H.H.G. Hansen

Noninvasive Carotid Strain Imaging Using Angular Compounding at Large Beam Steered Angles: Validation in Vessel Phantoms

Stroke and myocardial infarction are initiated by rupturing vulnerable atherosclerotic plaques. With noninvasive ultrasound elastography, these plaques might be detected in carotid arteries. However, since the ultrasound beam is generally not aligned with the radial direction in which the artery pulsates, radial and circumferential strains need to be derived from axial and lateral data. Conventional techniques to perform this conversion have the disadvantage that lateral strain is required. Since the lateral strain has relatively poor accuracy, the quality of the radial and circumferential strains is reduced. In this study, the radial and circumferential strain estimates are improved by combining axial strain data acquired at multiple insonification angles. Adaptive techniques to correct for grating lobe interference and other artifacts that occur when performing beam steering at large angles are introduced. Acquisitions at multiple angles are performed with a beam steered linear array. For each beam steered angle, there are two spatially restricted regions of the circular vessel cross section where the axial strain is closely aligned with the radial strain and two spatially restricted regions (different from the radial strain regions) where the axial strain is closely aligned with the circumferential strain. These segments with high quality strain estimates are compounded to form radial or circumferential strain images. Compound radial and circumferential strain images were constructed for a homogeneous vessel phantom with a concentric lumen subjected to different intraluminal pressures. Comparison of the elastographic signal-to-noise ratio (SNRe) and contrast-to-noise ratio ( CNRe) revealed that compounding increases the image quality considerably compared to images from 0deg information only. SNRe and CNRe increase up to 2.7 and 6.6 dB, respectively. The highest image quality was achieved by projecting axial data, completed with a small segment determined by either principal component analysis or by application of a rotation matrix.

Full 2D displacement vector and strain tensor estimation for superficial tissue using beam-steered ultrasound imaging.

Ultrasound strain imaging is used to measure local tissue deformations. Usually, only strains along the ultrasound beam are estimated, because those estimates are most precise, due to the availability of phase information. For estimating strain in other directions we propose to steer the ultrasound beam at an angle, which allows estimating different projections of the 2D strain tensor, while phase information remains available. This study investigates beam steering at maximally three different angles to determine the full 2D strain tensor. The method was tested on simulated and experimental data of an inclusion phantom and a vessel phantom. The combination of data from a non-steered acquisition and acquisitions at a large positive and an equally large but negative steering angle enabled the most precise estimation of the strain components. The method outperforms conventional methods that do not use beam steering.

Ultrafast vascular strain compounding using plane wave transmission

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° 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 (SNR(e) and CNR(e)) increased with 2.1 dB and 3.7 dB radially, and 5.6 dB 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.

Predicting Target Displacements Using Ultrasound Elastography and Finite Element Modeling

Soft tissue displacements during minimally invasive surgical procedures may cause target motion and subsequent misplacement of the surgical tool. A technique is presented to predict target displacements using a combination of ultrasound elastography and finite element (FE) modeling. A cubic gelatin/agar phantom with stiff targets was manufactured to obtain pre- and post-loading ultrasound radio frequency (RF) data from a linear array transducer. The RF data were used to compute displacement and strain images, from which the distribution of elasticity was reconstructed using an inverse FE-based approach. The FE model was subsequently used to predict target displacements upon application of different boundary and loading conditions to the phantom. The influence of geometry was investigated by application of the technique to a breast-shaped phantom. The distribution of elasticity in the phantoms as determined from the strain distribution agreed well with results from mechanical testing. Upon application of different boundary and loading conditions to the cubic phantom, the FE model-predicted target motion were consistent with ultrasound measurements. The FE-based approach could also accurately predict the displacement of the target upon compression and indentation of the breast-shaped phantom. This study provides experimental evidence that organ geometry and boundary conditions surrounding the organ are important factors influencing target motion. In future work, the technique presented in this paper could be used for preoperative planning of minimally invasive surgical interventions.

Three-dimensional ultrasound strain imaging of skeletal muscles

Muscle contraction is characterized by large deformation and translation, which requires a multi-dimensional imaging modality to reveal its behavior. Previous work on ultrasound strain imaging of the muscle contraction was limited to 2D and bi-plane techniques. In this study, a three-dimensional (3D) ultrasound strain imaging technique was tested against 2D strain imaging and used for quantifying deformation of skeletal muscles. A phantom compression study was conducted for an experimental validation of both 2D and 3D methods. The phantom was compressed 3% vertically and pre- and post-compression full volume radio frequency (RF) ultrasound data were acquired using a matrix array transducer. A cross-correlation-based algorithm using either 2D or 3D kernels was applied to obtain the displacement estimates. These estimates were converted to Cartesian space and subsequently, strain was derived using a least-squares strain estimator (LSQSE). The 3D results were compared with the 2D results and the theoretically predicted displacement and strain. Comparison between 2D and 3D kernels was performed on data from a plane with a large tilt angle to study the influence of out-of-plane motion on the two techniques. To demonstrate the in vivo feasibility, 3D strain was calculated from live 3D data, acquired during a 2 second isometric contraction and relaxation of the quadriceps muscle in a healthy volunteer. The phantom study showed good correlation between estimated displacements and the theoretically predicted displacements. Root-mean-squared errors (RMSE) were 0.16, 0.17 and 0.13 mm in the x-, y- and z-direction respectively. The absolute RMSE for the 3D strain values were 0.94, 1.2 and 0.41% in the x-, y- and z-direction respectively. The 2D method performed worse, with 3 (x-direction) to 6 (z-direction) times higher RMSE values. The larger errors in lateral and elevational direction with respect to the axial RMSE are potentially caused by the large angle between the ultrasound beams. Initial in vivo results revealed 3D strain curves which clearly visualized the contraction and relaxation of the quadriceps muscles. Muscle deformation estimation using real-time 3D ultrasound RF-data seems feasible and the use of 3D kernels improves displacement estimation in comparison to 2D techniques. Future work will focus on improving lateral and elevational displacement estimation, and investigating local differences of strain in skeletal muscles and its clinical relevance.

P3A-4 Compounding of Strain Data Non-Invasively Obtained at Large Beam Steered Angles

Non-invasive vulnerable plaque detection is beneficial for prevention of stroke and myocardial infarction. A recent study [1] has shown, that non-invasive vulnerable plaque detection in the carotid arteries is feasible by ultrasound elastography. However, the quality of the strain images is low, because contrary to intravascular strain imaging, lateral strain input is required for reconstructing radial and circumferential strain images. This study focuses on reducing the distortion caused by the lateral component by constructing radial and circumferential strain images from mainly axial strain data acquired at multiple insonification angles. Beam steering techniques [2] were applied to achieve insonification angles of up to 45deg. Most of the compound radial and circumferential strain images were estimated by projecting axial strain data in radial direction. A segment calculated by principal component analysis was added to obtain full 360deg compound strain images. Compound radial and circumferential strain images were constructed for three gelatin vessel phantoms; a homogeneous phantom with a concentric lumen, a homogeneous phantom with an eccentric lumen, and a heterogeneous phantom with an eccentric soft plaque. Three radial and circumferential strain images were constructed for the concentric phantom. The first was constructed by principal component analysis of 0deg data only. The second consisted of data obtained at -45, 0 and 45 degrees beam steering, and the third was made up of data acquired at -45, -30, -22, -15, 0, 15, 22, 30, and 45 degrees beam steering. Assuming a radial strain decay, elastographic signal-to-noise ratios (SNRe), and contrast-to-noise ratios (CNRe) of these images were determined and compared. Finally, compound radial and circumferential strain images of the two eccentric phantoms were constructed from data acquired at the complete range of insonification angles, and compared. The image quality of the radial and circumferential compound strain images improved considerably compared to the radial and circumferential strain image constructed from 0deg information only. SNRe and CNRe, increases of up to 4.4 dB and 9.9 dB were measured respectively. Image quality of the elastogram improved with the number of beam steered angles used for the compounding. By comparing the strain images of the two eccentric phantoms the soft plaque region could be identified.

Improved plane wave ultrasound image reconstruction using a deconvolution-based Fourier domain approach

Different from conventional focused ultrasound, ultrafast ultrasound imaging employs full-field transmission such as a plane wave to achieve frame rates in the order of 10 kHz. Image reconstruction of plane-wave ultrasound is more computational efficient to be processed in Fourier domain than in time domain. A widely-applied seismic wave migration technique, the Stolts f-k Fourier-domain method, was modified to fit the plane wave transmission-receiving process into the Exploding Reflector Model (ERM). In comparison with the ideal fitting in Lus f-k method, the fitting in the Stolts f-k is slightly imprecise for the higher lateral Fourier components that results in residual patterns that degrade the image quality. We propose a template that can be applied in Fourier domain directly to deconvolute the residual point spread function (residual PSF) induced by imprecise fitting.

Design of an angular weighting template for coherent plane wave compounding in Fourier domain

Coherent plane wave compound imaging has already shown to provide equal image quality as multi-focal conventional imaging at more than ten times higher frame rates. Furthermore, image reconstruction of plane wave ultrasound is more computational efficient in Fourier domain (e.g. Lus or Stolts f-k) than in spatial domain (e.g. Delay-and-Sum (DAS)). In this study we fully integrated the Stolts f-k method and coherent compounding in the Fourier domain to further increase its computational efficiency. Additionally, we introduced a weighting template for Fourier domain methods which is rotated as a function of plane wave transmission angle (angular weighting) and investigated how it affects contrast and resolution of coherently compounded images.

Estimation of Atherosclerotic Plaque Material Properties: A Mixed Method of Strain Imaging and Inverse Finite Element Analysis

Atherosclerosis is a cardiovascular disease characterized by plaque formation in the vessel wall. Plaque rupture initiates thrombus formation and may lead to myocardial infarction, stroke and eventually, to sudden death [1]. A plaque ruptures when the mechanical stress in the plaque exceeds its strength. Thus, biomechanical models have a great potential to predict plaque rupture. For reliable prediction models, correct material properties of plaque components at large strains are prerequisite. However, the data available in literature are limited and show a wide range.© 2013 ASME

Shear) Strain Imaging Used in Noninvasive Detection of Vulnerable Plaques in the Carotid Arterial Wall

The primary trigger for myocardial infarction and stroke is destabilization of atherosclerotic plaques. The chance of a plaque to rupture is related to its composition and geometry. Ultrasound (shear) strain imaging allows assessment of local tissue mechanics and possible risk assessment of vulnerable plaques. Intravascularly, in coronary arteries using a catheter, strain imaging has been demonstrated to be successful. At different intraluminal pressures, ultrasound data of the artery wall were recorded and local radial strains were estimated using cross-correlation methods. It has been shown in vitro and in vivo that softer lipidic plaques can be distinguished from harder fibrous and calcified plaques on basis of their strain values.