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Breast cancer is one of the most diagnosed types of cancer worldwide. Volumetric ultrasound breast imaging, combined with MRI can improve lesion detection rate, reduce examination time, and improve lesion diagnosis. However, to our knowledge, there are no 3D US breast imaging systems available that facilitate 3D US - MRI image fusion. In this paper, a novel Automated Cone-based Breast Ultrasound System (ACBUS) is introduced. The system facilitates volumetric ultrasound acquisition of the breast in a prone position without deforming it by the US transducer. Quality of ACBUS images for reconstructions at different voxel sizes (0.25 and 0.50 mm isotropic) was compared to quality of the Automated Breast Volumetric Scanner (ABVS) (Siemens Ultrasound, Issaquah, WA, USA) in terms of signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and resolution using a custom made phantom. The ACBUS image data were registered to MRI image data utilizing surface matching and the registration accuracy was quantified using an internal marker. The technology was also evaluated in vivo. The phantom-based quantitative analysis demonstrated that ACBUS can deliver volumetric breast images with an image quality similar to the images delivered by a currently commercially available Siemens ABVS. We demonstrate on the phantom and in vivo that ACBUS enables adequate MRI-3D US fusion. To our conclusion, ACBUS might be a suitable candidate for a second-look breast US exam, patient follow-up, and US guided biopsy planning.

The female pelvic floor (PF) muscles provide support to the pelvic organs. During delivery, some of these muscles have to stretch up to three times their original length to allow passage of the baby, leading frequently to damage and consequently later-life PF dysfunction (PFD). Three-dimensional (3D) ultrasound (US) imaging can be used to image these muscles and to diagnose the damage by assessing quantitative, geometric and functional information of the muscles through strain imaging. In this study we developed 3D US strain imaging of the PF muscles and explored its application to the puborectalis muscle (PRM), which is one of the major PF muscles.

Biopsy of breast lesions only visible on MRI may benefit from patient-specific finite-element modeling of the breast. This requires acquisition of the heterogeneous elasticity distribution. An acoustically transparent pad has been developed which enables strain elastography imaging using ultrasound without need for additional force or pressure sensors. The elasticity of the pad is known a priori while the change in thickness is detectable on ultrasound by edge detection. Based on the calculated pressure distribution across the pad, the pressure in inner tissue can be approximated. The strain is measured using a cross-correlation block matching algorithm and from these measurements the heterogeneous elasticity distribution can be estimated. The ultrasound probe with pad is moved over the breast surface by a robot arm. At sixteen sites on the surface of one phantom the probe with pad performed an inward push movement perpendicular to the breast surface. The ultrasound scans were post-processed to reconstruct a heterogeneous elasticity map in the local imaging plane. While it is challenging to accurately estimate the stress and strain in deeper tissue, certain features such as the skin can be effectively distinguished from the underlying soft tissue which demonstrates the potential of the system.

Atherosclerotic plaque rupture in coronary arteries, an important trigger of myocardial infarction, is shown to correlate with high levels of pressure-induced mechanical stresses in plaques. Finite element (FE) analyses are commonly used for plaque stress assessment. However, the required information of heterogenous material properties of atherosclerotic coronaries remains to be scarce. In this work, we characterized the component-wise mechanical properties of atherosclerotic human coronary arteries. To achieve this, we performed ex vivo inflation tests on post-mortem human coronary arteries and developed an inverse FE modeling (iFEM) pipeline, which combined high-frequency ultrasound deformation measurements, a high-field magnetic resonance-based artery composition characterization, and a machine learning-based Bayesian optimization (BO) with uniqueness assessment. By using the developed pipeline, 10 cross-sections from five atherosclerotic human coronary arteries were analyzed, and the Yeoh material model constants of the fibrous intima and arterial wall components were determined. This work outlines the developed pipeline and provides the knowledge of non-linear, multicomponent mechanical properties of atherosclerotic human coronary arteries.

Objective Plaque rupture in atherosclerotic carotid arteries is a main cause of ischemic stroke and it is correlated with high plaque stresses. Hence, analyzing stress patterns is essential for plaque specific rupture risk assessment. However, the critical information of the multicomponent material properties of atherosclerotic carotid arteries, is still lacking greatly. This work aims to characterize component-wise material properties of atherosclerotic human carotid arteries under (almost) physiological loading conditions. Methods An inverse finite element modeling (iFEM) framework was developed to characterize fibrous intima and vessel wall material properties of 13 cross sections from five carotids. The novel pipeline comprised ex-vivo inflation testing, pre-clinical high frequency ultrasound for deriving plaque deformations, pre-clinical high-magnetic field magnetic resonance imaging, finite element modeling, and a sample efficient machine learning based Bayesian Optimization. Results The nonlinear Yeoh constants for the fibrous intima and wall layers were successfully obtained. The optimization scheme of the iFEM reached the global minimum with a mean error of 3.8% in 133 iterations on average. The uniqueness of the results were confirmed with the inverted Gaussian Process (GP) model trained during the iFEM protocol. Conclusion The developed iFEM approach combined with the inverted GP model successfully predicted component-wise material properties of intact atherosclerotic human carotids ex-vivo under physiological-like loading conditions. Significance We developed a novel iFEM framework for the nonlinear, component-wise material characterization of atherosclerotic arteries and utilized it to obtain human atherosclerotic carotid material properties. The developed iFEM framework has great potential to be advanced for patient-specific in-vivo application.

Atherosclerotic plaque development in the carotid artery bifurcation elevates the risk for stroke, which is often initiated by plaque rupture. The risk-to-rupture of a plaque is related to its composition. Two-dimensional non-invasive carotid elastography studies have found a correlation between wall strain and plaque composition. This study introduces a technique to perform non-invasive volumetric elastography in vivo. Three-dimensional ultrasound data of carotid artery bifurcations were acquired in four asymptomatic individuals using an electrocardiogram-triggered multislice acquisition device that scanned over a length of 35 mm (350 slices) using a linear transducer (L11-3, fc = 9 MHz). For each slice, three-angle ultrasound plane wave data were acquired and beamformed. A correction for breathing-induced motion was applied to spatially align the slices, enabling 3-D cross-correlation-based compound displacement, distensibility and strain estimation. Distensibility values matched with previously published values, while the corresponding volumetric principal strain maps revealed locally elevated compressive and tensile strains. This study presents for the first time 3-D elastography of carotid arteries in vivo.

Visualization and quantification of blood flow are considered important for early detection of atherosclerosis and patient-specific diagnosis and intervention. As conventional Doppler imaging is limited to 1-D velocity estimates, 2-D and 3-D techniques are being developed. We introduce an adaptive velocity compounding technique that estimates the 2-D velocity vector field using predominantly axial displacements estimated by speckle tracking from dual-angle plane wave acquisitions. Straight-vessel experiments with a 7.8-MHz linear array transducer connected to a Verasonics Vantage ultrasound system revealed that the technique performed with a maximum velocity magnitude bias and angle bias of -3.7% (2.8% standard deviation) and -0.16° (0.41° standard deviation), respectively. In vivo, complex flow patterns were visualized in two healthy and three diseased carotid arteries and quantified using a vector complexity measure that increased with increasing wall irregularity. This measure could potentially be a relevant clinical parameter which might aid in early detection of atherosclerosis.

Although conventional Doppler ultrasound is widely used for quantifying blood flow, it is restricted by its low sensitivity to detect slow flow. The incorporation of ultrafast ultrasound and spatial-temporal clutter filters can not only extensively boost the Doppler sensitivity to low-velocity slow flow but also facilitate the development of advanced 3-D Doppler techniques. In this work, we propose a novel 3-D Doppler method which extends 2-D imaging to 3-D through the continuous mechanical translation of a linear transducer. The viability of this method is assessed by simulations with the aids of a theoretical model. The combination of simulations and the theoretical model provides unique insights into the inherent mechanisms involved in the performance of this 3-D Doppler method and the roles of factors, such as tissue vibration characteristics, blood flow velocity, elevational point-spread-function profile, probe translating speed, and signal energy ratios.

Adding volumetric ultrasound (3DUS) to MRI improves cancer detection rate in a breast. However, the fusion of 3DUS and MRI is challenging due to the different position (supine versus prone, respectively) and high deformation of a breast in the existing 3DUS scanners. In this study, we present and quantitatively evaluate a novel robot-assisted breast scanning system for 3D US acquisitions. Since the breast is scanned in a prone position with minimal and measurable deformation, this can facilitate MRI - 3DUS fusion. For quantitative evaluation, a breast-shaped rigid polyvinyl-alcohol phantom was constructed containing spherical lesions (15 mm diameter) with different echogenicity (-22 dB, -5 dB, -4 dB, 3 dB, 7 dB). First, the phantom was scanned with a Siemens Skyra 3T MRI (Siemens Healthcare, Erlangen, Germany). Next, the phantom was scanned by a robotic arm (KUKA, Augsburg, Germany) (Fig. 1a.) following a pre-planned spiral trajectory, determined from the MRI volume. The flange of the robot was equipped with an L10-5v US transducer attached to a P500 system (Siemens, Mountain View, CA, US). 2D B-mode US data were acquired for 3 minutes at 12 fps. The imaging depth was 5 cm, and the focal depth was set at 2 cm. A volume of 77 × 77 × 69 mm3 was reconstructed with an isotropic sampling distance of 0.2 mm utilizing a voxel nearest neighbor method with a subsequent 'hole filling' step, i.e. interpolation. The contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR) were calculated per lesion and compared to reference values from the originally acquired 2D B-mode images. The 3D US volume was registered to the MRI volume by using 4 lesions as landmark, using rigid registration. The distance between the centers of the remaining lesion in MRI and 3DUS after the registration was calculated. The measurement was repeated for 5 combinations between lesions and landmarks. The average distance was used as a measure of registration accuracy. For the reconstructed volume only 24 % of the data were obtained by interpolation. On average, the CNR and SNR were 24% and 6% higher, respectively, for the 3DUS compared to the reference. The registration accuracy was 3.4 mm; hence, the presented scanning approach enables ultrasound breast 3D imaging in prone position facilitating MRI - 3D US fusion.

In ultrasound elastography, plane-wave acquisitions and angular displacement compounding (ADC) are often used and combined to allow high frame rates and to improve accuracy of lateral displacement estimates, respectively. This study investigates the performance of displacement and strain estimation for ADC as a function of; the main-to-grating-lobe-amplitude ratio which decreases as a function of steering angle; plane-wave acquisition and Delay-and-Sum (DaS)-related parameters; and grating-lobe filter cut-off frequency. Three experiments were conducted with a block phantom to test ADC performance for displacement fields of varying complexity: a lateral transducer shift, phantom rotation and phantom deformation. Experiments were repeated for four linear array transducers (pitch-to-lambda ratios between 0.6 and 1.4). Best ADC performance was found for steering angles that resulted in a theoretically derived main-to-grating-lobe-amplitude ratio of 1.7 dB for pure lateral translation and 6 dB for predominately lateral strain or rotation. Temporal filtering to reduce grating lobe signal or shifting of the receive aperture to receive angles below or above the optimal angle, as dictated by the main-to-grating-lobe-amplitude ratio, did not improve results. The accuracy of lateral displacement and strain estimates was improved by apodization in transmission and a dedicated F-number in DaS (0.75) allowing incidence angles within ± 33° in the active aperture. ADC with the optimized settings as found in this study improves the accuracy of displacements and strain estimates up to 80.7% compared to non-ADC. Compared to ADC settings described in current literature, our optimization improved the accuracy by 11.9% to 75.3% for lateral displacement and strain, and by 89.3% to 96.2% for rotation. The accuracy of ADC in rotation seemed to depend highly on plane-wave and DaS-related parameters which may explain the major improvement compared to settings in current literature. The overall improvement by optimized ADC was statistically significant compared to non-ADC (p = 0.003) and literature (p = 0.002).

Contains fulltext : 225825.pdf (Publisher’s version ) (Open Access) Ischemic stroke is a leading cause of death and disability worldwide, so adequate prevention strategies are crucial. However, current stroke risk stratification is based on epidemiologic studies and is still suboptimal for individual patients. The aim of this systematic review was to provide a literature overview on the feasibility and diagnostic value of vascular shear wave elastography (SWE) using ultrasound (US) in (mimicked) human and non-human arteries affected by different stages of atherosclerotic diseases or diseases related to atherosclerosis. An online search was conducted on Pubmed, Embase, Web of Science and IEEE databases to identify studies using US SWE for the assessment of vascular elasticity. A quality assessment was performed using Quality Assessment of Diagnostic Accuracy Studies 2 (QUADAS-2) checklist, and relevant data were extracted. A total of 19 studies were included: 10 with human patients and 9 with non-human subjects (i.e., [excised] animal arteries and polyvinyl alcohol phantoms). All studies revealed the feasibility of using US SWE to assess individually stiffness of the arterial wall and plaques. Quantitative elasticity values were highly variable between studies. However, within studies, SWE could detect statistically significant elasticity differences in patient/subject characteristics and could distinguish different plaque types with good reproducibility. US SWE, with its unique ability to assess the elasticity of the vessel wall and plaque throughout the cardiac cycle, might be a good candidate to improve stroke risk stratification. However, more clinical studies have to be performed to assess this technique's exact clinical value.

The point spread function (PSF) is often analyzed to determine the image quality of an ultrasound system. The formation of PSF is determined by practical factors, such as transducer aperture, element directivity, apodization, pitch, imaging position, and steering angle. Conventional numerical simulations provide an iterative approach to examine those factors' effects but cannot explain the inherent mechanism of PSF formation. This article presents a theoretical approximation of PSF formation for plane-wave imaging throughout the Fourier-based reconstruction process. Aforementioned factors are incorporated in the theory. The proposed theory is used to analyze the effects of those factors and presents a high degree of consistency with numerical simulations and experiments.

Automated breast volume scanner (ABVS) is an ultrasound imaging modality used in breast cancer screening. It has high sensitivity but limited specificity as it is hard to discriminate between benign and malignant lesions by echogenic properties. Specificity might be improved by shear strain imaging as malignant lesions, firmly bonded to its host tissue, show different shear patterns compared to benign lesions, often loosely bonded. Therefore, 3-D quasi-static elastography was implemented in an ABVS-like system. Plane wave instead of conventional focused transmissions were used to reduce scan times within a single breath hold. A 3-D strain tensor was obtained and shear strains were reconstructed in phantoms containing firmly and loosely bonded lesions. Experiments were also simulated in finite-element models (FEMs). Experimental results, confirmed by FEM-results, indicated that loosely bonded lesions showed increased maximal shear strains (~2.5%) and different shear patterns compared to firmly bonded lesions (~0.9%). To conclude, we successfully implemented 3-D elastography in an ABVS-like system to assess lesion bonding by shear strain imaging.

Plaque strain and blood vector velocity imaging of stenosed arteries are expected to aid in diagnosis and prevention of cerebrovascular disease. Ultrafast plane wave imaging enables simultaneous strain and velocity estimation. Multiple ultrasound vendors are introducing high frequency ultrasound probes and systems. This study investigates whether the use of high frequency ultrafast ultrasound is beneficial for assessing blood velocities and strain in arteries. The performance of strain and blood flow velocity estimation was compared between a high frequency transducer (MS250, fc =21 MHz) and a clinically utilized transducer (L12-5, fc =9 MHz). Quantitative analysis based on straight tube phantom experiments revealed that the MS250 outperformed the L12-5 in the superficial region: low velocities near the wall were more accurately estimated and wall strains were better resolved. At greater than 2 cm echo depth, the L12-5 performed better, due to the high attenuation of the MS250 probe. Qualitative comparison using a perfused patient-specific carotid bifurcation phantom confirmed these findings. Thus, in conclusion, for strain and blood velocity estimation for depths up to ~2 cm a high frequency probe is recommended.

Conventional color Doppler imaging is limited, since it only provides velocity estimates along the ultrasound beam direction for a restricted field of view at a limited frame rate. High-frame-rate speckle tracking, using plane wave transmits, has shown potential for 2-D blood velocity estimation. However, due to the lack of focusing in transmit, image quality gets reduced, which hampers speckle tracking. Although ultrafast imaging facilitates improved clutter filtering, it still remains a major challenge in blood velocity estimation. Signal dropouts and poor velocity estimates are still present for high beam-to-flow angles and low blood flow velocities. In this paper, ultrafast plane wave imaging was combined with multiscale speckle tracking to assess the 2-D blood velocity vector in a common carotid artery (CCA) flow field. A multiangled plane wave imaging sequence was used to compare the performance of displacement compounding, coherent compounding, and compound speckle tracking. Zero-degree plane wave imaging was also evaluated. The performance of the methods was evaluated before and after clutter filtering for the large range of velocities (0-1.5 m/s) that are normally present in a healthy CCA during the cardiac cycle. An extensive simulation study was performed, based on a sophisticated model of the CCA, to investigate and evaluate the performance of the methods at different pulse repetition frequencies and signal-to-noise levels. In vivo data were acquired of a healthy carotid artery bifurcation to support the simulation results. In general, methods utilizing compounding after speckle tracking, i.e., displacement compounding and compound speckle tracking, were least affected by clutter filtering and provided the most robust and accurate estimates for the entire velocity range. Displacement compounding, which uses solely axial information to estimate the velocity vector, provided most accurate velocity estimates, although it required sufficiently high pulse repetition frequencies in high blood velocity phases and reliable estimates for all acquisition angles. When this latter requirement was not met, compound speckle tracking was most accurate, because it uses the possibility to discard angular velocity estimates corrupted by clutter filtering. Similar effects were observed for in vivo data obtained at the carotid artery bifurcation. Investigating a combination of these two compounding techniques is recommended for future research.

Item does not contain fulltext Cardiovascular disease (CVD) is a leading cause of death and is in the majority of cases due to the formation of atherosclerotic plaques in arteries. Initially, thickening of the inner layer of the arterial wall occurs. Continuation of this process leads to plaque formation. The risk of a plaque to rupture and thus to induce an ischemic event is directly related to its composition. Consequently, characterization of the plaque composition and its proneness to rupture are of crucial importance for risk assessment and treatment strategies. The carotid is an excellent artery to be imaged with ultrasound because of its superficial position. In this review, ultrasound-based methods for characterizing the mechanical properties of the carotid wall and atherosclerotic plaque are discussed. Using conventional echography, the intima media thickness (IMT) can be quantified. There is a wealth of studies describing the relation between IMT and the risk for myocardial infarction and stroke. Also the carotid distensibility can be quantified with ultrasound, providing a surrogate marker for the cross-sectional mechanical properties. Although all these parameters are associated with CVD, they do not easily translate to individual patient risk. Another technique is pulse wave velocity (PWV) assessment, which measures the propagation of the pressure pulse over the arterial bed. PWV has proven to be a marker for global arterial stiffness. Recently, an ultrasound-based method to estimate the local PWV has been introduced, but the clinical effectiveness still needs to be established. Other techniques focus on characterization of plaques. With ultrasound elastography, the strain in the plaque due to the pulsatile pressure can be quantified. This technique was initially developed using intravascular catheters to image coronaries, but recently noninvasive methods were successfully developed. A high correlation between the measured strain and the risk for rupture was established. Acoustic radiation force impulse (ARFI) imaging also provides characterization of local plaque components based on mechanical properties. However, both elastography and ARFI provide an indirect measure of the elastic modulus of tissue. With shear wave imaging, the elastic modulus can be quantified, although the carotid artery is one of the most challenging tissues for this technique due to its size and geometry. Prospective studies still have to establish the predictive value of these techniques for the individual patient. Validation of ultrasound-based mechanical characterization of arteries and plaques remains challenging. Magnetic resonance imaging is often used as the "gold" standard for plaque characterization, but its limited resolution renders only global characterization of the plaque. CT provides information on the vascular tree, the degree of stenosis, and the presence of calcified plaque, while soft plaque characterization remains limited. Histology still is the gold standard, but is available only if tissue is excised. In conclusion, elastographic ultrasound techniques are well suited to characterize the different stages of vascular disease.

Item does not contain fulltext In breast cancer screening, the automated breast volume scanner (ABVS) was introduced as an alternative for mammography since the latter technique is less suitable for women with dense breasts. Although clinical studies show promising results, clinicians report two disadvantages: long acquisition times (>90 s) introducing breathing artefacts, and high recall rates due to detection of many small lesions of uncertain malignant potential. Technical improvements for faster image acquisition and better discrimination between benign and malignant lesions are thus required. Therefore, the aim of this study was to investigate if 3D ultrasound elastography using plane-wave imaging is feasible. Strain images of a breast elastography phantom were acquired by an ABVS-mimicking device that allowed axial and elevational movement of the attached transducer. Pre- and post-deformation volumes were acquired with different constant speeds (between 1.25 and 40.0 mm s(-1)) and by three protocols: Go-Go (pre- and post-volumes with identical start and end positions), Go-Return (similar to Go-Go with opposite scanning directions) and Control (pre- and post-volumes acquired per position, this protocol can be seen as reference). Afterwards, 2D and 3D cross-correlation and strain algorithms were applied to the acquired volumes and the results were compared. The Go-Go protocol was shown to be superior with better strain image quality (CNRe and SNRe) than Go-Return and to be similar as Control. This can be attributed to applying opposite mechanical forces to the phantom during the Go-Return protocol, leading to out-of-plane motion. This motion was partly compensated by using 3D cross-correlation. However, the quality was still inferior to Go-Go. Since these results were obtained in a phantom study with controlled deformations, the effect of possible uncontrolled in vivo tissue motion artefacts has to be addressed in future studies. In conclusion, it seems feasible to implement 3D ultrasound quasi-static elastography on an ABVS-like system and to reduce scan times within one breath-hold (~10 s) by plane-wave acquisitions.

With an automated breast volume scanner (ABVS), a volumetric breast image can be constructed. Clinical studies show high sensitivity but also high recall-rates due to the detection of many lesions of uncertain malignant potential. It is known that breast lesions have different mechanical properties with respect to its surrounding tissue. Consequently, imaging the biomechanics of a breast might improve diagnosis in this disease. First, we improved the image quality by taking full advantage of the ultrafast acquisition and by applying dedicated signal processing techniques. Next, by imaging biomechanics we increased the specificity since compared to benign lesions, malignant lesions are often stiffer, and more grown into the surrounding tissue (firmly bonded). We extended conventional 2-D strain imaging to full 3-D (shear) strain imaging by combining automated breast volume scanning with ultrafast plane wave imaging (10 000 frames/s). Validation studies in breast phantoms revealed that lesions could be differentiated based on strain and shear strain imaging. Decreased strain values were found in stiff lesions and firmly bonded lesions demonstrated low shear strain values. Initial acquisitions in patients demonstrated decreasing strain values with increasing severity of the disease. Additionally, we developed an ultrasensitive 3-D Doppler method to detect neo-vascularization that might be present around malignant lesions. In conclusion, imaging the biomechanical properties in 3-D improves diagnosis breast cancer.With an automated breast volume scanner (ABVS), a volumetric breast image can be constructed. Clinical studies show high sensitivity but also high recall-rates due to the detection of many lesions of uncertain malignant potential. It is known that breast lesions have different mechanical properties with respect to its surrounding tissue. Consequently, imaging the biomechanics of a breast might improve diagnosis in this disease. First, we improved the image quality by taking full advantage of the ultrafast acquisition and by applying dedicated signal processing techniques. Next, by imaging biomechanics we increased the specificity since compared to benign lesions, malignant lesions are often stiffer, and more grown into the surrounding tissue (firmly bonded). We extended conventional 2-D strain imaging to full 3-D (shear) strain imaging by combining automated breast volume scanning with ultrafast plane wave imaging (10 000 frames/s). Validation studies in breast phantoms revealed that lesions could be diff...

In this paper, a novel processing framework is introduced for Fourier-domain beamforming of plane-wave ultrasound data, which incorporates coherent compounding and angular weighting in the Fourier domain. Angular weighting implies spectral weighting by a 2-D steering-angle-dependent filtering template. The design of this filter is also optimized as part of this paper. Two widely used Fourier-domain plane-wave ultrasound beamforming methods, i.e., Lu's f-k and Stolt's f-k methods, were integrated in the framework. To enable coherent compounding in Fourier domain for the Stolt's f-k method, the original Stolt's f-k method was modified to achieve alignment of the spectra for different steering angles in k-space. The performance of the framework was compared for both methods with and without angular weighting using experimentally obtained data sets (phantom and in vivo), and data sets (phantom) provided by the IEEE IUS 2016 plane-wave beamforming challenge. The addition of angular weighting enhanced the image contrast while preserving image resolution. This resulted in images of equal quality as those obtained by conventionally used delay-and-sum (DAS) beamforming with apodization and coherent compounding. Given the lower computational load of the proposed framework compared to DAS, to our knowledge it can, therefore, be concluded that it outperforms commonly used beamforming methods such as Stolt's f-k, Lu's f-k, and DAS.

Collecting high quality volumetric ultrasound (US) data using freehand scanning is challenging. The quality of the final 3DUS image is highly related to the applied scanning protocol and the subsequently used reconstruction method. The protocol should ensure the sonographer collects sufficient data of satisfactory quality for an accurate reconstruction. In this study we developed a real-time reconstruction method that provides visual feedback during scanning. The feedback indicates the areas, of which the sonographer should collect more data. The method was tested by acquiring US data of a breast phantom in a setup mimicking freehand scanning which consisted of a linear transducer mounted in a translation stage that also allowed rotation. To reconstruct the volume in real-time on a target grid of 0.5x0.5x0.5mm, we applied a simplified Voxel Nearest Neighbor (VNN) method, i.e., only the closest to B-mode plane voxels were updated. Furthermore, voxels were updated only when their projection on the B-mode plane was closer to the transducer surface than in the previous scan planes. Interpolation was performed within the acquired volume to fill in the holes where sufficient data were available. Sub-volumes with insufficient data were visualized in the reconstructed volume (update rate 50 Hz). This visual feedback can guide the sonographer during freehand scanning to improve the quality of the reconstructed 3DUS images. Cross-sections of the reconstructed data were compared to the independently acquired B-mode images and confirmed that our real-time method of low computational complexity provided accurate volumetric ultrasound images.

Quasi-static elastography is an ultrasound method which is widely used to assess displacements and strain in tissue by correlating ultrasound data at different levels of deformation. Ultrafast plane-wave imaging allows us to obtain ultrasound data at frame rates over 10 kHz, permitting the quantification and visualization of fast deformations. Currently, mainly three beam-forming strategies are used to reconstruct radio frequency (RF) data from plane-wave acquisitions: delay-and-sum (DaS), and Lu's-fk and Stolt's-fk operating in the temporal-spatial and Fourier spaces, respectively. However, the effect of these strategies on elastography is unknown. This study investigates the effect of these beam-forming strategies on the accuracy of displacement estimation in four transducers (L7-4, 12L4VF, L12-5, MS250) for various reconstruction line densities and apodization/filtering settings. A method was developed to assess the accuracy experimentally using displacement gradients obtained in a rotating phantom. A line density with multiple lines per pitch resulted in increased accuracy compared to one line per pitch for all transducers and strategies. The impact on displacement accuracy of apodization/filtering varied per transducer. Overall, Lu's-fk beam-forming resulted in the most accurate displacement estimates. Although DaS in some cases provided similar results, Lu's-fk is more computationally efficient, leading to the conclusion that Lu's-fk is most optimal for plane wave ultrasound-based elastography.

This paper presents a framework for motion estimation in ultrafast ultrasound data. It describes a novel approach for determining the sampling grid for ultrafast data based on the system’s point-spread-function (PSF). As a consequence, the cross-correlation functions (CCF) used in the speckle tracking (ST) algorithm will have circular-shaped peaks, which can be interpolated using a 2D interpolation method to estimate subsample displacements. Carotid artery wall motion and parabolic blood flow simulations together with rotating disk experiments using a Verasonics Vantage 256 are used for performance evaluation. Zero-degree plane wave data were acquired using an ATL L5-12 (fc = 9 MHz) transducer for a range of pulse repetition frequencies (PRFs), resulting in 0–600 µm inter-frame displacements. The proposed methodology was compared to data beamformed on a conventionally spaced grid, combined with the commonly used 1D parabolic interpolation. The PSF-shape-based beamforming grid combined with 2D cubic interpolation showed the most accurate and stable performance with respect to the full range of inter-frame displacements, both for the assessment of blood flow and vessel wall dynamics. The proposed methodology can be used as a protocolled way to beamform ultrafast data and obtain accurate estimates of tissue motion.

The lack of a standard format for storing ultrasound research data is hindering our ability to share and compare research results. This is slowing down the progress in our field, making it difficult to assess the relevance of new techniques. In October 2017 the Ultrasound File Format (UFF) initiative was formed with the aim of defining such a standard, with the support of eleven research groups. Here we present some of the components of the first draft of the UFF format.

Contains fulltext : 167164.pdf (Publisher’s version ) (Open Access) Biomechanical models have the potential to predict plaque rupture. For reliable models, correct material properties of plaque components are a prerequisite. This study presents a new technique, where high resolution ultrasound displacement imaging and inverse finite element (FE) modeling is combined, to estimate material properties of plaque components. Iliac arteries with plaques were excised from 6 atherosclerotic pigs and subjected to an inflation test with pressures ranging from 10 to 120 mmHg. The arteries were imaged with high frequency 40 MHz ultrasound. Deformation maps of the plaques were reconstructed by cross correlation of the ultrasound radiofrequency data. Subsequently, the arteries were perfusion fixed for histology and structural components were identified. The histological data were registered to the ultrasound data to construct FE model of the plaques. Material properties of the arterial wall and the intima of the atherosclerotic plaques were estimated using a grid search method. The computed displacement fields showed good agreement with the measured displacement fields, implying that the FE models were able to capture local inhomogeneities within the plaque. On average, nonlinear stiffening of both the wall and the intima was observed, and the wall of the atheroslcerotic porcine iliac arteries was markedly stiffer than the intima (877 +/- 459 vs. 100 +/- 68 kPa at 100 mmHg). The large spread in the data further illustrates the wide variation of the material properties. We demonstrated the feasibility of a mixed experimental-numerical framework to determine the material properties of arterial wall and intima of atherosclerotic plaques from intact arteries, and concluded that, due to the observed variation, plaque specific properties are required for accurate stress simulations.

Quasi-static elastography is a widely applied ultrasound method in which RF data acquired in tissue at different states of deformation are correlated to estimate displacements and strain (displacement gradient). A recent development is the introduction of ultrafast plane-wave imaging where element data are beam-formed after collection to reconstruct the image lines. Several beam-forming strategies are available: time-domain based delay-and-sum (DAS) based on time-of-flight, and frequency domain based techniques like Stolt's f-k (Stolt) and Lu's method (Lu) based on an exploding reflector model and ideal transmit-receive model, respectively. Plane-wave beam-forming allows arbitrary grid designs not limited by probe pitch etc. In this study, we investigated the influence of beam-forming strategies and grid designs on the performance of displacement estimation.

Atherosclerosis in the carotid artery (CA) elevates the risk for cerebral events. Strain imaging has demonstrated to be a technique capable of identifying plaque composition. For strain imaging of carotid cross-sections, compound techniques have been developed to solve the poor strain estimation quality perpendicular to the ultrasound beam direction. This study assesses the performance of radial and circumferential strain imaging in 4D derived from multi-plane acquisitions with a single transducer with and without compounding and using two orthogonally placed transducers.

Quantification of local arterial wall elasticity may assist in differentiating lipid rich rupture prone atherosclerotic plaques from stable fibrous plaques. Because lipid cores can be present anywhere along the circumference, we focus on developing a noninvasive shear wave elastography technique for transverse carotid cross-sections. Tracking the induced wave is not trivial because the circular geometry will result in particle motion in both the axial as well as the lateral direction. This study investigates the possibility to measure the minute 2D motion field using a single high frequency ultrasound transducer by combining axial displacements estimated at multiple beam-steering angles.

Contains fulltext : 137140.pdf (Publisher’s version ) (Open Access) Deformations of the atherosclerotic vascular wall induced by the pulsating blood can be estimated using ultrasound strain imaging. Because these deformations indirectly provide information on mechanical plaque composition, strain imaging is a promising technique for differentiating between stable and vulnerable atherosclerotic plaques. This paper first explains 1-D radial strain estimation as applied intravascularly in coronary arteries. Next, recent methods for noninvasive vascular strain estimation in a transverse imaging plane are discussed. Finally, a compounding technique that our group recently developed is explained. This technique combines motion estimates of subsequently acquired focused ultrasound images obtained at various insonification angles. However, because the artery moves and deforms during the multi-angle acquisition, errors are introduced when compounding. Recent advances in computational power have enabled plane wave ultrasound acquisition, which allows 100 times faster image acquisition and thus might resolve the motion artifacts. In this paper the performance of strain imaging using plane wave compounding is investigated using simulations of an artery with a vulnerable plaque and experimental data of a two-layered vessel phantom. The results show that plane wave compounding outperforms 0 degrees focused strain imaging. For the simulations, the root mean squared error reduced by 66% and 50% for radial and circumferential strain, respectively. For the experiments, the elastographic signal-to-noise and contrast-to-noise ratio (SNRe and CNRe) increased with 2.1dB and 3.7dB radially, and 5.6dB and 16.2dB circumferentially. Because of the high frame rate, the plane wave compounding technique can even be further optimized and extended to 3D in future.

Item does not contain fulltext BACKGROUND AND PURPOSE: Carotid artery vasculopathy is a long-term complication of radiotherapy (RT) of the neck. We investigated the change in carotid intima media thickness (IMT) and the incidence of ischemic stroke in the first 7years after radiotherapy (RT) of the neck. MATERIALS AND METHODS: A multicentre prospective cohort study among patients treated for Head and Neck Cancer (HNC) assessed carotid IMT at baseline (before RT) and after a median of 7years follow-up. We also screened for cerebrovascular risk factors and events. RESULTS: 48 patients underwent IMT measurement at baseline and follow-up (median age 61years, range 29-87). Mean IMT of the irradiated common carotid arteries was 0.64mm at baseline and 0.74mm at follow-up (p=0.002). Mean delta IMT in the irradiated and non-irradiated common carotid arteries were 0.11 and 0.02mm (p=0.03). Incidence rate of stroke in our cohort, compared to the Dutch population was 8.9 versus 1.5 per 1.000 person years. CONCLUSIONS: IMT in irradiated carotid arteries was significantly increased in the first 7years after RT. The incidence rate of stroke was six fold increased. Patients treated with RT for HNC have sustained risk for developing atherosclerosis of the carotid arteries and future stroke.

Item does not contain fulltext Local layer-specific myocardial deformation after myocardial infarction (MI) has not been studied extensively although the sub-endocardium is more vulnerable to ischemia and interstitial fibrosis deposition. Radiofrequency (RF) ultrasound-based analysis could provide superior layer-specific radial strain estimation compared with clinically available deformation imaging techniques. In this study, we used RF-based myocardial deformation measurements to investigate layer-specific differences between healthy and damaged myocardium in a porcine model of chronic MI. RF data were acquired epicardially in healthy (n = 21) and infarcted (n = 5) regions of a porcine chronic MI model 12 wk post-MI. Radial and longitudinal strains were estimated in the sub-endocardial, mid-wall and sub-epicardial layers of the left ventricle. Collagen content was quantified in three layers of healthy and infarcted regions in five pigs. An analytical geometric model of the left ventricle was used to theoretically underpin the radial deformation estimated in different myocardial layers. Means +/- standard errors of the peak radial and longitudinal strain estimates of the sub-endocardial, mid-wall and sub-epicardial layers of the healthy and infarcted tissue were: 82.7 +/- 5.2% versus 39.9 +/- 10.8% (p = 0.002), 63.6 +/- 3.3% versus 38.8 +/- 7.7% (p = 0.004) and 34.3 +/- 3.0% versus 35.1 +/- 5.2% (p = 0.9), respectively. The radial strain gradient between the sub-endocardium and the sub-epicardium had decreased 12 wk after MI, and histologic examination revealed the greatest increases in collagen in the sub-endocardial and mid-wall layers. Comparable normal peak radial strain values were found by geometric modeling when input values were derived from the in vivo measurements and literature. In conclusion, the estimated strain values are realistic and indicate that sub-endocardial radial strain in healthy tissue can amount to 80%. This high value can be explained by the cardiac geometry, as was illustrated by geometric modeling. After MI, strain values were decreased and collagen content was increased in the sub-endocardial and mid-wall layers. Layer-specific peak radial strain can be assessed by RF strain estimation and clearly differs between healthy and infarcted tissue. Although the relationship between tissue stiffness and tissue strain is not strictly local, this novel technique provides a valuable way to assess layer-specific regional cardiac function in a variety of myocardial diseases.

Contains fulltext : 170849.pdf (Publisher’s version ) (Closed access) Three-dimensional (3-D) strain estimation might improve the detection and localization of high strain regions in the carotid artery (CA) for identification of vulnerable plaques. This paper compares 2-D versus 3-D displacement estimation in terms of radial and circumferential strain using simulated ultrasound (US) images of a patient-specific 3-D atherosclerotic CA model at the bifurcation embedded in surrounding tissue generated with ABAQUS software. Global longitudinal motion was superimposed to the model based on the literature data. A Philips L11-3 linear array transducer was simulated, which transmitted plane waves at three alternating angles at a pulse repetition rate of 10 kHz. Interframe (IF) radio-frequency US data were simulated in Field II for 191 equally spaced longitudinal positions of the internal CA. Accumulated radial and circumferential displacements were estimated using tracking of the IF displacements estimated by a two-step normalized cross-correlation method and displacement compounding. Least-squares strain estimation was performed to determine accumulated radial and circumferential strain. The performance of the 2-D and 3-D methods was compared by calculating the root-mean-squared error of the estimated strains with respect to the reference strains obtained from the model. More accurate strain images were obtained using the 3-D displacement estimation for the entire cardiac cycle. The 3-D technique clearly outperformed the 2-D technique in phases with high IF longitudinal motion. In fact, the large IF longitudinal motion rendered it impossible to accurately track the tissue and cumulate strains over the entire cardiac cycle with the 2-D technique. 11 p.

Item does not contain fulltext There is increasing evidence that supports the hypothesis that elevated cyclic shear strain in the adventitia of the common carotid artery promotes plaque progression. In this article, we estimated cyclic shear strain in the carotid arterial wall in 16 asymptomatic human participants using radio-frequency (RF) ultrasound. In each participant, we acquired two separate RF ultrasound recordings. We correlated the cyclic shear strain with the distension waveform (representing the blood pressure waveform) of the carotid artery and the brachial blood pressure. There were no significant differences between the shear strains estimated from the two separate RF ultrasound recordings. The point-in-time of the maximum shear strain showed a significant correlation with that of the dicrotic notch in the distension waveform (Spearman's coefficient = 0.7, p < 0.001). The pulse shear strain (difference between maximum and minimum shear strain) was significantly correlated with the pulse pressure as measured in the brachial artery (Spearman's coefficient = 0.4, p < 0.01). In this study, we show that the cyclic shear strain in the adventitia of the common carotid artery can be estimated using RF ultrasound. We found indications that the estimated cyclic shear strain was induced by the pulsating blood pressure and it was found to be higher in participants with an elevated pulse pressure.

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Item does not contain fulltext The goal of this study was to assess whether it is viable to implement plane-wave imaging in the Automated Breast Volume Scanner (ABVS) to speed up the acquisition process. This would allow breath-hold examinations, thus reducing breathing artifacts without loss of imaging quality. A calibration phantom was scanned in an Automated Breast Volume Scanner-mimicking setup using both dynamic receive focusing with a fixed transmit focus and unfocused plane-wave compounding. Contrast-to-noise ratio and lateral resolution were compared using two beamforming schemes, delay-and-sum and Stolt's f-k algorithm. Plane-wave compounding using only 11 compounding angles and Stolt's f-k algorithm provided image quality similar to that of focused transmission with dynamic receive focusing (contrast-to-noise ratios = 10.3 and 10.8 dB for Stolt's f-k migration with Hann apodization and focused transmission, respectively; full width at half-maximum = 0.38 and 0.4 mm, respectively; all at 30-mm depth with transmit focus at 30 mm) with a higher signal-to-noise ratio at all depths. Furthermore, a full 3-D volume of a breast-mimicking phantom was scanned using this optimal set of compounding angles and different speeds (10, 20 and 50 mm/s) to assess the impact of scanning time on image quality. Only minor differences in contrast-to-noise ratio were found (cyst 1: 6.0 +/- 0.3 dB, cyst 2: 5.5 +/- 0.2 dB, cyst 3: 5.7 +/- 0.5 dB). These differences could not be correlated to the movement speeds, indicating that acquisition speed does not significantly affect image quality. Our results suggest that plane-wave imaging will enable breath-hold automated breast volume scanning examinations, eliminating breathing artifacts while otherwise preserving similar image quality.

Speckle tracking enables 2D blood velocity estimation; however, imaging at a high frame rate is crucial, since speckle quickly decorrelates due to high velocity gradients. Recently, ultrafast imaging has become possible, which allows imaging at very high frame rates. This improves the accuracy of high velocity blood flow estimation, but hinders the estimation of low velocities, since interframe displacements drop below the data sampling resolution. A lot of effort has already been put into the development of methods to estimate subsample displacements, but still it remains challenging. Ultrafast imaging provides the opportunity to generate ultrasound radio frequency (RF) data at any location covered by the transmit beam. In other words, the spatial sampling of the data can be reconsidered. In this work, a method is proposed which incorporates knowledge of the point spread function (PSF) of the imaging system into the data sampling grid, to improve subsample displacement estimation. By using the dimensions of the PSF to choose the data sampling grid, the peak in the cross-correlation function is captured in a standardized way, which facilitates an optimal match with a 2D cubic interpolation method. Performance of the new approach is evaluated based on simulated data from a carotid artery as well as on in vivo data from the common carotid artery of a healthy volunteer. For a large range of pulse repetition frequencies and beam-to-flow angles, simulation results showed improved performance with respect to results obtained using conventionally spaced RF and envelope data. These findings are corroborated by in vivo results, showing the most consistent flow using the proposed method.

Mammography is the gold standard for breast cancer screening but less suitable for women with dense breasts. Therefore, the ultrasound automated breast volume scanner (ABVS) was introduced as an alternative. Although clinical studies showed high sensitivity of ABVS to detect breast cancer, specificity and long acquisition times (>1 minute) remain an issue. Since specificity can be improved using elastography, the aim of this study was to develop 3-D strain imaging for ABVS scanning while also reducing acquisition times by plane wave imaging. To mimic the ABVS, an ATL L12-5 50mm transducer was attached to a motorized XZ-translational setup and connected to a Verasonics V1 ultrasound system. The aim was to obtain 150 equidistant (0.5 mm) 2-D strain images (60×25 mm2) of a breast phantom (model 059, CIRS) containing multiple inclusions. To obtain these images, beam-formed ultrasound radiofrequency (RF) data were acquired before and after deformation of the breast, followed by coarse-to-fine 2-D cross-correlation and 1-D least-squares strain estimation. The performance of strain estimation was compared for two transmission schemes at two frame rates of 2 Hz and 16 Hz (one breath hold) by calculating elastographic contrast-to-noise ratios (CNR e ) and signal-to-noise ratios (SNR e ). In transmission scheme 1 pre- and post-deformation RF data were acquired per position, whereas in scheme 2 (ABVS-like acquisition) RF data were acquired for all positions per state of deformation. All methods provided similar strain images upon visual inspection for most positions, although first and last 15 strain images (corresponding to rounder parts of the breast) resulted in reduced quality (SNR e 0±6 dB; CNR e 15±0 dB) in scheme 2 compared to scheme 1 (SNR e 16±1 dB; CNR e 28±4 dB). For the other 120 images, the two methods were similar (CNR e 33±1 dB; SNR e 15±1 dB). The frame rates did not seem to affect the CNR e and SNR e in both methods. In summary, scheme 1 performed similarly to the ABVS-like scheme 2 in the central part of the breast. Thus we might conclude that implementation of strain imaging in ABVS is feasible, although the reduced quality at the borders of the breast has to be improved. Moreover, plane wave imaging will enable high frame rates and lateral compounding and allow 3-D elastography within one breath hold.

Rupture of plaques in atherosclerotic carotid arteries is a main cause of stroke. Plaques with a large soft lipid-rich core are more prone to rupture than predominantly fibrous, stiff plaques. Detecting and monitoring the presence of lipids in plaques noninvasively and fast remains challenging. Ultrasound shear wave elastography is fast and noninvasive. Experimental studies showed that detection of locally softer regions in vessel-mimicking phantoms is possible. However, these studies only report imaging in a longitudinal imaging plane. Since it is not possible to detect lipids over the full vessel circumference when imaging in a longitudinal plane, this study investigates the feasibility of shear wave elastography in transverse imaging planes. A SuperSonic Imagine Aixplorer equipped with a linear array probe was used to induce shear waves in vessel-mimicking polyvinylalcohol phantoms of varying dimensions and shear moduli using a single focused ultrasonic push (300 µs). The push was followed by 0° plane wave acquisitions (n = 100, PRF ≥ 10 kHz) to detect the axial motion induced in the vessel wall. To visualize the generated circumferentially propagating shear waves, axial motion maps were created showing the axial phase shift as a function of circumferential position for circular paths in the vessel wall. Approximately linearly propagating wave fronts were observed for all phantoms as a function of circumferential position. For homogeneous phantoms, one wave was observed, which made a roundtrip, whereas a slow and a fast wave were observed in two-layered ‘soft plaque’ phantoms suggesting that soft lipid-rich regions can be detected. Finally, preliminary in vivo results were obtained in a volunteer using a Verasonics ultrasound system which confirmed that creation of circumferentially propagating waves is also possible in vivo. Based on these observations we conclude that shear wave elastography in transverse imaging planes is feasible and promising for lipid-core detection in plaques of superficial arteries.

Three-dimensional vascular strain estimation is crucial for assessment of the location of high strain regions in the carotid artery (CA) and the identification of vulnerable plaque features. This study compares 2D vs. 3D displacement estimation in terms of radial and circumferential strain using simulated ultrasound images of a 3D atherosclerotic CA model at the bifurcation embedded in surrounding tissue. The 3D finite element model (FEM) of a patient-specific, pulsating atherosclerotic CA (pulse pressure 60 mmHg) was generated with ABAQUS FEM software. Global longitudinal motion was superimposed to the model. Radiofrequency (RF) ultrasound data were simulated in Field II by moving point scatterers (vessel wall) according to the deformation patterns of the model. A linear array transducer (f c = 9 MHz, pitch = 198 µm, 192 elements) was used which transmitted plane waves at 3 alternating angles (+19.5°, 0°, −19.5°) at a pulse repetition rate of 10 kHz. Simulations with 20 ms (systole) and 100 ms (diastole) inter-frame (IF) time were performed for 191 equally spaced (0.1 mm) longitudinal positions of the internal CA containing fatty and calcified areas. After delay-and-sum beamforming, IF axial displacements were estimated using a coarse-to-fine normalized cross-correlation method. The axial displacement at 0° was used as the vertical displacement component. Projection of the −19.5° and +19.5° axial displacements yielded the horizontal displacement component. A polar grid and the lumen center were determined in the end-diastolic frame of each longitudinal position and used to convert the tracked vertical and horizontal displacements into radial and circumferential displacements. Least squares strain estimation was performed to determine accumulated radial and circumferential strain. The performance of the 2D and 3D method was compared by calculating the root-mean-squared error (RMSE) of the estimated strains with respect to the reference strains obtained from the model. More accurate strain images were obtained using the 3D displacement estimation for the entire cardiac cycle. The 3D technique clearly outperforms the 2D technique in phases with high IF longitudinal motion.

Strain imaging of the carotid artery (CA) has demonstrated to be a technique capable of identifying plaque composition. This study assesses the performance of volumetric strain imaging derived from multi-plane acquisitions with a single transducer, with and without displacement compounding. These methods were compared to a reference method using two orthogonally placed transducers. A polyvinyl alcohol phantom was created resembling a stenotic CA bifurcation. A realistic pulsatile flow was imposed on the phantom, resulting in fluid pressures inducing 10% strains. Two orthogonally aligned linear array transducers were connected to two Verasonics systems and fixed in a translation stage. For 120 equally spaced elevational positions, ultrasound series were acquired for a complete cardiac cycle and synchronized using a trigger. Each series consisted of ultrafast plane-wave acquisitions at 3 alternating angles. Inter-frame displacements were estimated using a 3D cross-correlation-based tracking algorithm. Horizontal displacements were acquired using the single probe lateral displacement estimate, the single probe compounded by axial displacement estimates obtained at angles of 19.47 and −19.47 degrees, and the dual probe registered axial displacement estimate. After 3D tracking, least squares strain estimations were performed to compare compressive and tensile principal strains in 3D for all methods. The compounding technique clearly outperformed the zero-degree method for the complete cardiac cycle and resulted in more accurate 3D strain estimates.

Contains fulltext : 87739.pdf (Publisher’s version ) (Closed access) This simulation study is concerned with: 1) the feasibility of measuring rotation and 2) the assessment of the performance of strain estimation in shearing and rotating structures. The performance of 3 different radio frequency (RF) based methods is investigated. Linear array ultrasound data of a deforming block were simulated (axial shear strain = 2.0, 4.0, and 6.0%, vertical strain = 0.0, 1.0, and 2.0%). Furthermore, data of a rotating block were simulated over an angular range of 0.5 degrees to 10 degrees . Local displacements were estimated using a coarse-to-fine algorithm using 1-D and 2-D precompression kernels. A new estimation method was developed in which axial displacements were used to correct the search area for local axial motion. The study revealed that this so-called free-shape 2-D method outperformed the other 2 methods and produced more accurate displacement images. For higher axial shear strains, the variance of the axial strain and the axial shear strain reduced by a factor of 4 to 5. Rotations could be accurately measured up to 4.0 to 5.0 degrees . Again, the free-shape 2-D method yielded the most accurate results. After reconstruction of the rotation angle, the mean angles were slightly underestimated. The precision of the strain estimates decreased with increasing rotation angles. In conclusion, the proposed free-shape 2-D method enhances the measurement of (axial shear) strains and rotation. Experimental validation of the new method still has to be performed. 01 april 2010

Contains fulltext : 81914.pdf (Publisher’s version ) (Closed access) The goal of this study was to investigate the applicability of conventional 2-D displacement and strain imaging techniques to phased array radiofrequency (RF) data. Furthermore, the possible advantages of aligning and stretching techniques for the reduction of decorrelation artefacts was examined. Data from both realistic simulations and phantoms were used in this study. Recently, the used processing concepts were successfully applied to linear array data. However, their applicability to sector scan data is not trivial because of the polar grid. Homogeneous and inhomogeneous tissue phantoms were simulated at a range of strains (0 to 5%) using Field II((c)). The inhomogeneous phantom, a commonly used tumor/lesion model, was also constructed using gelatin/agar solutions. A coarse-to-fine displacement algorithm was applied, using aligning and stretching to enhance re-correlation. Vertical and horizontal strains were reconstructed from the axial and lateral displacements. Results revealed that the error on displacement estimates was lower when using 2-D data windows rather than 1-D windows. For regions at large depths and large insonification angles, the allowed lateral window size was limited. Still, 1-D windows resulted in larger errors. The re-correlation techniques resulted in a significant increase in the elastographic signal-to-noise ratio (SNRe) and elastographic contrast-to-noise ratio (CNRe) of the vertical and horizontal strain components. An increase of the SNRe of 5-20 dB was observed over a range of strains (0.5 to 5.0%). In the inhomogeneous phantom, a vertical SNRe of 27.7 dB and a horizontal SNRe of 16.7 dB were measured in the background. The vertical and horizontal CNRe were 35 dB and 23.1 dB, respectively. For the experimental data, lower SNRe (vertical: 19.1 dB; horizontal: 11.4 dB) and CNRe (vertical: 33.3 dB; horizontal: 12.5 dB) were found. In conclusion, 2-D window matching of sector scan data is feasible and outperforms 1-D window matching. Furthermore, the use of re-correlation techniques enhances both precision and contrast of strain images.

In this study, the performances of one-dimensional and two-dimensional least-squares strain estimators (LSQSE) are compared. Furthermore, the effects of kernel size are examined using simulated raw frequency data of a widely-adapted hard lesion/soft tissue model. The performances of both methods are assessed in terms of root-mean-squared errors (RMSE), elastographic signal-to-noise ratio (SNRc) and contrast-to-noise ratio (CNRc). RMSE analysis revealed that the 2D LSQSE yields better results for phased array data, especially for larger insonification angles. Using a 2D LSQSE enabled the processing of unfiltered displacement data, in particular for the lateral/horizontal strain components. The SNRc and CNRc analysis showed an improvement in precision and almost no loss in contrast using 2D LSQSE. However, the RMSE images for different kernel sizes revealed that the optimal 2D kernel size depends on the region-of-interest and showed that the LSQ kernel size should be limited to avoid loss in resolution.

The material properties of atherosclerotic plaques govern the biomechanical environment, which is associated with rupture-risk. We investigated the feasibility of noninvasively estimating carotid plaque component material properties through simulating ultrasound (US) elastography and in vivo magnetic resonance imaging (MRI), and solving the inverse problem with finite element analysis. 2D plaque models were derived from endarterectomy specimens of nine patients. Nonlinear neo-Hookean models (tissue elasticity C-1) were assigned to fibrous intima, wall (i.e., media/adventitia), and lipid-rich necrotic core. Finite element analysis was used to simulate clinical cross-sectional US strain imaging. Computer-simulated, single-slice in vivo MR images were segmented by two MR readers. We investigated multiple scenarios for plaque model elasticity, and consistently found clear separations between estimated tissue elasticity values. The intima C-1 (160 kPa scenario) was estimated as 125.8 +/- 19.4 kPa (reader 1) and 128.9 +/- 24.8 kPa (reader 2). The lipid-rich necrotic core C-1 (5 kPa) was estimated as 5.6 +/- 2.0 kPa (reader 1) and 8.5 +/- 4.5 kPa (reader 2). A scenario with a stiffer wall yielded similar results, while realistic US strain noise and rotating the models had little influence, thus demonstrating robustness of the procedure. The promising findings of this computer-simulation study stimulate applying the proposed methodology in a clinical setting. (C) 2015 IPEM. Published by Elsevier Ltd. All rights reserved.

Quantification of complex flow patterns with variations in the blood flow direction throughout an image plane and strong fluctuations over the cardiac cycle, requires dedicated transmission schemes. Conventional Doppler-based velocity imaging techniques are limited, due to their angle dependence and limited frame rates. New methods have been suggested to estimate the full 2D velocity vector. These methods need to be combined with intelligent transmit schemes to increase the imaging frame rate. This study investigates the performance of two different techniques to estimate the 2D blood velocity vector, both using a 2D normalized cross-correlation based displacement estimation algorithm. Normal 0° velocity estimation is compared with compound vector velocity estimation, using plane wave transmissions. Combining finite element modeling with FIELD II provided ultrasound radio frequency data of a carotid artery flow field over the entire cardiac cycle. Ultrasound radiofrequency element data were simulated for a linear array transducer (fc = 9 MHz, fs = 36 MHz, pitch = 198 μm). Plane waves were transmitted at a PRF of 2 kHz at either 0° angle only, or at sequentially changing angles of 0°, -20° and 20°. RF data were beamformed in the direction of the steered plane waves by delay-and-sum beamforming. The 0° data were additionally beamformed at angles of -20° and +20°. The horizontal velocity component was determined in three different ways: 1) directly from the 0° acquisition (PW0), 2) by compounding the axial velocity component from the 0° data beamformed at +20° and -20° (PW0C), or 3) by compounding the axial velocity component from the multi-angle acquisitions (PWC). The vertical velocity component was determined directly from the 0° acquisition. The performance of the methods was compared by calculating the root mean squared error (RMSE) between estimated and true velocity components. The results showed the ability of the PW0 and PW0C methods to provide accurate 2D velocity estimates. Both methods perform similarly based on the calculated RMSEs. PWC showed less accurate velocity estimates, i.e., higher RMSEs, which are probably caused by the reduced effective frame rate and presence of grating lobes. At present frame rate and transducer choice, velocity compounding does not seem to be beneficial for 2D velocity estimation in a carotid artery flow field.

Carotid plaque rupture is a major cause of stroke and mainly occurs in plaques with a substantial necrotic/lipid core, whereas predominantly fibrous plaques are considered to be stable. Presence of local plaque features associated with vulnerability indicates an increased risk of future