Quantitative imaging of nonlinear shear modulus by combining static elastography and shear wave elastography

The study of new tissue mechanical properties such as shear nonlinearity could lead to better tissue characterization and clinical diagnosis. This work proposes a method combining static elastography and shear wave elastography to derive the nonlinear shear modulus by applying the acoustoelasticity theory in quasi-incompressible soft solids. Results demonstrate that by applying a moderate static stress at the surface of the investigated medium, and by following the quantitative evolution of its shear modulus, it is possible to accurately and quantitatively recover the local Landau (A) coefficient characterizing the shear nonlinearity of soft tissues.     

On the effects of reflected waves in transient shear wave elastography

In recent years, novel quantitative techniques have been developed to provide noninvasive and quantitative stiffness images based on shear wave propagation. Using radiation force and ultrafast ultrasound imaging, the supersonic shear imaging technique allows one to remotely generate and follow a transient plane shear wave propagating in vivo in real time. The tissue shear modulus, i.e., its stiffness, can then be estimated from the shear wave local velocity. However, because the local shear wave velocity is estimated using a time-of- flight approach, reflected shear waves can cause artifacts in the estimated shear velocity because the incident and reflected waves propagate in opposite directions. Such effects have been reported in the literature as a potential drawback of elastography techniques based on shear wave speed, particularly in the case of high stiffness contrasts, such as in atherosclerotic plaque or stiff lesions. In this letter, we present our implementation of a simple directional filter, previously used for magnetic resonance elastography, which separates the forward- and backward-propagating waves to solve this problem. Such a directional filter could be applied to many elastography techniques based on the local estimation of shear wave speed propagation, such as acoustic radiation force imaging (ARFI), shearwave dispersion ultrasound vibrometry (SDUV), needle-based elastography, harmonic motion imaging, or crawling waves when the local propagation direction is known and high-resolution spatial and temporal data are acquired.     

A computer-controlled ultrasound pulser-receiver system for transskull fluid detection using a shear wave transmission technique

The purpose of this study was to evaluate the performance of a computer-controlled ultrasound pulser-receiver system incorporating a shear mode technique for transskull fluid detection. The presence of fluid in the sinuses of an ex vivo human skull was examined using a pulse-echo method by transmitting an ultrasound beam through the maxilla bone toward the back wall on the other side of the sinus cavity. The pulser was programmed to generate bipolar pulse trains with 5 cycles at a frequency of 1 MHz, repetition frequency of about 20 Hz, and amplitude of 100 V to drive a 1-MHz piezoelectric transducer. Shear and longitudinal waves in the maxilla bone were produced by adjusting the bone surface incident angle to 45 degrees and 0 degrees, respectively. Computer tomography (CT) scans of the skull were performed to verify the ultrasound experiment. Using the shear mode technique, the echo waveform clearly distinguishes the presence of fluid, and the estimated distance of the ultrasound traveled in the sinus is consistent with the measurement from the CT images. Contrarily, using the longitudinal mode, no detectable back wall echo was observed under the same conditions. As a conclusion, this study demonstrated that the proposed pulser-receiver system with the shear mode technique is promising for transskull fluid detecting, such as mucus in a sinus.     

In vitro comminution of model renal calculi using histotripsy

Shock wave lithotripsy (SWL) suffers from the fact that it can produce residual stone fragments of significant size (>2 mm). Mechanistically, cavitation has been shown to play an important role in the reduction of such fragments to smaller debris. In this study, we assessed the feasibility of using cavitationally-based pulsed ultrasound therapy (histotripsy) to erode kidney stones. Previous work has shown that histotripsy is capable of mechanically fractionating soft tissue into fine, acellular debris. Here, we investigated the potential for translating this technology to renal calculi through the use of a commonly accepted stone model. Stone models were sonicated using a 1-MHz focused transducer, with 5-cycle pulses delivered at a rate of 1 kHz. Pulses having peak negative pressures ranging from 3 to 21 MPa were tested. Results indicate that histotripsy is capable of effectively eroding the stone model, achieving an average stone erosion rate of 26 mg/min at maximum treatment pressure; substantial stone erosion was only observed in the presence of a dense cavitational bubble cloud. Sequential sieving of residual stone fragments indicated that debris produced by histotripsy was smaller than 100 μm in size, and treatment monitoring showed that both the cavitational bubble cloud and model stone appear as hyperechoic regions on B-mode imaging. These preliminary results indicate that histotripsy shows promise in its use for stone comminution, and an optimized erosion process may provide a potential adjunct to conventional SWL procedures.     

Characterization of roll-drawn polypropylene using shear wave birefringence

Ultrasonic shear waves have been used to estimate the degree of anisotropy present within roll-drawn polypropylene samples, using shear wave birefringence. These measurements have been correlated with the physical properties that result from the draw ratio used during manufacture. It is shown that the technique has promise as a monitoring tool for this material.     

Axial strain calculation using a low-pass digital differentiator in ultrasound elastography

In ultrasound elastography, tissue axial strains are calculated from the gradient of the estimated axial displacements. However, the common differentiation operation amplifies the noises in the displacement estimation, especially at high frequencies. In this paper, a low-pass digital differentiator (LPDD) is proposed to calculate the axial strain from the estimated tissue displacement. Several LPDDs that have been well developed in the field of digital signal processing are presented. The corresponding performances are compared qualitatively and quantitatively in computer simulations and in preliminary phantom and in vitro experiments. The results are consistent with the theoretical analysis of the LPDDs.     

Evolution of bubble clouds induced by pulsed cavitational ultrasound therapy - histotripsy

Mechanical tissue fractionation can be achieved using successive, high-intensity ultrasound pulses in a process termed histotripsy. Histotripsy has many potential clinical applications where noninvasive tissue removal is desired. The primary mechanism for histotripsy is believed to be cavitation. Using fast-gated imaging, this paper studies the evolution of a cavitating bubble cloud induced by a histotripsy pulse (10 and 14 cycles) at peak negative pressures exceeding 21MPa. Bubble clouds are generated inside a gelatin phantom and at a tissue-water interface, representing two situations encountered clinically. In both environments, the imaging results show that the bubble clouds share the same evolutionary trend. The bubble cloud and individual bubbles in the cloud were generated by the first cycle of the pulse, grew with each cycle during the pulse, and continued to grow and collapsed several hundred microseconds after the pulse. For example, the bubbles started under 10 microm, grew to 50 microm during the pulse, and continued to grow 100 microm after the pulse. The results also suggest that the bubble clouds generated in the two environments differ in growth and collapse duration, void fraction, shape, and size. This study furthers our understanding of the dynamics of bubble clouds induced by histotripsy.     

Characterization of superhydrophobic materials using multiresonance acoustic shear wave sensors

Various superhydrophobic (SH) surfaces, with enhanced superhydrophobicity achieved by the use of nanoparticles, were characterized by a new acoustic sensing technique using multiresonance thickness-shear mode (MTSM) sensors. The MTSM sensors were capable of differentiating SH properties created by nano-scale surface features for film, exhibiting similar macroscopic contact angles.     

Visualizing the radial and circumferential strain distribution within vessel phantoms using synthetic-aperture ultrasound elastography

Noninvasive elastography (NIVE) produces elastograms that are difficult to interpret because NIVE visualizes strain in the transducer coordinate system. In this paper, we hypothesized that transforming normal and shear strain elastograms to the vessel coordinate system will produce better strain elastograms. To corroborate this hypothesis, we acquired synthetic-aperture (SA) ultrasound data from simulated and physical vessel phantoms. In both studies, SA echo frames were reconstructed from data acquired with a sparse transducer array. The simulation study was performed with homogeneous and heterogenous phantoms, but in the experimental study we used a modified ultrasound scanner to acquire SA data from homogeneous (n = 1) and heterogeneous (n = 3) vessel phantoms. Axial and lateral displacements were estimated by performing two-dimensional cross-correlation analysis on the beamformed RF echo frames. We generated radial and circumferential strain elastograms by transforming normal and shear strain elastograms to the vessel coordinate system. The results revealed: 1) radial and circumferential strain elastograms acquired from simulated data had a relative root mean squared error on the order of 0.1%; 2) experimentally acquired radial and circumferential strain elastograms had elastographic contrast-to-noise ratio (CNRe) between 10 and 40 dB, and elastographic signal-to-noise ratio (SNRe) between 10 and 35 dB, depending on the number of active transmission elements employed during imaging; 3) radial and circumferential strain elastograms produced with fewer than 8 active transmission elements were inferior to those computed with a greater number of active elements; and 4) plaques were evident in the strain elastograms, except in those obtained with the sparsest transducer array. This study demonstrated that a syntheticaperture ultrasound system could visualize radial and circumferential strain noninvasively.     

Improving thermal ablation delineation with electrode vibration elastography using a bidirectional wave propagation assumption

Thermal ablation procedures are commonly used to treat hepatic cancers and accurate ablation representation on shear wave velocity images is crucial to ensure complete treatment of the malignant target. Electrode vibration elastography is a shear wave imaging technique recently developed to monitor thermal ablation extent during treatment procedures. Previous work has shown good lateral boundary delineation of ablated volumes, but axial delineation was more ambiguous, which may have resulted from the assumption of lateral shear wave propagation. In this work, we assume both lateral and axial wave propagation and compare wave velocity images to those assuming only lateral shear wave propagation in finite element simulations, tissue-mimicking phantoms, and bovine liver tissue. Our results show that assuming bidirectional wave propagation minimizes artifacts above and below ablated volumes, yielding a more accurate representation of the ablated region on shear wave velocity images. Area overestimation was reduced from 13.4% to 3.6% in a stiff-inclusion tissue-mimicking phantom and from 9.1% to 0.8% in a radio-frequency ablation in bovine liver tissue. More accurate ablation representation during ablation procedures increases the likelihood of complete treatment of the malignant target, decreasing tumor recurrence.     

Robust estimation of time-of-flight shear wave speed using a radon sum transformation

Time-of-flight methods allow quantitative measurement of shear wave speed (SWS) from ultrasonically tracked displacements following impulsive excitation in tissue. However, application of these methods to in vivo data are challenging because of the presence of gross outlier data resulting from sources such as physiological motion or spatial inhomogeneities. This paper describes a new method for estimating SWS by considering a solution space of trajectories and evaluating each trajectory using a metric that characterizes wave motion along the entire trajectory. The metric used here is found by summing displacement data along the trajectory as in the calculation of projection data in the Radon transformation. The algorithm is evaluated using data acquired in calibrated phantoms and in vivo human liver. Results are compared with SWS estimates using a random sample consensus (RANSAC) algorithm described by Wang et al. Good agreement is found between the Radon sum and RANSAC SWS estimates with a correlation coefficient of greater than 0.99 for phantom data and 0.91 for in vivo liver data. The Radon sum transformation is suitable for use in situations requiring real-time feedback and is comparably robust to the RANSAC algorithm with respect to outlier data.     

Analysis of liquid-phase chemical detection using guided shear horizontal-surface acoustic wave sensors

Direct chemical sensing in liquid environments using polymer-guided shear horizontal surface acoustic wave sensor platforms on 36 degrees rotated Y-cut LiTaO3 is investigated. Design considerations for optimizing these devices for liquid-phase detection are systematically explored. Two different sensor geometries are experimentally and theoretically analyzed. Dual delay line devices are used with a reference line coated with poly (methyl methacrylate) (PMMA) and a sensing line coated with a chemically sensitive polymer, which acts as both a guiding layer and a sensing layer or with a PMMA waveguide and a chemically sensitive polymer. Results show the three-layer model provides higher sensitivity than the four-layer model. Contributions from mass loading and coating viscoelasticity changes to the sensor response are evaluated, taking into account the added mass, swelling, and plasticization. Chemically sensitive polymers are investigated in the detection of low concentrations (1-60 ppm) of toluene, ethylbenzene, and xylenes in water. A low-ppb level detection limit is estimated from the present experimental measurements. Sensor properties are investigated by varying the sensor geometries, coating thickness combinations, coating properties, and curing temperature for operation in liquid environments. Partition coefficients for polymer-aqueous analyte pairs are used to explain the observed trend in sensitivity for the polymers PMMA, poly(isobutylene), poly(epichlorohydrin), and poly(ethyl acrylate) used in this work.     

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

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

Array signal processing for local arterial pulse wave velocity measurement using ultrasound

A new signal processing approach to estimation of local arterial pulse wave velocity (PWV) in superficial arterial segments using long-axis ultrasound measurements is proposed. The method is designed to be resistant to estimation bias due to pulse wave reflections. It is evaluated using a laboratory test tank, and it appears to estimate local PWV with less bias than previously accepted methods, and with similar estimation variance to those methods.     

Arbitrary waveform coded excitation using bipolar square wave pulsers in medical ultrasound

This paper presents a new coded excitation scheme that efficiently synthesizes codes for arbitrary waveforms using a bipolar square wave pulser. In a coded excitation system, pulse compression is performed to restore the axial resolution. In order to maintain low range sidelobes, the system needs to transmit signals that have smooth spectra. However, such a transmitter requires the generation of arbitrary waveforms and, therefore, is more expensive. In other words, a trade-off is necessary between the compression performance and the transmitter cost. Here we propose a method that preserves the low-cost advantage of a bipolar pulser while achieving approximately the same compression performance as an arbitrary waveform generator. The key idea of the proposed method is the conversion of a nonbinary code (i.e., requiring an arbitrary waveform generator) with good compression performance into a binary code (i.e., requiring only a bipolar pulser) by code translation and code tuning. The code translation is implemented by sending the nonbinary code into a virtual one-bit, sigma-delta modulator, and the code tuning involves minimizing the root-mean-square error between the resultant binary code and the original nonbinary code by sequential and iterative tuning while taking the transducer response into account. Tukey-windowed chirps are known to have good compression performance. Such chirps of different durations (16, 20, and 24 micros), all with a taper ratio of 0.15, a center frequency of 2.5 MHz, and an equivalent bandwidth of 1.5 MHz, were converted into binary Tukey-windowed chirps that were compared with pseudochirps (i.e., direct binary approximations of the original chirp) over the same spectral band. The bit rate was 40 MHz. Simulation results show that the use of binary Tukey-windowed chirps can reduce the code duration by 20.6% or the peak sidelobe level by 6 dB compared to the commonly used pseudochirps. Experimental results obtained under the same settings were in agreement with the simulations. Our results demonstrate that arbitrary waveform coded excitation can be realized using bipolar square wave pulsers for applications in medical ultrasound.     

Damping of compression and shear piezoelectric accelerometers byelectromechanical feedback

It is shown that the very pronounced resonance peak in the frequency entirely by a simple modification of existing accelerometers, providing them with an electrical sensor output as well as an electrical actuator input, and using a charge amplifier in a feedback path between the sensor output and the actuator input. Because a piezoelectric accelerometer is normally read out by a charge amplifier, no extra circuitry (expense) is necessary to provide this electromechanical feedback. It is shown that a maximally flat response (Butterworth) can be obtained with little peaking (approximately 2 dB) and excellent dynamic stability, which makes the acceleration usable up to its resonance frequency