Plaque characterization using shear wave elastography—evaluation of differentiability and accuracy using a combined ex vivo and in vitro setup

Safety of Shear Wave Elastography as Evidenced From Carotid Artery Strain and Strain Rate Induced by Acoustic Radiation Force Impulse and Arterial Pulsations

OBJECTIVE

The aim of this study was to investigate the mechanical effects of carotid shear wave elastography (SWE) in vivo as its effects on the arterial wall have not been thoroughly examined.

METHODS

We evaluated the mechanical effects of carotid SWE in vivo in terms of the radial strain and strain rate to which acoustic radiation force impulses (ARFIs) expose the arterial wall, and compared them with the strain and strain rate induced by arterial pulsation in 13 healthy study subjects (seven individuals 20-35 y of age and six individuals 50-65 y of age). Additionally, we explored whether mechanical effects vary with timing of ARFI and subject age.

RESULTS

The young cohort was found to have, compared with the old cohort, a higher diastolic ARFI-induced peak strain (p = 0.002) and peak strain rate (p = 0.001), and a lower diastolic ARFI-induced peak negative strain rate (p = 0.013). When comparing cardiac phases, diastolic ARFIs were found to induce a lower peak negative strain rate than systolic ARFIs (p = 0.006). Importantly, ARFI-induced peak strain was lower than that caused by arterial pulsation in both age cohorts (p < 0.0001). The ARFI-induced peak strain rate was slightly higher than that caused by arterial pulsation at rest but lower than published exercise data. The ARFI-induced peak negative strain rate was similar to that caused by arterial pulsation.

CONCLUSION

Our results indicate that arterial SWE does not expose the arterial wall to any higher strain or strain rate than is experienced during normal arterial pulsation. Further research is required to validate the results in arteries containing vulnerable plaques.

Evaluating Carotid Plaque Stiffness with Ultrasound 2D Shear-Wave Elastography in Patients Undergoing Coronary Artery Bypass Grafting

Background: Coronary and carotid artery diseases are manifestations of a systemic atherosclerotic process, often coexisting in patients affected by both conditions. This association emphasizes the importance of evaluating both coronary and carotid atherosclerosis in high-risk individuals. Ultrasound 2D shear-wave elastography (2D-SWE) has shown promise as a noninvasive technique for assessing carotid plaque stiffness. This prospective pilot study aimed to assess carotid plaque stiffness in patients undergoing coronary artery bypass grafting (CABG) and those not scheduled for the procedure as a control group. Methods: 32 patients (17 CABG and 15 controls) were recruited, collectively presenting 43 carotid plaques. Bilateral carotid ultrasound was performed using a high-resolution linear transducer. Plaque stiffness was quantified via 2D-SWE, expressed in shear-wave velocity (SWV, m/s) and Young's modulus (YM, kPa). Plaque characteristics, including GSM, were quantified. Intra-observer reproducibility was evaluated with intraclass correlation coefficients (ICCs) and Bland-Altman plots. Statistical differences and correlations were assessed using Mann-Whitney U and Spearman's correlation tests. Results: Carotid plaques in the CABG group exhibited significantly lower stiffness compared to controls (median stiffness SWV: 3.64 m/s vs. 4.91 m/s, p < 0.0001; YM: 20.96 kPa vs. 72.54 kPa, p < 0.0001). ICCs demonstrated excellent reproducibility for stiffness measurements (SWV: ICC = 0.992; YM: ICC = 0.992), with minimal bias in measurements. A positive correlation was observed between 2D-SWE and GSM values (SWV: r = 0.343, p = 0.024; YM: r = 0.340, p = 0.026). Conclusions: Ultrasound 2D-SWE has shown promise as a reliable tool for quantifying carotid plaque stiffness, demonstrating high reproducibility and a significant correlation with GSM. The observed reduction in plaque stiffness among CABG patients highlights its potential as a valuable parameter for identifying high-risk plaques and assessing cerebrovascular risk in patients undergoing CABG.

Simulating atherosclerotic plaque mechanics using polyvinyl alcohol (PVA) cryogel artery phantoms, ultrasound imaging and inverse finite element analysis

The clinical decision to establish if a patient with carotid disease should undergo surgical intervention is primarily based on the percent stenosis. Whilst this applies for high-grade stenosed vessels (>70%), it falls short for other cases. Due to the heterogeneity of plaque tissue, probing the mechanics of the tissue would likely provide further insights into why some plaques are more prone to rupture. Mechanical characterization of such tissue is nontrivial, however, due to the difficulties in collecting fresh, intact plaque tissue and using physiologically relevant mechanical testing of such material. The use of polyvinyl alcohol (PVA) cryogel is thus highly convenient because of its acoustic properties and tunable mechanical properties.Methods.The aim of this study is to demonstrate the potential of PVA phantoms to simulate atherosclerotic features. In addition, a testing and simulation framework is developed for full PVA vessel material characterization using ring tensile testing and inflation testing combined with non-invasive ultrasound imaging and computational modeling.Results.Strain stiffening behavior was observed in PVA through ring tensile tests, particularly at high (n= 6) freeze-thaw cycles (FTCs). Inflation testing of bi-layered phantoms featuring lipid pool inclusions demonstrated high strains at shoulder regions. The application of an inverse finite element framework successfully recovered boundaries and determined the shear moduli for the PVA wall to lie within the range 27-53 kPa.Conclusion.The imaging-modeling framework presented facilitates the use and characterization of arterial mimicking phantoms to further explore plaque rupture. It also shows translational potential for non-invasive mechanical characterization of atherosclerotic plaques to improve the identification of clinically relevant metrics of plaque vulnerability.

Simulation of stress in a blood vessel due to plaque sediments in coronary artery disease

Atherosclerosis is a cardiovascular disease mainly caused by plaque deposition in blood vessels. Plaque comprises components such as thrombosis, fibrin, collagen, and lipid core. It plays an essential role in inducing rupture in a blood vessel. Generally, Plaque could be described as three kinds of elastic models: cellular Plaque, hypocellular Plaque, and calcified Plaque. The present study aimed to investigate the behavior of atherosclerotic plaque rupture according to different lipid cores using Fluid-Structure Interaction (FSI). The blood vessel was also varied with different thicknesses (0.05, 0.25, and 0.5 mm). In this study, FSI simulation with a cellular plaque model with various thicknesses was investigated to obtain information on plaque rupture. Results revealed that the blood vessel with Plaque having a lipid core represents higher stresses than those without a lipid core. Blood vessels' thin thickness, like a thin cap, results in more considerable than Von Mises stress. The result also suggests that even at low fracture stress, the risk of rupture due to platelet decomposition at the gap was more significant for cellular plaques.

Performance evaluation of commercial and non-commercial shear wave elastography implementations for vascular applications

BACKGROUND

Shear wave elastography (SWE) is mainly used for stiffness estimation of large, homogeneous tissues, such as the liver and breasts. However, little is known about its accuracy and applicability in thin (∼0.5-2 mm) vessel walls. To identify possible performance differences among vendors, we quantified differences in measured wave velocities obtained by commercial SWE implementations of various vendors over different imaging depths in a vessel-mimicking phantom. For reference, we measured SWE values in the cylindrical inclusions and homogeneous background of a commercial SWE phantom. Additionally, we compared the accuracy between a research implementation and the commercially available clinical SWE on an Aixplorer ultrasound system in phantoms and in vivo in patients.

METHODS

SWE measurements were performed over varying depths (0-35 mm) using three ultrasound machines with four ultrasound probes in the homogeneous 20 kPa background and cylindrical targets of 10, 40, and 60 kPa of a multi-purpose phantom (CIRS-040GSE) and in the anterior and posterior wall of a homogeneous polyvinyl alcohol vessel-mimicking phantom. These phantom data, along with in vivo SWE data of carotid arteries in 23 patients with a (prior) head and neck neoplasm, were also acquired in the research and clinical mode of the Aixplorer ultrasound machine. Machine-specific estimated phantom stiffness values (CIRS phantom) or wave velocities (vessel phantom) over all depths were visualized, and the relative error to the reference values and inter-frame variability (interquartile range/median) were calculated. Correlations between SWE values and target/vessel wall depth were explored in phantoms and in vivo using Spearman's correlations. Differences in wave velocities between the anterior and posterior arterial wall were assessed with Wilcoxon signed-rank tests. Intra-class correlation coefficients were calculated for a sample of ten patients as a measure of intra- and interobserver reproducibility of SWE analyses in research and clinical mode.

RESULTS

There was a high variability in obtained SWE values among ultrasound machines, probes, and, in some cases, with depth. Compared to the homogeneous CIRS-background, this variation was more pronounced for the inclusions and the vessel-mimicking phantom. Furthermore, higher stiffnesses were generally underestimated. In the vessel-mimicking phantom, anterior wave velocities were (incorrectly) higher than posterior wave velocities (3.4-5.6 m/s versus 2.9-5.9 m/s, p ≤ 0.005 for 3/4 probes) and remarkably correlated with measurement depth for most machines (Spearman's ρ = -0.873-0.969, p < 0.001 for 3/4 probes). In the Aixplorer's research mode, this difference was smaller (3.3-3.9 m/s versus 3.2-3.6 m/s, p = 0.005) and values did not correlate with measurement depth (Spearman's ρ = 0.039-0.659, p ≥ 0.002). In vivo, wave velocities were higher in the posterior than the anterior vessel wall in research (left p = 0.001, right p < 0.001) but not in clinical mode (left: p = 0.114, right: p = 0.483). Yet, wave velocities correlated with vessel wall depth in clinical (Spearman's ρ = 0.574-0.698, p < 0.001) but not in research mode (Spearman's ρ = -0.080-0.466, p ≥ 0.003).

CONCLUSIONS

We observed more variation in SWE values among ultrasound machines and probes in tissue with high stiffness and thin-walled geometry than in low stiffness, homogeneous tissue. Together with a depth-correlation in some machines, where carotid arteries have a fixed location, this calls for caution in interpreting SWE results in clinical practice for vascular applications.

Extending arterial stiffness assessment along the circumference using beam-steered ARFI and wave-tracking: A proof-of-principle study in phantoms and ex vivo

Background

To fully quantify arterial wall and plaque stiffness, acoustic radiation force impulse (ARFI)-induced wave-tracking along the entire vessel circumference is desired. However, attenuation and guided wave behavior in thin vessel walls limits wave-tracking to short trajectories. This study investigated the potential of beam-steered ARFI and wave-tracking to extend group velocity estimation over a larger proportion of the circumference compared to conventional 0° ARFI-induced wave-tracking.

Methods

Seven vessel-mimicking polyvinyl alcohol cryogel phantoms with various dimensions and compositions and an ex vivo human carotid artery were imaged in a dedicated setup. For every 20⁰ phantom rotation, transverse group wave velocity measurements were performed with an Aixplorer Ultimate system and SL18-5 transducer using 0⁰/20⁰/-20⁰-angled ultrasound pushes. Transmural angular wave velocities were derived along 60⁰-trajectories. A 360⁰-angular velocity map was composed from the top-wall 60⁰-trajectories 0°-data, averaged over all physical phantom rotations (reference). For each phantom rotation, 360⁰-angular velocity maps were composed using 0°-data (0⁰-approach) or data from all angles (beam-steered approach). Percentages of rotations with visible waves and relative angular velocity errors compared to the reference map as function of the circumferential angle were determined for both approaches.

Results

Reference 360°-angular velocity maps could be derived for all samples, representing their stiffness. Beam-steering decreased the proportion of the circumference where waves were untraceable by 20% in phantoms and 10% ex vivo, mainly at 0° push locations. Relative errors were similar for both approaches (phantoms: 10-15%, ex vivo: 15-35%).

Conclusion

Beam-steering enables wave-tracking along a higher proportion of the wall circumference than 0⁰ ARFI-induced wave-tracking.

Novel ultrasound techniques in the identification of vulnerable plaques-an updated review of the literature

Atherosclerosis is an inflammatory disease partly mediated by lipoproteins. The rupture of vulnerable atherosclerotic plaques and thrombosis are major contributors to the development of acute cardiovascular events. Despite various advances in the treatment of atherosclerosis, there has been no satisfaction in the prevention and assessment of atherosclerotic vascular disease. The identification and classification of vulnerable plaques at an early stage as well as research of new treatments remain a challenge and the ultimate goal in the management of atherosclerosis and cardiovascular disease. The specific morphological features of vulnerable plaques, including intraplaque hemorrhage, large lipid necrotic cores, thin fibrous caps, inflammation, and neovascularisation, make it possible to identify and characterize plaques with a variety of invasive and non-invasive imaging techniques. Notably, the development of novel ultrasound techniques has introduced the traditional assessment of plaque echogenicity and luminal stenosis to a deeper assessment of plaque composition and the molecular field. This review will discuss the advantages and limitations of five currently available ultrasound imaging modalities for assessing plaque vulnerability, based on the biological characteristics of the vulnerable plaque, and their value in terms of clinical diagnosis, prognosis, and treatment efficacy assessment.

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.

Downregulation of GSK-3β Expression via Ultrasound-Targeted Microbubble Destruction Enhances Atherosclerotic Plaque Stability in New Zealand Rabbits

Accumulating evidence suggests that atherosclerosis (AS) is the underlying cause of vascular diseases, including heart disease and stroke. Ultrasound-targeted microbubble destruction (UTMD) technology provides a tolerable, efficient and effective system for drug delivery and gene transfection, which has broad application prospects in the treatment of AS. In addition, glycogen synthase kinase (GSK)-3β has been implicated as a potentially valuable therapeutic agent for AS treatment; however, the specific molecular mechanisms remain unknown. Therefore, this study was conducted to explore the effect of downregulation of GSK-3β expression via UTMD on atherosclerotic plaque stability. We established a THP-1 macrophage-derived foam cell model in vitro and an atherosclerotic plaque model in the right common carotid artery of New Zealand rabbits. We determined levels of the relevant vulnerable plaque stability elements. The results indicate that GSK-3β was upregulated in the foam cells and in atherosclerotic rabbits. Downregulation of GSK-3β expression by UTMD suppressed vulnerable plaque factors and inflammation in vitro and in vivo, changed the cytoskeleton of the foam cells in vitro, increased Young's modulus and decreased the peak intensity of atherosclerotic plaque in vivo. Moreover, GSK-3β inhibition by UTMD did not influence the viability of the foam cells. Collectively, our results indicate that GSK-3β could be a potential target for anti-atherogenic interventions and, in particular, can improve the stability of AS plaques in combination with UTMD.

Vortex dynamics and transport phenomena in stenotic aortic models using Echo-PIV

Atherosclerosis is the most fatal cardiovascular disease. As disease progresses, stenoses grow inside the arteries blocking their lumen and altering blood flow. Analysing flow dynamics can provide a deeper insight on the stenosis evolution. In this work we combined Eulerian and Lagrangian descriptors to analyze blood flow dynamics and fluid transport in stenotic aortic models with morphology, mechanical and optical properties close to those of real arteries. To this end, vorticity, particle residence time (PRT), particle's final position (FP) and finite time Lyapunov's exponents (FTLE) were computed from the experimental fluid velocity fields acquired using ultrasonic particle imaging velocimetry (Echo-PIV). For the experiments, CT-images were used to create morphological realistic models of the descending aorta with 0%, 35% and 50% occlusion degree with same mechanical properties as real arteries. Each model was connected to a circuit with a pulsatile programmable pump which mimics physiological flow and pressure conditions. The pulsatile frequency was set to ≈0.9 Hz (55 bpm) and the upstream peak Reynolds number (Re) was changed from 1100 to 2000. Flow in the post-stenotic region was composed of two main structures: a high velocity jet over the stenosis throat and a recirculation region behind the stenosis where vortex form and shed. We characterized vortex kinematics showing that vortex propagation velocity increases withRe. Moreover, from the FTLE field we identified Lagrangian coherent structures (i.e. material barriers) that dictate transport behind the stenosis. The size and strength of those barriers increased withReand the occlusion degree. Finally, from the PRT and FP maps, we showed that independently ofRe, the same amount of fluid remains on the stenosis over more than a pulsatile period.

A parametric evaluation of shear wave speeds estimated with time-of-flight calculations in viscoelastic media

Shear wave elasticity imaging (SWEI) uses an acoustic radiation force to generate shear waves, and then soft tissue mechanical properties are obtained by analyzing the shear wave data. In SWEI, the shear wave speed is often estimated with time-of-flight (TOF) calculations. To characterize the errors produced by TOF calculations, three-dimensional (3D) simulated shear waves are described by time-domain Green's functions for a Kelvin-Voigt model evaluated for multiple combinations of the shear elasticity and the shear viscosity. Estimated shear wave speeds are obtained from cross correlations and time-to-peak (TTP) calculations applied to shear wave particle velocities and shear wave particle displacements. The results obtained from these 3D shear wave simulations indicate that TTP calculations applied to shear wave particle displacements yield effective estimates of the shear wave speed if noise is absent, but cross correlations applied to shear wave particle displacements are more robust when the effects of noise and shear viscosity are included. The results also show that shear wave speeds estimated with TTP methods and cross correlations using shear wave particle velocities are more sensitive to increases in shear viscosity and noise, which suggests that superior estimates of the shear wave speed are obtained from noiseless or noisy shear wave particle displacements.

Vascular Shear Wave Elastography in Atherosclerotic Arteries: A Systematic Review

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.

A parametric evaluation of shear wave speeds estimated with time-of-flight calculations in viscoelastic media

Shear wave elasticity imaging (SWEI) uses an acoustic radiation force to generate shear waves, and then soft tissue mechanical properties are obtained by analyzing the shear wave data. In SWEI, the shear wave speed is often estimated with time-of-flight (TOF) calculations. To characterize the errors produced by TOF calculations, three-dimensional (3D) simulated shear waves are described by time-domain Green's functions for a Kelvin-Voigt model evaluated for multiple combinations of the shear elasticity and the shear viscosity. Estimated shear wave speeds are obtained from cross correlations and time-to-peak (TTP) calculations applied to shear wave particle velocities and shear wave particle displacements. The results obtained from these 3D shear wave simulations indicate that TTP calculations applied to shear wave particle displacements yield effective estimates of the shear wave speed if noise is absent, but cross correlations applied to shear wave particle displacements are more robust when the effects of noise and shear viscosity are included. The results also show that shear wave speeds estimated with TTP methods and cross correlations using shear wave particle velocities are more sensitive to increases in shear viscosity and noise, which suggests that superior estimates of the shear wave speed are obtained from noiseless or noisy shear wave particle displacements.

Combined spatiotemporal and frequency-dependent shear wave elastography enables detection of vulnerable carotid plaques as validated by MRI

Fatal cerebrovascular events are often caused by rupture of atherosclerotic plaques. However, rupture-prone plaques are often distinguished by their internal composition rather than degree of luminal narrowing, and conventional imaging techniques might thus fail to detect such culprit lesions. In this feasibility study, we investigate the potential of ultrasound shear wave elastography (SWE) to detect vulnerable carotid plaques, evaluating group velocity and frequency-dependent phase velocities as novel biomarkers for plaque vulnerability. In total, 27 carotid plaques from 20 patients were scanned by ultrasound SWE and magnetic resonance imaging (MRI). SWE output was quantified as group velocity and frequency-dependent phase velocities, respectively, with results correlated to intraplaque constituents identified by MRI. Overall, vulnerable lesions graded as American Heart Association (AHA) type VI showed significantly higher group and phase velocity compared to any other AHA type. A selection of correlations with intraplaque components could also be identified with group and phase velocity (lipid-rich necrotic core content, fibrous cap structure, intraplaque hemorrhage), complementing the clinical lesion classification. In conclusion, we demonstrate the ability to detect vulnerable carotid plaques using combined SWE, with group velocity and frequency-dependent phase velocity providing potentially complementary information on plaque characteristics. With such, the method represents a promising non-invasive approach for refined atherosclerotic risk prediction.