI is proposed, providing the rationale for indirect estimation of local dispersion by deriving the analytical relation between dispersion and the correlation coefficient among neighboring time-intensity curves obtained at each pixel.
Abstract: The major role of angiogenesis in cancer development has driven many researchers to investigate the prospects of noninvasive cancer imaging based on assessment of microvascular perfusion. The limited results so far may be caused by the complex and contradictory effects of angiogenesis on perfusion. Alternatively, assessment of ultrasound contrast agent dispersion kinetics, resulting from features such as density and tortuosity, has shown a promising potential to characterize angiogenic effects on the microvascular structure. This method, referred to as contrast-ultrasound dispersion imaging (CUDI), is based on contrast-enhanced ultrasound imaging after an intravenous contrast agent bolus injection. In this paper, we propose a new spatiotemporal correlation analysis to perform CUDI. We provide the rationale for indirect estimation of local dispersion by deriving the analytical relation between dispersion and the correlation coefficient among neighboring time-intensity curves obtained at each pixel. This robust analysis is inherently normalized and does not require curve-fitting. In a preliminary validation of the method for localization of prostate cancer, the results of this analysis show superior cancer localization performance (receiver operating characteristic curve area of 0.89) compared with those of previously reported CUDI implementations and perfusion estimation methods.
TL;DR: A detailed evaluation of the two most suitable modes, S0 and A0, is presented to compare their performance using both numerical and experimental data, and it was shown that both modes could achieve a similar level of resolution in the plane of the plate surface.
Abstract: Guided wave tomography offers a method to accurately quantify wall thickness losses in pipes and vessels caused by corrosion, using ultrasonic waves transmitted over distances of approximately 1 to 2 m, and measured by an array of transducers. These measurements are then used to reconstruct a map of wall thickness throughout the inspected region. To achieve accurate estimations of remnant wall thickness, it is vital that a suitable Lamb mode is chosen. This paper presents a detailed evaluation of the two most suitable modes, S0 and A0, to compare their performance using both numerical and experimental data. The sensitivity of A0 to thickness variations was shown to be superior to S0; however, the attenuation from A0 when a liquid loading was present was much higher than S0. A0 was less sensitive to the presence of coatings on the surface than was S0. Finally, it was shown that both modes could achieve a similar level of resolution in the plane of the plate surface.
TL;DR: In this article, both axial and transverse components of the force exerted on a silicone-oil sphere are obtained for a zero-and first-order Bessel vortex beam.
Abstract: Acoustic Bessel beams are known to produce an axial radiation force on a sphere centered on the beam axis (on-axial configuration) that exhibits both repulsor and tractor behaviors. The repulsor and the tractor forces are oriented along the beam's direction of propagation and opposite to it, respectively. The behavior of the acoustic radiation force generated by Bessel beams when the sphere lies outside the beam's axis (off-axial configuration) is unknown. Using the 3-D radiation force formulas given in terms of the partial wave expansion coefficients for the incident and scattered waves, both axial and transverse components of the force exerted on a silicone- oil sphere are obtained for a zero- and a first-order Bessel vortex beam. As the sphere departs from the beam's axis, the tractor force becomes weaker. Moreover, the behavior of the transverse radiation force field may vary with the sphere's size factor ka (where k is the wavenumber and a is the sphere radius). Both stable and unstable equilibrium regions around the beam's axis are found, depending on ka values. These results are particularly important for the design of acoustical tractor beam devices operating with Bessel beams.
TL;DR: A prototype of a 2-D cavitation imager capable of producing images of the dominant cavitation state and activity level in a region of interest is presented and results for cavitation states and their changes as a function of acoustic amplitude are presented.
Abstract: Ultrasound cavitation of microbubble contrast agents has a potential for therapeutic applications such as sonothrombolysis (STL) in acute ischemic stroke. For safety, efficacy, and reproducibility of treatment, it is critical to evaluate the cavitation state (moderate oscillations, stable cavitation, and inertial cavitation) and activity level in and around a treatment area. Acoustic passive cavitation detectors (PCDs) have been used to this end but do not provide spatial information. This paper presents a prototype of a 2-D cavitation imager capable of producing images of the dominant cavitation state and activity level in a region of interest. Similar to PCDs, the cavitation imaging described here is based on the spectral analysis of the acoustic signal radiated by the cavitating microbubbles: ultraharmonics of the excitation frequency indicate stable cavitation, whereas elevated noise bands indicate inertial cavitation; the absence of both indicates moderate oscillations. The prototype system is a modified commercially available ultrasound scanner with a sector imaging probe. The lateral resolution of the system is 1.5 mm at a focal depth of 3 cm, and the axial resolution is 3 cm for a therapy pulse length of 20 μs. The maximum frame rate of the prototype is 2 Hz. The system has been used for assessing and mapping the relative importance of the different cavitation states of a microbubble contrast agent. In vitro (tissue-mimicking flow phantom) and in vivo (heart, liver, and brain of two swine) results for cavitation states and their changes as a function of acoustic amplitude are presented.
TL;DR: The generation of omnidirectional Lamb waves by a new magnetostrictive patch transducer (MPT) is presented and the mechanism of the Lamb wave generation and its frequency characteristics are investigated.
Abstract: This work presents the generation of omnidirectional Lamb waves by a new magnetostrictive patch transducer (MPT) and investigates its generation mechanism. Although MPTs have been widely used for wave transduction in plates and pipes, no investigation reports the generation of omnidirectional Lamb waves in a plate by an MPT. For the generation, we propose an axisymmetrically-configured MPT that installs multiple axisymmetric turns of coil outside of a permanent cylindrical magnet located above the center of a circular magnetostrictive patch. After confirming the omnidirectivity of the proposed MPT experimentally, the mechanism of the Lamb wave generation and its frequency characteristics are investigated. It is also shown that the Lamb wave is most efficiently generated in a test plate when its wavelength is equal to two-thirds of the magnetostrictive patch diameter. If this wavelength?patch diameter relation holds, the second radial extensional vibration mode of the patch of the proposed MPT is shown to be the mode responsible for generating the Lamb wave in a plate.
TL;DR: LFM and NLFM signals were used for the insonation of microbubble populations and the NLFM chirp with a customized window was used as an excitation signal to perform subharmonic imaging in an ultrasound flow phantom.
Abstract: Subharmonic generation from ultrasound contrast agents depends on the spectral and temporal properties of the excitation signal. The subharmonic response can be improved by using wideband and long-duration signals. However, for sinusoidal tone-burst excitation, the effective bandwidth of the signal is inversely proportional to the signal duration. Linear frequency-modulated (LFM) and nonlinear frequencymodulated (NLFM) chirp excitations allow independent control over the signal bandwidth and duration; therefore, in this study LFM and NLFM signals were used for the insonation of microbubble populations. The amplitude modulation of the excitation waveform was achieved by applying different window functions. A customized window was designed for the NLFM chirp excitation by focusing on reducing the spectral leakage at the subharmonic frequency and increasing the subharmonic generation from microbubbles. Subharmonic scattering from a microbubble population was measured for various excitation signals and window functions. At a peak negative pressure of 600 kPa, the generated subharmonic energy by ultrasound contrast agents was 15.4 dB more for NLFM chirp excitation with 40% fractional bandwidth when compared with tone-burst excitation. For this reason, the NLFM chirp with a customized window was used as an excitation signal to perform subharmonic imaging in an ultrasound flow phantom. Results showed that the NLFM waveform with a customized window improved the subharmonic contrast by 4.35 ± 0.42 dB on average over a Hann-windowed LFM excitation.
TL;DR: In situ measurements of the pulse-echo response of YCa 4O(BO3, LiNbO
Abstract: High-temperature piezoelectric crystals, including YCa4O(BO3)3, LiNbO3, and AlN, have been studied for use in ultrasonic transducers under continuous operation for 55 h at 550°C. Additionally, thermal ratcheting tests were performed on the transducers by subjecting the crystals to heat treatments followed by ultrasonic performance testing at room temperature and 500°C. The changes resulting from the heat treatments were less than the statistical spread obtained in repeated experiments and were thus considered negligible. Finally, in situ measurements of the pulse-echo response of YCa4O(BO3)3 were performed at temperatures up to 950°C for the first time, showing stable characteristics up to these high temperatures.
TL;DR: A protocol is described which uses ultrasonic waves to achieve simultaneous bidirectional communication through the metallic enclosures and sufficient power is harvested to allow for the continuous operation of internal electronics which require an aggregate of less than 100 mW.
Abstract: This paper presents a method for two-way ultrasonic communication and power delivery through thick metallic enclosures without physical penetration. Acousticelectric channels are implemented using a pair of coaxially aligned piezoelectric transducers having 25.4 mm diameters and 1 MHz nominal resonant frequencies, mounted on steel walls having lengths in the range of 57.15 to 304.8 mm. A protocol is described which uses ultrasonic waves to achieve simultaneous bidirectional communication through the metallic enclosures. It is shown that such channels are very frequency selective, and a carrier frequency selection and tracking algorithm is presented to choose a frequency of operation at which both adequate power delivery and reliable full-duplex communication are achieved. Using this algorithm, sufficient power is harvested to allow for the continuous operation of internal electronics which require an aggregate of less than 100 mW. Reliable communication of sensor data is achieved at rates in excess of 30 kbps.
TL;DR: Overall, TFM is the most robust algorithm across a range of different types of defects, and it is shown that the detection limit of all three imaging algorithms is almost equal for weakly scattering defects.
Abstract: Ultrasonic array imaging algorithms have been widely used and developed in nondestructive evaluation in the last 10 years. In this paper, three imaging algorithms [total focusing method (TFM), phase-coherent imaging (PCI), and spatial compounding imaging (SCI)] are compared through both simulation and experimental measurements. In the simulation, array data sets were generated using a hybrid forward model containing a single defect among a multitude of randomly distributed point scatterers to represent backscatter from material microstructure. The number of point scatterers per unit area and their scattering amplitude were optimized to reduce computation cost. The SNR of the final images and their resolution were used to indicate the quality of the different imaging algorithms. The images of different types of defects (point reflectors and planar cracks) were used to investigate the robustness of the imaging algorithms. It is shown that PCI can yield higher image resolution and higher SNR for defects in material with weak backscatter than TFM, but that the images of cracks are distorted. Overall, TFM is the most robust algorithm across a range of different types of defects. It is also shown that the detection limit of all three imaging algorithms is almost equal for weakly scattering defects.