Diverging Wave Imaging Research Articles

Complex Residual Attention U-Net for Fast Ultrasound Imaging from a Single Plane-Wave Equivalent to Diverging Wave Imaging.

Plane wave imaging persists as a focal point of research due to its high frame rate and low complexity. However, in spite of these advantages, its performance can be compromised by several factors such as noise, speckle, and artifacts that affect the image quality and resolution. In this paper, we propose an attention-based complex convolutional residual U-Net to reconstruct improved in-phase/quadrature complex data from a single insonification acquisition that matches diverging wave imaging. Our approach introduces an attention mechanism to the complex domain in conjunction with complex convolution to incorporate phase information and improve the image quality matching images obtained using coherent compounding imaging. To validate the effectiveness of this method, we trained our network on a simulated phased array dataset and evaluated it using in vitro and in vivo data. The experimental results show that our approach improved the ultrasound image quality by focusing the network's attention on critical aspects of the complex data to identify and separate different regions of interest from background noise.

Adaptive higher-order singular value decomposition clutter filter for ultrafast Doppler imaging of coronary flow under non-negligible tissue motion

Background and Objective:With the development of advanced clutter-filtering techniques by singular value decomposition (SVD) and leveraging favorable acquisition settings such as open-chest imaging by a linear high-frequency probe and plane waves, several studies have shown the feasibility of cardiac flow measurements during the entire cardiac cycle, ranging from coronary flow to myocardial perfusion. When applying these techniques in a routine clinical setting, using transthoracic ultrasound imaging, new challenges emerge. Firstly, a smaller aperture is needed that can fit between ribs. Consequently, diverging waves are employed instead of plane waves to achieve an adequate field of view. Secondly, to ensure imaging at a larger depth, the maximum pulse repetition frequency has to be reduced. Lastly, in comparison to the open-chest scenario, tissue motion induced by the heartbeat is significantly stronger. The latter complicates substantially the distinction between clutter and blood signals. Methods:This study investigates a strategy to overcome these challenges by diverging wave imaging with an optimal number of tilt angles, in combination with dedicated clutter-filtering techniques. In particular, a novel, adaptive, higher-order SVD (HOSVD) clutter filter, which utilizes spatial, temporal, and angular information of the received ultrasound signals, is proposed to enhance clutter and blood separation. Results:When non-negligible tissue motion is present, using fewer tilt angles not only reduces the decorrelation between the received waveforms but also allows for collecting more temporal samples at a given ensemble duration, contributing to improved Doppler performance. The addition of a third angular dimension enables the application of HOSVD, providing greater flexibility in selecting blood separation thresholds from a 3-D tensor. This differs from the conventional threshold selection method in a 2-D spatiotemporal space using SVD. Exhaustive threshold search has shown a significant improvement in Contrast and Contrast-to-Noise ratio for Power Doppler images filtered with HOSVD compared to the SVD-based clutter filter. Conclusion:With the improved settings, the obtained Power Doppler images show the feasibility of measuring coronary flow under the influence of non-negligible tissue motion in both in vitro and ex vivo.

A study of wide unfocused wavefront for convex-array ultrasound imaging

Ultrafast ultrasound imaging modalities have attracted a lot of attention in the ultrasound community. It breaks the compromise between the frame rate and the region of interest by insonifying the whole medium with wide unfocused waves. Coherent compounding can be performed to enhance the image quality at a cost of frame rate. Ultrafast imaging has wide clinical applications, such as vector Doppler imaging and shear elastography. On the other hand, the use of unfocused waves is still marginal with convex-array transducers. For convex array, plane wave imaging is limited by the complicated transmission delay calculation, limited field-of-view, and inefficient coherent compounding. In this article, we study three wide unfocused wavefronts, namely, lateral virtual-source defined diverging wave imaging (latDWI), tilt virtual-source defined diverging wave imaging (tiltDWI), and Archimedean-spiral-based imaging (AMI) for convex-array imaging using the full-aperture transmission. The analytical monochromatic wave solutions to this three imaging are given. The mainlobe width and grating lobe position are given explicitly. Theoretical −6 dB beamwidth and synthetic transmit field response are studied. Simulation studies are carried on with the point targets and hypoechoic cysts. Time-of-flight formulas are given explicitly for beamforming. The conclusions are in good agreement with the theory: latDWI provides the finest lateral resolution but generates the severest axial lobe level for scatterers with large obliquities (i.e., for scatterers located at the image border) which degrades the image contrast. This effect gets worsen as the compound number increases. The tiltDWI and AMI give a very close performance on resolution and image contrast. AMI displays better contrast with a small compound number.

A High-Performance 3-D Imaging Technique Using Simultaneous Azimuth and Elevation Compounding.

A new technique for 3-D imaging with a row-column array (RCA) configuration has been developed. The technique requires an electrostrictive piezoelectric for the active substrate. While the top set of electrodes is connected to RF transmit and receive channels for conventional diverging wave imaging (DWI), the orthogonal bottom set of electrodes is connected to independently controlled variable dc bias channels. By implementing modulated bias patterns compounded across multiple pulses, fine delay control across the bottom elements can be achieved simultaneously with imaging with the top set of electrodes. This resulted in a high-quality two-way focus in both azimuth and elevation. A 20-MHz electrostrictive composite substrate was fabricated, and 64 top ×64 bottom electrodes were patterned and connected to custom beamforming and biasing electronics. The point spread functions were generated in all dimensions, and the -6 dB resolution was measured to be 93 [Formula: see text] axially, [Formula: see text] in the azimuth, and 328 [Formula: see text] in the elevation dimension. This was in good agreement with the simulated resolutions of 80, 273, and 280 [Formula: see text], respectively.

Predictors of shear wave propagation speed assessed by shear wave elastography

Abstract Background Cardiac shear wave elastography (SWE) is a novel ultrasound-based method that assesses the propagation speed of shear waves travelling in the myocardium induced by e.g. mitral valve closure (MVC). The propagation speed of these waves is related to the stiffness of the myocardium. The capability of SWE to evaluate myocardial stiffness has already been demonstrated in several publications. However, which factors beside the intrinsic mechanical properties of the myocardium influence shear wave speed in vivo has not been extensively investigated. Purpose The aim of this study was to investigate the influence of clinical parameters as well as echocardiographic indicators of myocardial tissue properties and left ventricular (LV) filling pressures on shear wave propagation speed after MVC. Methods 219 subjects (59±17 years; 140 males) were included in the study. Of those, 49 were healthy volunteers, 25 were patients with cardiac amyloidosis, 42 with hypertrophic cardiomyopathy, 35 with hypertensive heart disease and 68 with coronary artery disease. Subjects were scanned with an experimental ultrasound scanner using diverging wave imaging to acquire images at a high temporal resolution (average frame rate: 1167±414 Hz). Shear waves after MVC were visualized on M-mode maps along the interventricular septum which were colour coded for tissue acceleration (Figure 1). The propagation speed was calculated by semi-automatically measuring the spatiotemporal slope of the shear wave. Univariate and multivariate linear regression analysis was performed to identify variables associated with shear wave propagation speed. Results Univariate regression analysis revealed an association of the following variables with shear wave speed: age, BMI, systolic blood pressure, E wave, average e', E/e', left atrial volume index (LAVI), grade of diastolic dysfunction, isovolumic relaxation time, septal wall thickness, LV wall mass and presence of cardiomyopathy. These variables were added to a multivariate model. Predictors of shear wave speed after MVC were BMI, E wave, average e', LAVI, LV wall mass and presence of cardiomyopathy (R2=0.53) (Table 1). Conclusions The results of this study indicate that both myocardial structural properties (reflected by predictors LV wall mass and presence of cardiomyopathy) and LV filling pressures (reflected by predictors E wave, average e' and LAVI) affect shear wave speed. These findings suggest that SWE has the potential to assess structural as well as functional changes to the LV, which should be further explored. Funding Acknowledgement Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Research foundation Flanders - FWO

Fourier Beamformation for Convex-Array Diverging Wave Imaging Using Virtual Sources.

Convex probes have been widely used in clinical abdominal imaging for providing deep penetration and wide field of view. Ultrafast imaging modalities have been studied extensively in the ultrasound community. Specifically, broader wavefronts, such as plane wave and spherical wave, are used for transmission. For convex array, spherical wavefront can be simply synthesized by turning all elements simultaneously. Due to the lack to transmit focus, the image quality is suboptimal. One solution is to adopt virtual sources behind the transducer and compound corresponding images. In this work, we propose two novel Fourier-domain beamformers (vs1 and vs2) for nonsteered diverging wave imaging and an explicit interpolation scheme for virtual-source-based steered diverging wave imaging using a convex probe. The received echoes are first beamformed using the proposed beamformers and then interpolated along the range axis. A total of 31 virtual sources located on a circular line are used. The lateral resolution, the contrast ( C ), and the contrast-to-noise ratio (CNR) are evaluated in simulations, phantom experiments, ex vivo imaging of the bovine heart, and in vivo imaging of the liver. The results show that the two proposed Fourier-domain beamformers give higher contrast than dynamic receive focusing (DRF) with better resolution. In vitro results demonstrate the enhancement on CNR: 6.7-dB improvement by vs1 and 5.9-dB improvement by vs2. Ex vivo imaging experiments on the bovine heart validate the CNR enhancements by 8.4 dB (vs1) and 8.3 dB (vs2). In vivo imaging on the human liver also reveals 6.7- and 5.5-dB improvements of CNR by vs1 and vs2, respectively. The computation time of vs1 and vs2, depending on the image pixel number, is shortened by 2-73 and 4-216 times than the DRF.

Convex Array With Variable Spacings (CAVS) for Ultrafast Ultrasound Diverging-Wave Imaging

The synthetic focusing of coherent plane-wave compounding (CPWC) has gradually developed from ultrasound linear array to convex array. The field of view (FOV) has been expanded compared to that of the linear array. However, the existing technology does not make the most of the convex structural advantages and FOV is greatly limited by the element number of a convex array. In this paper, based on the convex array of diverging wave (DW) imaging theory and the special structure design principles, the elements of the convex array probe are unequally spaced for better imaging quality and wider FOV. Firstly, the sound field pattern was simulated for sidelobe comparison. Then, DW imaging of the new convex array was simulated and the experiment was carried out on the tissue-mimicking ultrasound phantom using the commercial probe. Comparisons of lateral and axial variations illustrated that the convex array with variable spacings (CAVS) does improve the imaging quality when compared to the given convex array with uniform spacings. Finally, we conclude that CAVS can further extend the FOV of DW imaging by taking advantage of the structure of convex array and variable element spacings.

High frame rate multi-perspective cardiac ultrasound imaging using phased array probes

Ultrasound (US) imaging is used to assess cardiac disease by assessing the geometry and function of the heart utilizing its high spatial and temporal resolution. However, because of physical constraints, drawbacks of US include limited field-of-view, refraction, resolution and contrast anisotropy. These issues cannot be resolved when using a single probe. Here, an interleaved multi-perspective 2-D US imaging system was introduced, aiming at improved imaging of the left ventricle (LV) of the heart by acquiring US data from two separate phased array probes simultaneously at a high frame rate.In an ex-vivo experiment of a beating porcine heart, parasternal long-axis and apical views of the left ventricle were acquired using two phased array probes. Interleaved multi-probe US data were acquired at a frame rate of 170 frames per second (FPS) using diverging wave imaging under 11 angles. Image registration and fusion algorithms were developed to align and fuse the US images from two different probes. First- and second-order speckle statistics were computed to characterize the resulting probability distribution function and point spread function of the multi-probe image data.First-order speckle analysis showed less overlap of the histograms (reduction of 34.4%) and higher contrast-to-noise ratio (CNR, increase of 27.3%) between endocardium and myocardium in the fused images. Autocorrelation results showed an improved and more isotropic resolution for the multi-perspective images (single-perspective: 0.59 mm × 0.21 mm, multi-perspective: 0.35 mm × 0.18 mm). Moreover, mean gradient (MG) (increase of 74.4%) and entropy (increase of 23.1%) results indicated that image details of the myocardial tissue can be better observed after fusion.To conclude, interleaved multi-perspective high frame rate US imaging was developed and demonstrated in an ex-vivo experimental setup, revealing enlarged field-of-view, and improved image contrast and resolution of cardiac images.

Abstract 11452: Full 3D Single Heartbeat Electromechanical Wave Imaging of the Heart in Vivo with Coherent Compounding of Diverging Waves

Introduction: Electromechanical wave imaging (EWI) is a new ultrasound-based technique that can noninvasively map myocardial activation of all four chambers at high temporal resolution (500-2000 frames per second). Localization of atrial and ventricular arrhythmias with 3D-rendered EWI maps has been demonstrated and validated in heart rhythm disorders using multiple 2D echocardiographic views at different heartbeats. Objective: The objective of this study was to assess the in vivo performance of single heartbeat 3D EWI using coherent compounding of high volume-rate 3D anatomical echocardiography. Methods: A 32x32-element matrix array (Vermon, France) was connected to four synchronized research ultrasound scanners (Vantage 256, Verasonics, WA). High volume rate acquisitions were performed with diverging wave imaging at a compounded imaging rate of 840 volumes per second. Three healthy volunteers were imaged during normal sinus rhythm and one open chest canine was imaged during left ventricular pacing. Timing of zero-crossing of temporal inter-volume axial strain curves, corresponding to local onset of contraction, was determined for EWI activation. Results: A normal activation sequence was consistently observed in all volunteers (Figure 1). Early activation was mapped in the right atrium, followed by the left atrium, followed by the ventricles. Epicardial ventricular pacing was performed alternatively in the basal anterior-lateral region and in the apical anterior-septal region. The earliest activated region with 3D EWI was observed in the same location as the pacing electrode (Figure 1). Good agreement between 3D EWI and electro-anatomical mapping was obtained. Conclusions: Ultrasound-based full 3D EWI is capable of consistently mapping myocardial activation of the full heart in a single heartbeat in healthy volunteers noninvasively and is capable of identifying early activation during epicardial ventricular pacing in an open-chest canine.

In Vivo Comparison of Multiline Transmission and Diverging Wave Imaging for High-Frame-Rate Speckle-Tracking Echocardiography.

High-frame-rate (HFR) speckle-tracking echocardiography (STE) assesses myocardial function by quantifying motion and deformation at high temporal resolution. Among the proposed HFR techniques, multiline transmission (MLT) and diverging wave (DW) imaging have been used in this context both being characterized by specific advantages and disadvantages. Therefore, in this article, we directly contrast both approaches in an in vivo setting while operating at the same frame rate (FR). First, images were recorded at baseline (resting condition) from healthy volunteers and patients. Next, additional acquisitions during stress echocardiography were performed on volunteers. Each scan was contoured and processed by a previously proposed 2-D HFR STE algorithm based on cross correlation. Then, strain curves and their end-systolic (ES) values were extracted for all myocardial segments for further statistical analysis. The baseline acquisitions did not reveal differences in estimated strain between the acquisition modes ( ); myocardial segments ( ); or an interaction between imaging mode and depth ( ). Similarly, during stress testing, no difference ( p = 0.7 ) was observed for the two scan sequences, stress levels or an interaction sequence-stress level ( p = 0.94 ). Overall, our findings show that MLT and DW compoundings give comparable HFR STE strain values and that the choice for using one method or the other may thus rather be based on other factors, for example, system requirements or computational cost.

In Vivo Evaluation of Plane Wave Imaging for Abdominal Ultrasonography.

Although plane wave imaging (PWI) has been extensively employed for ultrafast ultrasound imaging, its potential for sectorial B-mode imaging with a convex array transducer has not yet been widely recognized. Recently, we reported an optimized PWI approach for sector scanning that exploits the dynamic transmit focusing capability. In this paper, we first report the clinical applicability of the optimized PWI for abdominal ultrasonography by in vivo image and video evaluations and compare it with conventional focusing (CF) and diverging wave imaging (DWI), which is another dynamic transmit focusing technique generally used for sectorial imaging. In vivo images and videos of the liver, kidney, and gallbladder were obtained from 30 healthy volunteers using PWI, DWI, and CF. Three radiologists assessed the phantom images, 156 in vivo images, and 66 in vivo videos. PWI showed significantly enhanced (p < 0.05) spatial resolution, contrast, and noise and artifact reduction, and a 4-fold higher acquisition rate compared to CF and provided similar performances compared to DWI. Because the computations required for PWI are considerably lower than that for DWI, PWI may represent a promising technique for sectorial imaging in abdominal ultrasonography that provides better image quality and eliminates the need for focal depth adjustment.

The nonlinear field produced by 1-D and 2-D imaging arrays with plane and diverging wave modes for ultrafast imaging

Combining plane and diverging wave imaging (PWI/DWI) with tissue harmonic imaging (THI) may offer improvements in image quality in applications requiring very high frame rates (ultrafast). The beam shape and magnitude of the 2nd harmonic in tissue are important design considerations. We developed a numerical model based on the KZK equation and modeled three 1-D diagnostic arrays, L11-4v linear array, P4-2v phased array, and C5-2 convex array operating in PWI/DWI. Our numerical code predicts the nonlinear field of nonaxisymmetric sources and source configurations with elevation and no azimuthal foci (plane/diverging fields), which have not been modeled before. We showed that the second harmonic produced by ultrafast THI is 2-16 dB lower than that of focused beams for the imaging arrays considered when operated at the same maximum MI. This moderate difference of the second harmonic between PWI/DWI and focused ultrasound suggests that it is feasible to combine PWI/DWI and THI. We have also investigated harmonic generation produced by diagnostic 2-D arrays for 4-D THI. From our predictions for the second harmonic field we propose beamforming approaches for 4-D cardiac THI with focused ultrasound, PWI, and DWI that would result in frame rates of 10, 81, and 1275 Hz, respectively.

Reconstruction for Diverging-Wave Imaging Using Deep Convolutional Neural Networks.

In recent years, diverging wave (DW) ultrasound imaging has become a very promising methodology for cardiovascular imaging due to its high temporal resolution. However, if they are limited in number, DW transmits provide lower image quality compared with classical focused schemes. A conventional reconstruction approach consists in summing series of ultrasound signals coherently, at the expense of frame rate, data volume, and computation time. To deal with this limitation, we propose a convolutional neural network (CNN) architecture, Inception for DW Network (IDNet), for high-quality reconstruction of DW ultrasound images using a small number of transmissions. In order to cope with the specificities induced by the sectorial geometry associated with DW imaging, we adopted the inception model composed of the concatenation of multiscale convolution kernels. Incorporating inception modules aims at capturing different image features with multiscale receptive fields. A mapping between low-quality images and corresponding high-quality compounded reconstruction was learned by training the network using in vitro and in vivo samples. The performance of the proposed approach was evaluated in terms of contrast ratio (CR), contrast-to-noise ratio (CNR), and lateral resolution (LR), and compared with standard compounding method and conventional CNN methods. The results demonstrated that our method could produce high-quality images using only 3 DWs, yielding an image quality equivalent to that obtained with compounding of 31 DWs and outperforming more conventional CNN architectures in terms of complexity, inference time, and image quality.

Ultrafast Phased-Array Imaging Using Sparse Orthogonal Diverging Waves.

We present a new transmit pulse encoding scheme for ultrafast phased-array imaging called sparse orthogonal diverging wave imaging (SODWI). In SODWI, Hadamard encoding is used to selectively invert transmit pulse phases beamformed with a diverging wave delay profile. This approach has the advantage of delivering energy to a much wider field of view than conventional Hadamard-encoded multielement synthetic transmit aperture (HMSTA), making it more suitable for phased-array applications. With SODWI, we use a synthetic transmit element delay insertion (STEDI) approach which produces significant improvements in resolution, grating lobe level, and signal-to-noise ratio (SNR) over HMSTA. We also show how in SODWI a subset of the Hadamard codes can be sparsely selected to increase the imaging frame rate at the expense of image quality. SODWI is then compared with a variety of beamforming schemes for phased-array applications, including HMSTA, STEDI-HMSTA, diverging wave imaging (DWI), synthetic aperture (SA), and focused imaging. We present the results by implementing this technique on a 64-channel custom beamforming platform with a 40-MHz phased array. When a full set of codes is used, SODWI outperforms focused imaging contrast and SNR by 2.7 and 1.8 dB in addition to an 8× increase in frame rate, respectively.

A comparison between unfocused and focused transmit strategies in cardiac strain imaging

Unfocused ultrasound imaging, particularly coherent compounding with diverging waves, is a commonly employed high-frame rate transmit strategy in cardiac strain imaging. However, the accuracy and precision of diverging wave imaging compared to focused-beam transmit approaches in human subjects is unknown. Three transmit strategies—coherent compounding imaging, composite focused imaging with ECG gating and narrow-beams, and focused imaging with wide-beams—were compared in simulation and in transthoracic imaging of healthy human subjects (n = 7). The focused narrow-beam sequence estimated radial end-systolic cumulative strains of a simulated left ventricular deformation with 26% ± 1.5% and 34% ± 1.5% greater accuracy compared with compounding and wide-beam imaging, respectively. Strain estimation precision in transthoracic imaging was then assessed with the Strain Filter on cumulative end-systolic radial strains. Within the strain values where statistically significant differences in precision (E(SNRe|ε)) were found between transmit strategies, the narrow-beam sequence estimated radial strain 13% ± 0.71% and 34% ± 8.9% more precisely on average compared to compounding or wide-beam imaging, respectively.

Wide Field-of-View Ultrafast Curved Array Imaging Using Diverging Waves.

Ultrafast ultrasound imaging provides great opportunities for very high frame rate applications, such as shear wave elastography and microvascular imaging. However, ultrafast imaging with curved array transducers remains challenging in terms of element directivity and a limited field-of-view (FOV) for a fully synthetic area. In this paper, a wide FOV ultrafast curved array imaging method based on diverging wave transmissions is presented for high frame rate abdominal ultrasound applications. For this method, a theoretical model for a diverging wave solution based on a virtual point source originating from a circular line is proposed, and the FOV and element directivity are analyzed by this model. Furthermore, an integrated model for plane wave and diverging wave imaging along the location of the virtual point source is derived. The proposed method was evaluated with simulation, phantom, and in vivo studies. In the simulation and phantom studies, the image quality (i.e., spatial resolution, cystic resolution, and contrast-to-noise ratio), and effective FOV were assessed. For the in vivo study, a preliminary result from abdominal microvascular imaging, where diverging wave excitation was utilized to depict the vasculature, was also presented. In the renal cortex microvessels, the diverging wave imaging yielded a higher signal-to-clutter ratio value than the plane wave imaging, i.e., 6.35 vs. 4.26dB, due to the wider synthetic field. These studies demonstrated that the proposed ultrafast curved array imaging technique based on diverging wave excitation allowed for an extended FOV with high spatiotemporal resolution.

Comparison Between Multiline Transmission and Diverging Wave Imaging: Assessment of Image Quality and Motion Estimation Accuracy.

High frame rate imaging is particularly important in echocardiography for better assessment of the cardiac function. Several studies showed that diverging wave imaging (DWI) and multiline transmission (MLT) are promising methods for achieving a high temporal resolution. The aim of this study was to compare MLT and compounded motion compensation (MoCo) DWI for the same transmitted power, same frame rates [image quality and speckle tracking echocardiography (STE) assessment], and same packet size [tissue Doppler imaging (TDI) assessment]. Our results on static images showed that MLT outperforms DW in terms of resolution (by 30% on average). However, in terms of contrast, MLT outperforms DW only for the depth of 11 cm (by 40% on average), the result being reversed at a depth of 4 cm (by 27% on average). In vitro results on a spinning phantom at nine different velocities showed that similar STE axial errors (up to 2.3% difference in median errors and up to 2.1% difference in the interquartile ranges) are obtained with both ultrafast methods. On the other hand, the median lateral STE estimates were up to 13% more accurate with DW than with MLT. On the contrary, the accuracy of TDI was only up to ~3% better with MLT, but the achievable DW Doppler frame rate was up to 20 times higher. However, our overall results showed that the choice of one method relative to the other is therefore dependent on the application. More precisely, in terms of image quality, DW is more suitable for imaging structures at low depths, while MLT can provide an improved image quality at the focal point that can be placed at higher depths. In terms of motion estimation, DW is more suitable for color Doppler-related applications, while MLT could be used to estimate velocities along selected lines of the image.

Modeling of the Acoustic Field Produced by Diagnostic Ultrasound Arrays in Plane and Diverging Wave Modes.

Recent advances in ultrafast contrast imaging have facilitated innovations, such as superresolution imaging and ultrafast contrast-enhanced Doppler imaging. Combining plane and diverging wave imaging (PWI/DWI) with tissue harmonic imaging (THI) may offer improvements in image quality in applications such as 3-D THI and harmonic color flow. However, no studies have reported simulations of the nonlinear acoustic fields produced by diagnostic arrays in either plane or diverging wave mode. The aim of this study is to model three typical diagnostic arrays that are used in clinical practice and research, Verasonics L11-4v linear array, C5-2v convex array, and P4-2v phased array with the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation. We have two specific objectives: first, to investigate whether there is increased bubble destruction due to the nature of the plane and diverging fields in contrast imaging; and second, to investigate the feasibility of combining PWI/DWI and THI by quantifying the second harmonic generated by these fields. We showed in linear simulations that using such arrays for ultrafast contrast imaging produced pressures that are greater in the near field and lower in the far field than those of focused beams and thus may induce more near-field bubble destruction. In nonlinear simulations, the second harmonic produced by ultrafast THI was found to be 2-16 dB lower than that of focused beams for all arrays considered when operated at the same MI. This moderate difference of the second harmonic between PWI/DWI and focused ultrasound suggests that it is feasible to combine PWI/DWI and THI.

A Physical Model of Nonstationary Blur in Ultrasound Imaging

While restoration methods have been extensively studied in ultrasound (US) imaging, only few recent works have focused on modeling and understanding the blur from a physical point of view even in simple configurations, such as lossless homogeneous media. Despite a highly nonstationary blur due to diffraction effects, many techniques rely on simplistic approximations based on shift-invariant models or sectional methods and their efficiency has not been demonstrated for plane-wave (PW) and diverging-wave (DW) imaging. In this paper, we first propose a physical model of nonstationary blur in the context of PW and DW imaging. The blur operation is expressed as a composition of a U.S. propagation operator and a delay-and-sum operator, each of which has derived in previous works. We show that such a composition leads to a standard model of nonstationary blur as a Fredholm integral of the first kind. Second, we describe an approximation of the blur in the discrete domain based on the above decomposition coupled with an appropriate discretization of the latent element-raw-data space. We show theoretically and empirically that its evaluation, using such an approximation, scales linearly instead of quadratically with respect to the grid size, better than shift-invariant approaches. Through simulations and in vivo experimental data, we demonstrate that using the proposed model in the context of maximum-a-posteriori image restoration results in a higher image quality than using state-of-the-art shift-invariant models.

Compressed sensing reconstruction of synthetic transmit aperture dataset for volumetric diverging wave imaging

A high volume rate and high performance ultrasound imaging method based on a matrix array is proposed by using compressed sensing (CS) to reconstruct the complete dataset of synthetic transmit aperture (STA) from three-dimensional (3D) diverging wave transmissions (i.e. 3D CS-STA). Hereto, a series of apodized 3D diverging waves are transmitted from a fixed virtual source, with the ith row of a Hadamard matrix taken as the apodization coefficients in the ith transmit event. Then CS is used to reconstruct the complete dataset, based on the linear relationship between the backscattered echoes and the complete dataset of 3D STA. Finally, standard STA beamforming is applied on the reconstructed complete dataset to obtain the volumetric image. Four layouts of element numbering for apodizations and transmit numbers of 16, 32 and 64 are investigated through computer simulations and phantom experiments. Furthermore, the proposed 3D CS-STA setups are compared with 3D single-line-transmit (SLT) and 3D diverging wave compounding (DWC). The results show that, (i) 3D CS-STA has competitive lateral resolutions to 3D STA, and their contrast ratios (CRs) and contrast-to-noise ratios (CNRs) approach to those of 3D STA as the number of transmit events increases in noise-free condition. (ii) the tested 3D CS-STA setups show good robustness in complete dataset reconstruction in the presence of different levels of noise. (iii) 3D CS-STA outperforms 3D SLT and 3D DWC. More specifically, the 3D CS-STA setup with 64 transmit events and the Random layout achieves ~31% improvement in lateral resolution, ~14% improvement in ratio of the estimated-to-true cystic areas, a higher volume rate, and competitive CR/CNR when compared with 3D DWC. The results demonstrate that 3D CS-STA has great potential of providing high quality volumetric image with a higher volume rate.