Ultrasound speckle tracking strain estimation of in vivo carotid artery plaque with in vitro sonomicrometry validation

Comparing Focused-Tracked and Plane Wave-Tracked ARFI Log(VoA) In Silico and in Application to Human Carotid Atherosclerotic Plaque, Ex Vivo

A significant risk factor for ischemic stroke is carotid atherosclerotic plaque that is susceptible to rupture, with rupture potential conveyed by plaque morphology. Human carotid plaque composition and structure have been delineated noninvasively and in vivo by evaluating log(VoA), a parameter derived as the decadic log of the second time derivative of displacement induced by an acoustic radiation force impulse (ARFI). In prior work, ARFI-induced displacement was measured using conventional focused tracking; however, this requires a long data acquisition period, thereby reducing framerate. We herein evaluate if ARFI log(VoA) framerate can be increased without a reduction in plaque imaging performance using plane wave tracking instead. In silico, both focused- and plane wave-tracked log(VoA) decreased with increasing echobrightness, quantified as signal-to-noise ratio (SNR), but did not vary with material elasticity for SNRs below 40 dB. For SNRs of 40-60 dB, both focused- and plane wave-tracked log(VoA) varied with SNR and material elasticity. Above 60 dB SNR, both focused- and plane wave-tracked log(VoA) varied with material elasticity alone. This suggests that log(VoA) discriminates features according to a combination of their echobrightness and mechanical property. Further, while both focused- and plane-wave tracked log(VoA) values were artifactually inflated by mechanical reflections at inclusion boundaries, plane wave-tracked log(VoA) was more strongly impacted by off-axis scattering. Applied to three excised human cadaveric carotid plaques with spatially aligned histological validation, both log(VoA) methods detected regions of lipid, collagen, and calcium (CAL) deposits. These findings support that plane wave tracking performs comparably to focused tracking for log(VoA) imaging and that plane wave-tracked log(VoA) is a viable approach to discriminating clinically relevant atherosclerotic plaque features at a 30-fold higher framerate than by focused tracking.

The impact of geometry, intramural friction, and pressure on the antegrade longitudinal motion of the arterial wall: A phantom and finite element study

Longitudinal motion of the carotid arterial wall, as measured with ultrasound, has shown promise as an indicator of vascular health. The underlying mechanisms are however not fully understood. We have found, in in vivo studies, that blood pressure has a strong relation to the antegrade longitudinal displacement in early systole. Further, we have identified that a tapered geometry and the intramural friction in-between two parts of a vessel wall influence the longitudinal displacement. We therefore studied the interaction between pressure, vessel geometry and intramural friction, tapered and straight ultrasound phantoms in a paralleled hydraulic bench study and corresponding numerical models. Profound antegrade longitudinal motion was induced in the innermost part of both tapered phantoms and the numerical models, but to a lesser extent when intramural friction was increased in the simulations. Strong correlations (R = 0.82-0.96; p < 1e-3; k = 9.3-14 μm/mmHg) between longitudinal displacement and pulse pressure were found in six of seven regions of interest in tapered phantoms. The motion of the straight phantom and the corresponding numerical model was smaller, on average zero or close to zero. This study demonstrates that tapering of the lumen, low intramural friction, and pressure might be important conducive features to the antegrade longitudinal motion of the arterial wall in vivo.

Interframe Echo Intensity Variation of Subregions and Whole Plaque in Two-Dimensional Carotid Ultrasonography: Simulations and In Vivo Observations

OBJECTIVES

The risk of cardiovascular disease is associated with the echo intensity of carotid plaques in ultrasound images and their cardiac cycle-induced intensity variations. In this study, we aimed to 1) explore the underlying origin of echo intensity variations by using simulations and 2) evaluate the association between the two-dimensional (2D) spatial distribution of these echo intensity variations and plaque vulnerability.

METHODS

First, we analyzed how out-of-plane motion and compression of simulated scattering spheres of different sizes affect the ultrasound echo intensity. Next, we propose a method to analyze the features of the 2D spatial distribution of interframe plaque echo intensity in carotid ultrasound image sequences and explore their associations with plaque vulnerability in experimental data.

RESULTS

The simulations showed that the magnitude of echo intensity changes was similar for both the out-of-plane motion and compression, but for scattering objects smaller than 1 mm radius, the out-of-plane motion dominated. In experimental data, maps of the 2D spatial distribution of the echo intensity variations had a low correlation with standard B-mode echo intensity distribution, indicating complementary information on plaque tissue composition. In addition, we found the existence of ∼1 mm diameter subregions with pronounced echo intensity variations associated with plaque vulnerability.

CONCLUSIONS

The results indicate that out-of-plane motion contributes to intra-plaque regions of high echo intensity variation. The 2D echo intensity variation maps may provide complementary information for assessing plaque composition and vulnerability. Further studies are needed to verify this method's role in identifying vulnerable plaques and predicting cardiovascular disease risk.

Use of 3D anatomical models in mock circulatory loops for cardiac medical device testing

INTRODUCTION

Mock circulatory loops (MCLs) are mechanical representations of the cardiovascular system largely used to test the hemodynamic performance of cardiovascular medical devices (MD). Thanks to 3 dimensional (3D) printing technologies, MCLs can nowadays also incorporate anatomical models so to offer enhanced testing capabilities. The aim of this review is to provide an overview on MCLs and to discuss the recent developments of 3D anatomical models for cardiovascular MD testing.

METHODS

The review first analyses the different techniques to develop 3D anatomical models, in both rigid and compliant materials. In the second section, the state of the art of MCLs with 3D models is discussed, along with the testing of different MDs: implantable blood pumps, heart valves, and imaging techniques. For each class of MD, the MCL is analyzed in terms of: the cardiovascular model embedded, the 3D model implemented (the anatomy represented, the material used, and the activation method), and the testing applications.

DISCUSSIONS AND CONCLUSIONS

MCLs serve the purpose of testing cardiovascular MDs in different (patho-)physiological scenarios. The addition of 3D anatomical models enables more realistic connections of the MD with the implantation site and enhances the testing capabilities of the MCL. Current attempts focus on the development of personalized MCLs to test MDs in patient-specific hemodynamic and anatomical scenarios. The main limitation of MCLs is the impossibility to assess the impact of a MD in the long-term and at a biological level, for which animal experiments are still needed.

Safety of arterial shear wave elastography-ex-vivoassessment of induced strain and strain rates

Shear wave elastography (SWE) is a promising technique for characterizing carotid plaques and assessing local arterial stiffness. The mechanical stress to which the tissue is subjected during SWE using acoustic radiation force (ARF), leading to strain at a certain strain rate, is still relatively unknown. Because SWE is increasingly used for arterial applications where the mechanical stress could potentially lead to significant consequences, it is important to understand the risks of SWE- induced strain and strain rate. The aim of this study was to investigate the safety of SWE in terms of induced arterial strain and strain rateex-vivoand in a human carotid arteryin-vivo. SWE was performed on six porcine aortae as a model of the human carotid artery using different combinations of ARF push parameters (push voltage: 60/90 V, aperture width: f/1.0/1.5, push length: 100/150/200 μs) and distance to push position. The largest induced strain and strain rate were 1.46 % and 54 s^-1(90 V, f/1.0, 200 μs), respectively. Moreover, the SWE-induced strains and strain rates increased with increasing push voltage, aperture, push length, and decreasing distance between the region of interest and the push. In the human carotid artery, the SWE-induced maximum strain was 0.06 % and the maximum strain rate was 1.58 s^-1, compared with the maximum absolute strain and strain rate of 12.61 % and 5.12 s^-1, respectively, induced by blood pressure variations in the cardiac cycle. Our results indicate thatex-vivoarterial SWE does not expose the artery to higher strain rate than normal blood pressure variations, and to strain one order of magnitude higher than normal blood pressure variations, at the push settings and distances from the region of interest used in this study.

Dynamic carotid plaque imaging using ultrasonography

OBJECTIVE

Dynamic image analysis of carotid plaques has demonstrated that during systole and early diastole, all plaque components will move in the same direction (concordant motion) in some plaques. However, in others, different parts of the plaque will move in different directions (discordant motion). The aim of our study was (1) to determine the prevalence of discordant motion in symptomatic and asymptomatic plaques, (2) to develop a measurement of the severity of discordant motion, and (3) to determine the correlation between the severity of discordant motion and symptom prevalence.

METHODS

A total of 200 patients with 204 plaques resulting in 50% to 99% stenosis (112 asymptomatic and 92 symptomatic plaques) had video recordings available of the plaque motion during 10 cardiac cycles. Video tracking was performed using Farneback's method, which relies on frame comparisons. In our study, these were performed at 0.1-second intervals. The maximum angular spread (MAS) of the motion vectors at 10-pixel intervals in the plaque area was measured in degrees. Plaques were classified as concordant (MAS, <70°), moderately discordant (MAS, 70°-120°), and discordant (MAS, >120°).

RESULTS

Motion was discordant in 89.1% of the symptomatic plaques but only in 17.9% of asymptomatic plaques (P < .001). The prevalence of symptoms increased with increasing MAS. For a MAS >120°, the hazard ratio for the presence of symptoms was 47.7 (95% confidence interval, 18.1-125.6) compared with the rest of the plaques after adjustment for the degree of stenosis and mean pixel motion. The area under the receiver operating characteristic curve for the prediction of the presence of symptoms using the MAS was 0.876 (95% confidence interval, 0.823-0.929). The use of the median MAS (120°) as a cutoff point classified 86% of the plaques correctly (sensitivity, 81.4%; specificity, 91.2%; positive predictive value, 90.2%; and negative predictive value, 83.0%).

CONCLUSIONS

The use of the MAS value to identify asymptomatic plaques at increased risk of developing symptoms and, in particular, stroke should be tested in prospective studies.

Von Mises Strain as a Risk Marker for Vulnerability of Carotid Plaque: Preliminary Clinical Evaluation of Cerebral Infarction

Non-invasive assessment of carotid artery plaque vulnerability is a key issue for cerebrovascular disease. This study investigates Von Mises strain imaging in patients by relating Von Mises strain to cerebral infarction presentation. Ultrasonography was performed in patients evaluated for carotid artery stenosis. Strains were estimated by a flow-driven diffusion method and least-squares regression applying Kalman filtering. Von Mises strains ɛ_VM^sys and ɛ_VM^dia were calculated by averaging four or five cardiac cycles in systole and diastole, respectively. Von Mises strain (peak, coefficient of variance, skewness and kurtosis) in patients with cerebral infarction was compared with that in the control group. Higher Von Mises peak strain localized to echolucent areas on B-mode imaging. Higher peak strain was found in patients with cerebral infarction compared with the control group (p = 0.02 for ɛ_VM^dia and p = 0.001 for ɛ_VM^sys). The area under the receiver operating characteristic curve for peak ɛ_VM^sys was 0.761 (p = 0.001) with high sensitivity and specificity. Peak strain also correlated with homocysteine (r = 0.345, p = 0.007, for ɛ_VM^dia; r = 0.287, p = 0.036, for ɛ_VM^sys) and hypersensitive C-reactive protein (r = 0.399, p = 0.043, for ɛ_VM^dia; r = 0.195, p = 0.034, for ɛ_VM^sys) levels. The coefficient of variance, skewness and kurtosis of ɛ_VM^dia or ɛ_VM^sys were also associated with homocysteine levels. In conclusion, this study indicates that peak Von Mises strain is a potential clinical risk marker for carotid plaque vulnerability and cerebral infarction.

Interoperator Reproducibility of Carotid Elastography for Identification of Vulnerable Atherosclerotic Plaques

Ultrasound-based carotid elastography has been developed to evaluate the vulnerability of carotid atherosclerotic plaques. The aim of this study was to investigate the in vivo interoperator reproducibility of carotid elastography for the identification of vulnerable plaques, with high-resolution magnetic resonance imaging (MRI) as reference. Ultrasound radio-frequency data of 45 carotid arteries (including 53 plaques) from 32 volunteers were acquired separately by two experienced operators in the longitudinal view and then were used to estimate the interframe axial strain rate (ASR) with a two-step optical flow method. The maximum 99th percentile of absolute ASR of all plaques in a carotid artery was used as the elastographic index. MRI scanning was also performed on each volunteer to identify the vulnerable plaque. The results showed no systematic bias in the Bland-Altman plot and an intraclass correlation coefficient of 0.66 between the two operators. In addition, no statistical significance was found between the receiver operating characteristic (ROC) curves from the two operators ( ), and their areas under the ROC curves were 0.83 and 0.77, respectively. Using the mean measurements of the two operators as the classification criterion, a sensitivity of 71.4%, a specificity of 87.1%, and an accuracy of 82.2% were obtained with a cutoff value of 1.37 [Formula: see text]. This study validates the interoperator reproducibility of ultrasound-based carotid elastography for identifying vulnerable carotid plaques.

Investigation of out-of-plane motion artifacts in 2D noninvasive vascular ultrasound elastography

Ultrasound noninvasive vascular elastography (NIVE) has shown its potential to measure strains of carotid arteries to predict plaque instability. When two-dimensional (2D) strain estimation is performed, either in longitudinal or cross-sectional view, only in-plane motions are considered. The motions in elevation direction (i.e. perpendicular to the imaging plane), can induce estimation artifacts affecting the accuracy of 2D NIVE. The influence of such out-of-plane motions on the performance of axial strain and axial shear strain estimations has been evaluated in this study. For this purpose, we designed a diseased carotid bifurcation phantom with a 70% stenosis and an in vitro experimental setup to simulate orthogonal out-of-plane motions of 1 mm, 2 mm and 3 mm. The Lagrangian speckle model estimator (LSME) was used to estimate axial strains and shears under pulsatile conditions. As anticipated, in vitro results showed more strain estimation artifacts with increasing magnitudes of motions in elevation. However, even with an out-of-plane motion of 2.0 mm, strain and shear estimations having inter-frame correlation coefficients higher than 0.85 were obtained. To verify findings of in vitro experiments, a clinical LSME dataset obtained from 18 participants with carotid artery stenosis was used. Deduced out-of-plane motions (ranging from 0.25 mm to 1.04 mm) of the clinical dataset were classified into three groups: small, moderate and large elevational motions. Clinical results showed that pulsatile time-varying strains and shears remained reproducible for all motion categories since inter-frame correlation coefficients were higher than 0.70, and normalized cross-correlations (NCC) between radiofrequency (RF) images were above 0.93. In summary, the performance of LSME axial strain and shear estimations appeared robust in the presence of out-of-plane motions (<2 mm) as encountered during clinical ultrasound imaging.

Comparison of in vivo vs. ex situ obtained material properties of sheep common carotid artery

Patient-specific biomechanical modelling can improve preoperative surgical planning. This requires patient-specific geometry as well as patient-specific material properties as input. The latter are, however, still quite challenging to estimate in vivo. This study focuses on the estimation of the mechanical properties of the arterial wall. Firstly, in vivo pressure, diameter and thickness of the arterial wall were acquired for sheep common carotid arteries. Next, the animals were sacrificed and the tissue was stored for mechanical testing. Planar biaxial tests were performed to obtain experimental stress-stretch curves. Finally, parameters for the hyperelastic Mooney-Rivlin and Gasser-Ogden-Holzapfel (GOH) material model were estimated based on the in vivo obtained pressure-diameter data as well as on the ex situ experimental stress-stretch curves. Both material models were able to capture the in vivo behaviour of the tissue. However, in the ex situ case only the GOH model provided satisfactory results. When comparing different fitting approaches, in vivo vs. ex situ, each of them showed its own advantages and disadvantages. The in vivo approach estimates the properties of the tissue in its physiological state while the ex situ approach allows to apply different loadings to properly capture the anisotropy of the tissue. Both of them could be further enhanced by improving the estimation of the stress-free state, i.e. by adding residual circumferential stresses in vivo and by accounting for the flattening effect of the tested samples ex vivo. • Competing interests: none declared • Word count: 4716.

Quantification of carotid artery plaque stability with multiple region of interest based ultrasound strain indices and relationship with cognition

Vulnerability and instability in carotid artery plaque has been assessed based on strain variations using noninvasive ultrasound imaging. We previously demonstrated that carotid plaques with higher strain indices in a region of interest (ROI) correlated to patients with lower cognition, probably due to cerebrovascular emboli arising from these unstable plaques. This work attempts to characterize the strain distribution throughout the entire plaque region instead of being restricted to a single localized ROI. Multiple ROIs are selected within the entire plaque region, based on thresholds determined by the maximum and average strains in the entire plaque, enabling generation of additional relevant strain indices. Ultrasound strain imaging of carotid plaques, was performed on 60 human patients using an 18L6 transducer coupled to a Siemens Acuson S2000 system to acquire radiofrequency data over several cardiac cycles. Patients also underwent a battery of neuropsychological tests under a protocol based on National Institute of Neurological Disorders and Stroke and Canadian Stroke Network guidelines. Correlation of strain indices with composite cognitive index of executive function revealed a negative association relating high strain to poor cognition. Patients grouped into high and low cognition groups were then classified using these additional strain indices. One of our newer indices, namely the average L  -  1 norm with plaque (AL1NWP) presented with significantly improved correlation with executive function when compared to our previously reported maximum accumulated strain indices. An optimal combination of three of the new indices generated classifiers of patient cognition with an area under the curve (AUC) of 0.880, 0.921 and 0.905 for all (n  =  60), symptomatic (n  =  33) and asymptomatic patients (n  =  27) whereas classifiers using maximum accumulated strain indices alone provided AUC values of 0.817, 0.815 and 0.813 respectively.

Imaging of high-risk carotid plaques: ultrasound

Duplex ultrasonography has a well-established role in the assessment of the degree of stenosis caused by carotid atherosclerosis. This assessment is derived from Doppler velocity changes induced by the narrowing lumen of the artery. New research into the mechanisms for plaque rupture and atheroembolic stroke indicates that the degree of narrowing is an imperfect predictor of stroke risk, and that other factors, such as plaque composition and remodeling and biomechanical forces acting on the plaque, can play a role. New advances in ultrasound imaging technology have made it possible to investigate these measures of plaque vulnerability to identify pre-embolic unstable carotid plaques. Efforts have been made to quantify the morphologic appearance of the plaque in B-mode images and to correlate them with histology. Additional research has resulted in the first generation of clinically available 3-dimensional ultrasound transducers that reduce operator-dependence and variability. Finally, ultrasonography provides real-time imaging and physiologic information that can be utilized to measure disruptive forces acting on carotid plaques. We review some of these exciting developments in ultrasonography and discuss how these may impact clinical practice.

Non-Invasive Identification of Vulnerable Atherosclerotic Plaques Using Texture Analysis in Ultrasound Carotid Elastography: An In Vivo Feasibility Study Validated by Magnetic Resonance Imaging

The aims of this study were to quantify the textural information of strain rate images in ultrasound carotid elastography and evaluate the feasibility of using the textural features in discriminating stable and vulnerable plaques with magnetic resonance imaging as an in vivo reference. Ultrasound radiofrequency data were acquired in 80 carotid plaques from 52 patients, mainly in the longitudinal imaging view, and axial strain rate images were estimated with an ultrasound carotid elastography technique based on an optical flow algorithm. Four textural features of strain rate images-contrast, homogeneity, correlation and angular second moment-were derived based on the gray-level co-occurrence matrix in plaque regions to quantify the deformation distribution pattern. Conventional elastographic indices based on the magnitude of the absolute strain rate, such as the maximum, mean, median, standard deviation and 99th percentile of the axial strain rate, were also obtained for comparison. Composition measurement with magnetic resonance imaging identified 30 plaques as vulnerable and the other 50 as stable. The four textural features, as well as the magnitude of strain rate images, significantly differed between the two groups of plaques. The best performing features for plaque classification were found to be the contrast and 99th percentile of the absolute strain rate, with a comparative area under the receiver operating characteristic curve of 0.81; a slightly higher maximum accuracy of plaque classification can be achieved by the textural feature of contrast (83.8% vs. 81.3%). The results indicate that the use of texture analysis in plaque classification is feasible and that larger local deformations and higher level of complexity in deformation patterns (associated with the elastic or stiffness heterogeneity of plaque tissues) are more likely to occur in vulnerable plaques.

Clinical viability of carotid plaque strain estimation using B-mode ultrasound image sequences

It is estimated that approximately 30% of ischemic strokes are caused by rupture of plaque in the carotid artery. Development of techniques focusing on identifying plaques that are vulnerable to rupture is thus indispensable for stroke prevention. Recent studies have demonstrated that motion analysis of plaques from B-mode and RF ultrasound (US) image sequences can be used to estimate plaque strain. However, viability of these methods in a clinical setting, with variable acquisition protocols, has not been demonstrated yet. In this paper, we explore the viability of estimating plaque strain from B-mode US images of asymptomatic patients, acquired in a real clinical setting with different acquisition settings, frame rates, and operators. Our proposed strain measures, shear strain rate entropy and variance, combined with the recently reported maximum absolute shear strain rate, show that the plaques fall into two distinct clusters. Moreover, these clusters show good correlations with plaque echolucency and echogenicity. We conclude that B-mode US imaging is a viable tool for characterizing plaque dynamics in clinical environments. In future studies, we plan to implement this method on multi-center studies for longitudinal monitoring of plaque.

Noninvasive characterization of carotid plaque strain

OBJECTIVE

Current risk stratification of internal carotid artery plaques based on diameter-reducing percentage stenosis may be unreliable because ischemic stroke results from plaque disruption with atheroembolization. Biomechanical forces acting on the plaque may render it vulnerable to rupture. The feasibility of ultrasound-based quantification of plaque displacement and strain induced by hemodynamic forces and their relationship to high-risk plaques have not been determined. We studied the feasibility and reliability of carotid plaque strain measurement from clinical B-mode ultrasound images and the relationship of strain to high-risk plaque morphology.

METHODS

We analyzed carotid ultrasound B-mode cine loops obtained in patients with asymptomatic ≥50% stenosis during routine clinical scanning. Optical flow methods were used to quantify plaque motion and shear strain during the cardiac cycle. The magnitude (maximum absolute shear strain rate [MASSR]) and variability (entropy of shear strain rate [ESSR] and variance of shear strain rate [VSSR]) of strain were combined into a composite shear strain index (SSI), which was assessed for interscan repeatability and correlated with plaque echolucency.

RESULTS

Nineteen patients (mean age, 70 years) constituting 36 plaques underwent imaging; 37% of patients (n = 7) showed high strain (SSI ≥0.5; MASSR, 2.2; ESSR, 39.7; VSSR, 0.03) in their plaques; the remaining clustered into a low-strain group (SSI <0.5; MASSR, 0.58; ESSR, 21.2; VSSR, 0.002). The area of echolucent morphology was greater in high-strain plaques vs low-strain plaques (28% vs 17%; P = .018). Strain measurements showed low variability on Bland-Altman plots with cluster assignment agreement of 76% on repeated scanning. Two patients developed a stroke during 2 years of follow-up; both demonstrated high SSI (≥0.5) at baseline.

CONCLUSIONS

Carotid plaque strain is reliably computed from routine B-mode imaging using clinical ultrasound machines. High plaque strain correlates with known high-risk echolucent morphology. Strain measurement can complement identification of patients at high risk for plaque disruption and stroke.

Design and Fabrication of a Conceptual Arterial Ultrasound Phantom Capable of Exhibiting Longitudinal Wall Movement

The longitudinal movement of the arterial wall of large human arteries has shown promise to be an independent indicator of vascular health. Despite growing interest in this movement, its nature, causes, and implications are not fully understood, and existing phantoms have failed to show a pure longitudinal movement that is not secondary to the distension. An often overlooked aspect of the arterial wall is the interaction between the different layers. The longitudinal movement of the innermost layers, the intima and media, can be several hundred micrometers in the direction of flow during early systole. This is markedly larger than that of the adventitia, indicating that sliding occurs between the two layers. This feature was incorporated into a phantom by casting it in two parts. The molds were developed in-house using mainly a 3-D printer, a versatile and easy production method. Additionally, the phantom contains a tapered region. Using the phantom, we were able to demonstrate a pure longitudinal movement; when it was subjected to a pulsatile pressure, the wall displaced 220 [Formula: see text] (SD 40) radially and 560 [Formula: see text] (SD 74) longitudinally distal to the tapering. The motion followed the pressure variations. This paper serves as a guide for phantom production, explaining each step of the process.

Shear Wave Elastography Quantifies Stiffness in Ex Vivo Porcine Artery with Stiffened Arterial Region

Five small porcine aortas were used as a human carotid artery model, and their stiffness was estimated using shear wave elastography (SWE) in the arterial wall and a stiffened artery region mimicking a stiff plaque. To optimize the SWE settings, shear wave bandwidth was measured with respect to acoustic radiation force push length and number of compounded angles used for motion detection with plane wave imaging. The mean arterial wall and simulated plaque shear moduli varied from 41 ± 5 to 97 ± 10 kPa and from 86 ± 13 to 174 ± 35 kPa, respectively, over the pressure range 20-120 mmHg. The results revealed that a minimum bandwidth of approximately 1500 Hz is necessary for consistent shear modulus estimates, and a high pulse repetition frequency using no image compounding is more important than a lower pulse repetition frequency with better image quality when estimating arterial wall and plaque stiffness using SWE.

Ultrasound-Based Carotid Elastography for Detection of Vulnerable Atherosclerotic Plaques Validated by Magnetic Resonance Imaging

Ultrasound-based carotid elastography has been developed to estimate the mechanical properties of atherosclerotic plaques. The objective of this study was to evaluate the in vivo capability of carotid elastography in vulnerable plaque detection using high-resolution magnetic resonance imaging as reference. Ultrasound radiofrequency data of 46 carotid plaques from 29 patients (74 ± 5 y old) were acquired and inter-frame axial strain was estimated with an optical flow method. The maximum value of absolute strain rate for each plaque was derived as an indicator for plaque classification. Magnetic resonance imaging of carotid arteries was performed on the same patients to classify the plaques into stable and vulnerable groups for carotid elastography validation. The maximum value of absolute strain rate was found to be significantly higher in vulnerable plaques (2.15 ± 0.79 s(-1), n = 27) than in stable plaques (1.21 ± 0.37 s(-1), n = 19) (p < 0.0001). Receiver operating characteristic curve analysis was performed, and the area under the curve was 0.848. Therefore, the in vivo capability of carotid elastography to detect vulnerable plaques, validated by magnetic resonance imaging, was proven, revealing the potential of carotid elastography as an important tool in atherosclerosis assessment and stroke prevention.

Inflation and Bi-Axial Tensile Testing of Healthy Porcine Carotid Arteries

Knowledge of the intrinsic material properties of healthy and diseased arterial tissue components is of great importance in diagnostics. This study describes an in vitro comparison of 13 porcine carotid arteries using inflation testing combined with functional ultrasound and bi-axial tensile testing. The measured tissue behavior was described using both a linear, but geometrically non-linear, one-parameter (neo-Hookean) model and a two-parameter non-linear (Demiray) model. The shear modulus estimated using the linear model resulted in good agreement between the ultrasound and tensile testing methods, GUS = 25 ± 5.7 kPa and GTT = 23 ± 5.4 kPa. No significant correspondence was observed for the non-linear model aUS = 1.0 ± 2.7 kPa vs. aTT = 17 ± 8.8 kPa, p ∼ 0); however, the exponential parameters were in correspondence (bUS = 12 ± 4.2 vs. bTT = 10 ± 1.7, p > 0.05). Estimation of more complex models in vivo is cumbersome considering the sensitivity of the model parameters to small changes in measurement data and the absence of intraluminal pressure data, endorsing the use of a simple, linear model in vivo.

Quantification of carotid plaque elasticity and intraplaque neovascularization using contrast-enhanced ultrasound and image registration-based elastography

It is valuable for evaluation of carotid plaque vulnerability to investigate the relation between intraplaque neovascularization (IPN) and plaque elasticity. The contrast-enhanced ultrasound (CEUS) has been used in IPN measurement, but it cannot assess plaque elasticity. The aim of this study was to develop an ultrasound elastography technique based on registration of CEUS sequential images and to use this technique for direct comparison between IPN and plaque elasticity. We employed a nonrigid image registration method using the free-form deformation model to register a pair of clinical CEUS images at systole and diastole. The 2D displacement field of the plaque was estimated and then utilized to calculate the axial and lateral strain distributions within the plaque, from which quantitative strain parameters were obtained. The IPN was measured semiquantitatively with visual assessment and quantitatively with the time-intensity curve analysis and the analysis of contrast agent spatial distributions. Histopathology with CD34 staining for quantification of microvessel density (MVD) was performed on plaques excised by carotid endarterectomy. Simulation experiments showed that the mean absolute error and the root mean squared error of the displacement estimation were 0.325±0.180 pixel (7.2%±3.8%) and 0.556±0.284 pixel (12.3%±6.1%), respectively, demonstrating high accuracy of the elastography technique. Thirty-eight plaques in 29 patients met the inclusion criteria for the elastography and image analysis, where ten plaques underwent endarterectomy. The 95th percentile (A95) and standard deviation (Asd) of the axial strains exhibited significant differences between the low and high grades of IPN visually assessed (p<0.01). A95 (R=0.579; p<0.001) and Asd (R=0.609; p<0.001) were correlated with the enhanced intensity of plaque, and also correlated with the MVD (R=0.793 and 0.817, respectively; p<0.01), suggesting that plaque became softer and more elastically heterogeneous as IPN increased. These findings provide direct and quantitative evidence for the associations between plaque strains and IPN and might be helpful for evaluation of carotid plaque vulnerability and for plaque risk stratification.

Ultrasound imaging for risk assessment in atherosclerosis

Atherosclerosis and its consequences like acute myocardial infarction or stroke are highly prevalent in western countries, and the incidence of atherosclerosis is rapidly rising in developing countries. Atherosclerosis is a disease that progresses silently over several decades before it results in the aforementioned clinical consequences. Therefore, there is a clinical need for imaging methods to detect the early stages of atherosclerosis and to better risk stratify patients. In this review, we will discuss how ultrasound imaging can contribute to the detection and risk stratification of atherosclerosis by (a) detecting advanced and early plaques; (b) evaluating the biomechanical consequences of atherosclerosis in the vessel wall;