Optimized Virtual Sources Distributions for 3-D Ultrafast Diverging Wave Compounding Imaging: A Simulation Study.

Ultrafast ultrasound imaging allows observing rapid phenomena; combined with 3-D imaging it has the potential to provide a more accurate analysis of organs which leads, in the end, to better diagnosis. Coherent compounding using diverging waves is commonly used to reconstruct high-quality images on large volumes while keeping the frame rate high enough to allow dynamic analysis. In practice, the virtual sources (VSs) that drive the diverging waves are often distributed in a deterministic way: following a regular grid, concentric rings, and spirals. Even though those deterministic distributions can offer various tradeoffs in terms of imaging performance, other distributions can be considered to improve imaging performance. It is herein suggested to look at alternative VSs distributions for optimizing the lateral resolution and the secondary lobes level (SLL) on several point spread functions (PSFs) by means of a multiobjective genetic algorithm. The optimization framework has led to seven pseudo-irregular distributions of VSs distributions that have not yet been found in the literature. An analysis of the imaging performance with a simulated phantom shows that these new distributions offer different tradeoffs between lateral resolution and contrast, respectively, measured on point-like reflectors and anechoic cysts. As an example, one of these optimized distributions improves the lateral resolution by 16% and gives equivalent contrast values on cysts and PSF isotropy properties, when compared to a concentric-rings-based distribution.

Regularization-Based 2D Strain Tensor Imaging in Quasi-Static Ultrasound Elastography SAGE Publications.

Accurately estimating all strain components in quasi-static ultrasound elastography is crucial for the full analysis of biological media. In this study, 2D strain tensor imaging was investigated, focusing on the use of a regularization method to improve strain images. This method enforces the tissue property of (quasi-) incompressibility, while penalizing strong field variations, to smooth the displacement fields and reduce the noise in the strain components. The performance of the method was assessed with numerical simulations, phantoms, and in vivo breast tissues. For all the media examined, the results showed a significant improvement in both lateral displacement and strain, while axial fields were only slightly modified by the regularization. The introduction of penalty terms allowed us to obtain shear strain and rotation elastograms where the patterns around the inclusions/lesions were clearly visible. In phantom cases, the findings were consistent with the results obtained from the modeling of the experiments. Finally, the easier detectability of the inclusions/lesions in the final lateral strain images was associated with higher elastographic contrast-to-noise ratios (CNRs), with values in the range of [0.54-9.57] versus [0.08-0.38] before regularization.

2D tissue strain tensor imaging in quasi-static ultrasound elastography.

Accurately estimating all strain components in quasi-static ultrasound elastography is crucial for the full analysis of biological media. In this paper, 2D strain tensor imaging is investigated, using a partial differential equation (PDE)-based regularization method. More specifically, this method employs the tissue property of incompressibility to smooth the displacement fields and reduce the noise in the strain components. The performance of the method is assessed with phantoms and in vivo breast tissues. For all the media examined, the results showed a significant improvement in both lateral displacement and strain but also, to a lesser extent, in the shear strain. Moreover, axial displacement and strain were only slightly modified by the regularization, as expected. Finally, the easier detectability of the inclusion/lesion in the final lateral strain images is associated with higher elastographic contrast-to-noise ratios (CNRs), with values in the range [0.68 - 9.40] vs [0.09 - 0.38] before regularization.

Quantitative assessment of media concentration using the Homodyned K distribution.

The Homodyned K distribution has been used successfully as a tool in the ultrasound characterization of sparse media, where the scatterer clustering parameter α accurately discriminates between media with different numbers of scatterers per resolution cell. However, as the number of scatterers increases and the corresponding amplitude statistics become Rician, the reliability of the α estimates decreases rapidly. In the present study, we assess the usefulness of α for the characterization of both sparse and concentrated media, using simulated independent and identically distributed (i.i.d.) samples from Homodyned K distributions, ultrasound images of media with up to 68 scatterers per resolution cell and ultrasound signals acquired from particle phantoms with up to 101 scatterers per resolution cell. All parameter estimates are obtained using the XU estimator (Destrempes et al., 2013). Results suggest that the parameter α can be used to distinguish between media with up to 40 scatterers per resolution cell at 22 MHz, provided that parameter estimation can be performed on very large sample sizes (i.e., >10,000 i.i.d. samples).

Spectral Doppler Measurements With 2-D Sparse Arrays.

The 2-D sparse arrays, in which a few hundreds of elements are distributed on the probe surface according to an optimization procedure, represent an alternative to full 2-D arrays, including thousands of elements usually organized in a grid. Sparse arrays have already been used in B-mode imaging tests, but their application to Doppler investigations has not been reported yet. Since the sparsity of the elements influences the acoustic field, a corresponding influence on the mean frequency (Fm), bandwidth (BW), and signal-to-noise ratio (SNR) of the Doppler spectra is expected. This article aims to assess, by simulations and experiments, to what extent the use of a sparse rather than a full gridded 2-D array has an impact on spectral Doppler measurements. Parabolic flows were investigated by a 3 MHz, 1024-element gridded array and by a sparse array; the latter was obtained by properly selecting a subgroup of 256 elements from the full array. Simulations show that the mean Doppler frequency does not change between the sparse and the full array while there are significant differences on the BW (average reduction of 17.2% for the sparse array, due to different apertures of the two probes) and on the signal power (Ps) (22 dB, due to the different number of active elements). These results are confirmed by flow phantom experiments, which also highlight that the most critical difference between sparse and full gridded array in Doppler measurements is in terms of SNR (-16.8 dB).

Quantitative Ultrasound in Ex Vivo Fibrotic Rabbit Livers.

Liver fibrosis is the common result of chronic liver disease. Diagnosis and grading liver fibrosis for patient management is mainly based on blood tests and hepatic puncture-biopsy, which is particularly invasive. Quantitative ultrasound (QUS) techniques provide insight into tissue microstructure and are based on the frequency-based analysis of the signals from biologic tissues. This study aims to quantify how spectral-based QUS parameters change with fibrosis grade. The changes in QUS parameters of healthy and fibrotic rabbit liver samples were investigated and were compared with the changes in liver stiffness, using shear wave elastography. Overall, the acoustic concentration was found to decrease with increasing fibrosis grade, and the effective scatterer size was found to be higher in fibrotic livers when compared with normal liver. The result of this study indicates that the combination of three QUS parameters (stiffness, effective scatterer size and acoustic concentration) provides the best classification performance, especially for classifying healthy and fibrotic livers.

Experimental Implementation of a Pulse Compression Technique Using Coherent Plane-Wave Compounding.

The axial resolution of an ultrasound imaging system is inversely proportional to the bandwidth of the emitted signal. When conventional pulsing (CP) is used, the impulse response of the transducer and the excitation signal determine together the shape of the emitted pulse and its bandwidth. A way to increase the ultrasound image resolution is to increase the transducer's limited passband. The resolution enhancement compression (REC) is a coding technique that boosts the signal energy in the transition frequency bands, where the energy transduction of the ultrasound probe is less efficient. Consequently, image quality metrics including axial resolution, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) can be improved. In this paper, the objective is to combine REC with coherent plane-wave compounding (CPWC) in order to achieve better image quality at an ultrafast acquisition rate. Promising results are obtained from both wire and cyst phantoms using an excitation signal designed to provide a 54% increase in bandwidth over the one obtained with a broadband pulse excitation at -6 dB. The experimental bandwidth measured from the backscattered echoes was improved by 49% for the wire phantom, when using the CPWC-REC technique compared to CPWC-CP. Furthermore, the axial resolution as derived from the modulation transfer function of the envelope of the wire target was enhanced by 29%. The CNR and SNR were improved up to 9 and up to 4 dB, respectively, in the cyst phantom. These results reveal that CPWC-REC is able to achieve higher spatial resolution, compared to CPWC-CP, with better SNR and CNR. Moreover, experimental results show that an effective implementation on a research scanner of REC using plane-wave imaging is possible. Consistent in vivo acquisition results on rabbit are presented and discussed.

Radiofrequency ultrasound data acquisition with a 640-element array transducer for strain imaging: Experimental results with phantoms and biological tissue samples.

This paper presents ultrasound elastography results obtained with a 640-element array transducer we have recently developed. This probe allows the acquisition of series of three adjacent imaging planes over time and therefore makes possible the computation of 2-D elastograms, with consideration of out-of-plane motion. In this study, elastography experiments were conducted on phantoms and bovine tissue samples, and compression was manually applied to the media via the hand-held ultrasound transducer. The results obtained with the proposed data acquisition and 3-D processing are presented and compared to those from a classical 2-D approach.

Specific Ultrasound Data Acquisition for Tissue Motion and Strain Estimation: Initial Results.

Ultrasound applications such as elastography can benefit from 3-D data acquisition and processing. In this article, we describe a specific ultrasound probe, designed to acquire series of three adjacent imaging planes over time. This data acquisition makes it possible to consider the out-of-plane motion that can occur at the central plane during medium scanning, and is proposed with the aim of improving the results of strain imaging. In this first study, experiments were conducted on phantoms, and controlled axial and elevational displacements were applied to the probe using a motorized system. Radiofrequency ultrasound data were acquired at a 40-MHz sampling frequency with an Ultrasonix ultrasound scanner, and processed using a 3-D motion estimation method. For each of the 2-D regions of interest of the central plane in pre-compression data, a 3-D search was run to determine its corresponding version in post-compression data, with this search taking into account the region-of-interest deformation model chosen. The results obtained with the proposed ultrasound data acquisition and strain estimation were compared with results from a classic approach and illustrate the improvement produced by considering the medium's local displacements in elevation, with notably an increase in the mean correlation coefficients achieved.

Fast Nonlinear Ultrasound Propagation Simulation Using a Slowly Varying Envelope Approximation.

Medical systems usually consider linear propagation of ultrasound, an approximation of reality. However, numerous studies have attempted to accurately simulate the nonlinear pressure wave distortion and to evaluate the contribution of harmonic frequencies. In such simulations, the computation time is very large, except for the method based on the angular spectrum scheme where the derivative order is reduced using the Fourier transform. However, the harmonic computation is usually limited to the second harmonic because of quasi-linear approximation. In this paper, a slowly varying envelope approximation (SVEA) is used in the Fourier domain to compute the entire nonlinear distortion induced, including high harmonics and nonlinear mixing frequencies. The simulation by SVEA is evaluated by comparison with other simulation tools. The obtained deviation and difference remain low enough to fully validate such an approximation. Moreover, the simulator is implemented on a GPU to obtain a very fast tool, where the full nonlinear distorted [Formula: see text] field is computed in less than 10 s.

Evaluation of NADPH oxidases as drug targets in a mouse model of familial amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) is an incurable neurodegenerative disease characterized by progressive loss of motor neurons, gliosis, neuroinflammation and oxidative stress. The aim of this study was to evaluate the involvement of NADPH oxidases (NOX) in the oxidative damage and progression of ALS neuropathology. We examined the pattern of NOX expression in spinal cords of patients and mouse models of ALS and analyzed the impact of genetic deletion of the NOX1 and 2 isoforms as well as pharmacological NOX inhibition in the SOD1(G93A) ALS mouse model. A substantial (10-60 times) increase of NOX2 expression was detected in three etiologically different ALS mouse models while up-regulation of some other NOX isoforms was model-specific. In human spinal cord samples, high NOX2 expression was detected in microglia. In contrast to previous publications, survival of SOD1(G93A) mice was not modified upon breeding with constitutive NOX1 and NOX2 deficient mice. As genetic deficiency of a single NOX isoform is not necessarily predictive of a pharmacological intervention, we treated SOD1(G93A) mice with broad-spectrum NOX inhibitors perphenazine and thioridazine. Both compounds reached in vivo CNS concentrations compatible with NOX inhibition and thioridazine significantly decreased superoxide levels in the spinal cord of SOD1(G93A) mice in vivo. Yet, neither perphenazine nor thioridazine prolonged survival. Thioridazine, but not perphenazine, dampened the increase of microglia markers in SOD1(G93A) mice. Thioridazine induced an immediate and temporary enhancement of motor performance (rotarod) but its precise mode of action needs further investigation. Additional studies using specific NOX inhibitors will provide further evidence on the relevance of NOX as drug targets for ALS and other neurodegenerative disorders.

Generalization of Multipulse Transmission Techniques for Ultrasound Imaging.

To increase the contrast-to-tissue ratio (CTR) in contrast imaging or the signal-to-noise ratio (SNR) in tissue harmonic imaging, many multipulse transmission techniques have been suggested. This article first recalls the various imaging techniques proposed in the literature and then presents a mathematical background to synthesize and generalize most of the multipulse ultrasound imaging techniques. The formulation presented can be used to predict the relative amplitude of the nonlinear components in each frequency band and to design new transmission sequences to either increase or decrease specified nonlinear components in each harmonic band. Simulation results on several multipulse techniques agree with the results from previous studies.

Thomson's multitaper approach combined with coherent plane-wave compounding to reduce speckle in ultrasound imaging.

In ultrasound imaging, the speckle pattern limits the image quality. Spatial and frequency compounding are commonly used to reduce speckle noise or improve the contrast. Although recent implementations can preserve a frame rate that is compatible with real-time imaging (e.g., synthetic aperture compounding), most classic compounding techniques are based on the coherent combination of several radiofrequency images from the same investigated area, which reduces the frame rate. Furthermore, Thomson's multitaper approach aims to smooth the speckle by incoherently combining the obtained B-mode images after applying different apodization windows on the same original data. With only one acquisition, the frame rate remains high, but the spatial resolution is decreased. To improve the resolution and contrast while reducing the speckle noise, this paper proposes combining the coherent plane-wave compounding technique (CPWC) with Thomson's multitaper method. The resulting multitaper coherent plane-wave compounding (MCPWC) takes advantage of coherent and incoherent approaches. Simulations and experimental results demonstrate that in terms of the signal-to-noise ratio, contrast, and resolution, the image quality is increased using plane wave emissions at approximately ten steering angles with three Thomson's tapers. Outside the focal area, the lateral resolution is improved by a factor of 2, and the contrast is increased by approximately 2dB compared with images obtained using a single focalization technique and Thomson's multitaper approach.

Evaluation of a frequency-domain ultrasonic imaging attenuation compensation technique.

Ultrasound attenuation is typically compensated for in clinical scanners by using time gain compensation (TGC). However, TGC operates in a frequency-independent fashion and therefore the spatial resolution of the echographic images degrades as the examination depth increases. In the current study, the capability of a multi-band attenuation compensation (MBAC) TGC technique to recover both magnitude and spatial resolution in lossy media was evaluated. Simulations were performed using a 5-MHz transducer for imaging point targets embedded in a medium with attenuation coefficient slope (ACS) of 0.5 dB/(cm.MHz). For performance assessment, the magnitude and spatial resolution of the reflected point spread functions (PSFs) were compared to the ones obtained from point targets embedded in a lossless medium. The results showed a complete recovery of the spectral content when using MBAC for all depths when compared to the lossless case. Both the magnitude and spatial resolution of the compensated PSFs were in agreement with the lossless result (i.e., less than 1 dB and 3 % difference in PSF magnitude and spatial resolution, respectively). The MBAC was then applied to in vivo liver imaging using a scanner equipped with a 5-MHz linear array. Attenuation compensation was performed using ACSs reported in the literature for skin, fat and muscle, and experimentally estimated ACS using the spectral log difference technique for the liver. The lateral and axial extent of the autocorrelation function was estimated in the liver tissue. The experimental MBAC image exhibited only 6 % and 11 % variation in speckle magnitude and lateral autocorrelation length for depths between 2.5 and 4 cm. These results suggest that MBAC technique may enhance speckle uniformity in homogeneous tissue regions.

Experimental evaluation of spectral-based quantitative ultrasound imaging using plane wave compounding.

Quantitative ultrasound (QUS) based on backscatter coefficient (BSC) estimation has shown potential for tissue characterization. Beamforming using plane wave compounding has advantages for echographic, Doppler, and elastographic imaging; however, to date, plane wave compounding has not been experimentally evaluated for the purpose of BSC estimation. In this study, two BSC-derived parameters (i.e., the BSC midband fit and intercept) were estimated from experimental data obtained using compound plane wave beamforming. For comparison, QUS parameters were also estimated from data obtained using both fixed focus and dynamic receive beamforming. An ultrasound imaging system equipped with a 9-MHz center frequency, 64-element array was used to collect data up to a depth of 45 mm. Two gelatin phantoms with randomly distributed 20-µm inclusions with a homogeneous scatterer concentration and a two-region scatterer concentration were used for assessing the precision and lateral resolution of QUS imaging, respectively. The use of plane wave compounding resulted in accurate QUS estimation (i.e., bias in the BSC parameters of less than 2 dB) and relatively constant lateral resolution (i.e., BSC midband fit 10% to 90% rise distance ranging between 1.0 and 1.5 mm) throughout a 45 mm field of view. Although both fixed focus and dynamic receive beamforming provided the same performance around the focal depth, the reduction in SNR away from the focus resulted in a reduced field of view in the homogeneous phantom (i.e., only 28 mm). The lateral resolution also degraded away from the focus, with up to a 2-fold and 10-fold increase in the rise distance at 20 mm beyond the focal depth for dynamic receive and fixed focus beamforming, respectively. These results suggest that plane wave compounding has the potential to improve the performance of spectral-based quantitative ultrasound over other conventional beamforming strategies.

Ultrasound contrast imaging: influence of scatterer motion in multi-pulse techniques.

In ultrasound contrast imaging, many techniques based on multiple transmissions have been proposed to increase the contrast-to-tissue ratio (CTR). They are generally based on the response of static scatterers inside the imaged region. However, scatterer motion, for example in blood vessels, has an inevitable influence on multi-pulse techniques, which can either enhance or degrade the technique involved. This paper investigates the response of static nonlinear media insonated by multi-pulses with various phase shifts, and the influence of scatterer motion on multi-pulse techniques. Simulations and experimental results from a single bubble and clouds of bubbles show that the phase shift of the echoes backscattered from bubbles is dependent on the transmissions' phase shift, and that the bubble motion influences the efficiency of multi-pulse techniques: fundamental and second-harmonic amplitudes of the processed signal change periodically, exhibiting maximum or minimum values, according to scatterer motion. Furthermore, experimental results based on the second-harmonic inversion (SHI) technique reveal that bubble motion can be taken into account to regulate the pulse repetition frequency (PRF). With the optimal PRF, the CTR of SHI images can be improved by about 12 dB compared with second-harmonic images.

CREANUIS: a non-linear radiofrequency ultrasound image simulator.

Nonlinear ultrasound methods are widely used in clinical applications for tissue or contrast harmonic imaging. Accurate non-linear imaging simulation tools are required in research studies for the development of new methods. However, in existing simulators, the possible inhomogeneity of the coefficient of non-linearity, which is generally observed in tissue and in particular when contrast agents are involved, has not yet been implemented. This article describes a new ultrasound simulator, called CREANUIS, devoted to the computation of B-mode images where both linear and non-linear propagation in media is considered, with a possible inhomogeneous coefficient of non-linearity. The resulting fundamental images, based on a spatially variant and non-linear point spread function, are in accordance with those obtained through the reference linear FieldII simulator, with computation time reduced by a factor of at least 1.8. Non-linear images of media exhibiting inhomogeneous coefficients of non-linearity are also provided. The simulation software can be freely downloaded from our website.

Frequency-domain-based strain estimation and high-frame-rate imaging for quasi-static elastography.

In freehand elastography, quasi-static tissue compression is applied through the ultrasound probe, and the corresponding axial strain is estimated by calculating the time shift between consecutive echo signals. This calculation typically suffers from a poor signal-to-noise ratio or from the decorrelation between consecutive echoes resulting from an erroneous axial motion impressed by the operator. This paper shows that the quality of elastograms can be improved through the integration of two distinct techniques in the strain estimation procedure. The first technique evaluates the displacement of the tissue by analyzing the phases of the echo signal spectra acquired during compression. The second technique increases the displacement estimation robustness by averaging multiple displacement estimations in a high-frame-rate imaging system, while maintaining the typical elastogram frame-rate. The experimental results, obtained with the Ultrasound Advanced Open Platform (ULA-OP) and a cyst phantom, demonstrate that each of the proposed methods can independently improve the quality of elastograms, and that further improvements are possible through their combination.

A restoration framework for ultrasonic tissue characterization.

Ultrasonic tissue characterization has become an area of intensive research. This procedure generally relies on the analysis of the unprocessed echo signal. Because the ultrasound echo is degraded by the non-ideal system point spread function, a deconvolution step could be employed to provide an estimate of the tissue response that could then be exploited for a more accurate characterization. In medical ultrasound, deconvolution is commonly used to increase diagnostic reliability of ultrasound images by improving their contrast and resolution. Most successful algorithms address deconvolution in a maximum a posteriori estimation framework; this typically leads to the solution of l(2)-norm or (1)-norm constrained optimization problems, depending on the choice of the prior distribution. Although these techniques are sufficient to obtain relevant image visual quality improvements, the obtained reflectivity estimates are, however, not appropriate for classification purposes. In this context, we introduce in this paper a maximum a posteriori deconvolution framework expressly derived to improve tissue characterization. The algorithm overcomes limitations associated with standard techniques by using a nonstandard prior model for the tissue response. We present an evaluation of the algorithm performance using both computer simulations and tissue-mimicking phantoms. These studies reveal increased accuracy in the characterization of media with different properties. A comparison with state-of-the-art Wiener and l(1)-norm deconvolution techniques attests to the superiority of the proposed algorithm.

Fundamental and second-harmonic ultrasound field computation of inhomogeneous nonlinear medium with a generalized angular spectrum method.

The simulation of nonlinear propagation of ultrasound waves is typically based on the Kuznetsov-Zabolotskaya- Khokhlov equation. A set of simulators has been proposed in the literature but none of them takes into account a possible spatial 3-D variation of the nonlinear parameter in the investigated medium. This paper proposes a generalization of the angular spectrum method (GASM) including the spatial variation of the nonlinear parameter. The proposed method computes the evolution of the fundamental and second-harmonic waves in four dimensions (spatial 3-D and time). For validation purposes, the one-way fields produced by the GASM are first compared with those produced by established reference simulators and with experimental one-way fields in media with a homogeneous nonlinear parameter. The same simulations are repeated for media having an axial variation of the nonlinear parameter. The mean errors estimated in the focal region are less than 4.0% for the fundamental and 5.4% for the second harmonic in all cases. Finally, the fundamental and second-harmonic fields simulated for media having nonlinear parameter variations in the axial, lateral, and elevation directions, which cannot be simulated with other currently available methods, are presented. The new approach is also shown to yield a reduction in computation time by a factor of 13 with respect to the standard nonlinear simulator.