Material properties of components in human carotid atherosclerotic plaques: a uniaxial extension study

Ultrasound Strain Imaging for Characterizing Atherosclerotic Plaque in the Carotid Arteries of Asymptomatic Subjects

OBJECTIVE

The rupture of vulnerable plaques in the carotid artery is a leading cause of strokes. While magnetic resonance imaging (MRI) is the standard for quantifying plaque composition, its high costs and lengthy procedure times limit large-scale use. Compound ultrasound strain imaging (CUSI) ultrasound offers a non-invasively alternative by assessing tissue deformation/strain within the arterial wall. Previous studies have demonstrated the relationship between strain values and plaque composition, primarily focusing on longitudinal acquisitions, which only image the proximal and distal parts of the wall. This study examines CUSI in transverse imaging planes, which enables visualization of the entire cross-section of the vessel wall. CUSI has already been validated on a symptomatic population. Therefore, the aim is to determine whether CUSI can differentiate plaque composition in asymptomatic individuals with an increased cardiovascular risk profile, validated by MRI.

METHODS

This cross-sectional study included 42 plaques in 28 participants. Ultrasound data were acquired with a Samsung Medison Accuvix V10 with an L5-13IS transducer and RF-interface. An experienced neuroradiologist classified plaques with MRI using the Siemens 3.0T MAGNETOM Skyra MRI (Erlangen, Germany) with a dedicated coil. Strain differences were compared across four plaque categories (calcified, lipid, hemorrhagic, fibrous/aspecific) and for vulnerable versus stable plaques.

RESULTS

A difference in strain values was found between lipid and calcified plaques (Kruskal-Wallis test, p < 0.05). Additionally, vulnerable plaques exhibited higher strain values than stable plaques (independent samples test, p < 0.05).

CONCLUSION

CUSI values differ between lipid and calcified and between stable and vulnerable plaques. Minor differences were found between the other plaque types.

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.

Predicting vulnerable coronary arteries: A combined radiomics-biomechanics approach

BACKGROUND AND OBJECTIVE

Nowadays, vulnerable coronary plaque detection from coronary computed tomography angiography (CCTA) is suboptimal, although being crucial for preventing major adverse cardiac events. Moreover, despite the suggestion of various vulnerability biomarkers, encompassing image and biomechanical factors, accurate patient stratification remains elusive, and a comprehensive approach integrating multiple markers is lacking. To this aim, this study introduces an innovative approach for assessing vulnerable coronary arteries and patients by integrating radiomics and biomechanical markers through machine learning methods.

METHODS

The study included 40 patients (7 high-risk and 33 low-risk) who underwent both CCTA and coronary optical coherence tomography (OCT). The dataset comprised 49 arteries (with 167 plaques), 7 of which (with 28 plaques) identified as vulnerable by OCT. Following image preprocessing and segmentation, CCTA-based radiomic features were extracted and a finite element analysis was performed to compute the biomechanical features. A novel machine learning pipeline was implemented to stratify coronary arteries and patients. For each stratification task, three independent predictive models were developed: a radiomic, a biomechanical and a combined radiomic-biomechanical model. Both k-nearest neighbors (KNN) and decision tree (DT) classifiers were considered.

RESULTS

The best radiomic model (KNN) detected all 7 vulnerable arteries and patients and was associated with a balanced accuracy of 0.86 (sensitivity=1, specificity=0.71) for the artery model and of 0.83 (sensitivity=1, specificity=0.67) for the patient model. The best biomechanical model (DT) detected 6 over 7 vulnerable arteries and patients and remarkably increased the specificity, resulting in a balanced accuracy of 0.89 (sensitivity=0.86, specificity=0.93) for the artery model and of 0.88 (sensitivity=0.86, specificity=0.91) for the patient model. Notably, the combined approach optimized the performance, with an increase in the balance accuracy up to 0.94 for the artery model and up to 0.92 for the patient model, being associated with sensitivity=1 and high specificity (0.88 and 0.85 for artery and patient models, respectively).

CONCLUSION

This investigation highlights the promise of radio-mechanical coronary artery phenotyping for patient stratification. If confirmed from larger studies, our approach enables a more personalized management of the disease, with the early identification of high-risk individuals and the reduction of unnecessary interventions for low-risk individuals.

Deciphering the complex mechanics of atherosclerotic plaques: A hybrid hierarchical theory-microrheology approach

Understanding the viscoelastic properties of atherosclerotic plaques at rupture-prone scales is crucial for assessing their vulnerability. Here, we develop a Hybrid Hierarchical theory-Microrheology (HHM) approach, enabling the analysis of multiscale mechanical variations and distribution changes in regional tissue viscoelasticity within plaques across different spatial scales. We disclose a universal two-stage power-law rheology in plaques, characterized by distinct power-law exponents (αshort and αlong), which serve as mechanical indexes for plaque components and assessing mechanical gradients. We further propose a self-similar hierarchical theory that effectively delineates plaque heterogeneity from the cytoplasm, cell, to tissue levels. Moreover, our proposed multi-layer perceptron model addresses the viscoelastic heterogeneity and gradients within plaques, offering a promising diagnostic strategy for identifying unstable plaques. These findings not only advance our understanding of plaque mechanics but also pave the way for innovative diagnostic approaches in cardiovascular disease management. STATEMENT OF SIGNIFICANCE: Our study pioneers a Hybrid Hierarchical theory-Microrheology (HHM) approach to dissect the intricate viscoelasticity of atherosclerotic plaques, focusing on distinct components including cap fibrosis, lipid pools, and intimal fibrosis. We unveil a universal two-stage power-law rheology capturing mechanical variations across plaque structures. The proposed hierarchical model adeptly captures viscoelasticity changes from cytoplasm, cell to tissue levels. Based on the newly proposed markers, we further develop a machine learning (ML) diagnostic model that sets precise criteria for evaluating plaque components and heterogeneity. This work not only reveals the comprehensive mechanical heterogeneity within plaques but also introduces a mechanical marker-based ML strategy for assessing plaque conditions, offering a significant leap towards understanding and diagnosing atherosclerotic risks.

Combining Computational Fluid Dynamics, Structural Analysis, and Machine Learning to Predict Cerebrovascular Events: A Mild ML Approach

Background/Objectives: Cerebrovascular events, such as strokes, are often preceded by the rupture of atherosclerotic plaques in the carotid arteries. This work introduces a novel approach to predict the occurrence of such events by integrating computational fluid dynamics (CFD), structural analysis, and machine learning (ML) techniques. The objective is to develop a predictive model that combines both imaging and non-imaging data to assess the risk of carotid atherosclerosis and subsequent cerebrovascular events, ultimately improving clinical decision-making. Methods: A multidisciplinary approach was employed, utilizing 3D reconstruction techniques and blood-flow simulations to extract key plaque characteristics. These were combined with patient-specific clinical data for risk evaluation. The study involved 134 asymptomatic individuals diagnosed with carotid artery disease. Data imbalance was addressed using two distinct approaches, with the optimal method chosen for training a Gradient Boosting Tree (GBT) classifier. The model's performance was evaluated in terms of accuracy, sensitivity, specificity, and ROC AUC. Results: The best-performing GBT model achieved a balanced accuracy of 88%, with a ROC AUC of 0.92, a sensitivity of 0.88, and a specificity of 0.91. This demonstrates the model's high predictive power in identifying patients at risk for cerebrovascular events. Conclusions: The proposed method effectively combines CFD, structural analysis, and ML to predict cerebrovascular event risk in patients with carotid artery disease. By providing clinicians with a tool for better risk assessment, this approach has the potential to significantly enhance clinical decision-making and patient outcomes.

Effect of variability of mechanical properties on the predictive capabilities of vulnerable coronary plaques

BACKGROUND AND OBJECTIVE

Coronary plaque rupture is a precipitating event responsible for two thirds of myocardial infarctions. Currently, the risk of plaque rupture is computed based on demographic, clinical, and image-based adverse features. However, using these features the absolute event rate per single higher-risk lesion remains low. This work studies the power of a novel framework based on biomechanical markers accounting for material uncertainty to stratify vulnerable and non-vulnerable coronary plaques.

METHODS

Virtual histology intravascular ultrasounds from 55 patients, 29 affected by acute coronary syndrome and 26 affected by stable angina pectoris, were included in this study. Two-dimensional vessel cross-sections for finite element modeling (10 sections per plaque) incorporating plaque structure (medial tissue, loose matrix, lipid core and calcification) were reconstructed. A Montecarlo finite element analysis was performed on each section to account for material variability on three biomechanical markers: peak plaque structural stress at diastolic and systolic pressure, and peak plaque stress difference between systolic and diastolic pressures, together with the luminal pressure. Machine learning decision tree classifiers were trained on 75% of the dataset and tested on the remaining 25% with a combination of feature selection techniques. Performance against classification trees based on geometric markers (i.e., luminal, external elastic membrane and plaque areas) was also performed.

RESULTS

Our results indicate that the plaque structural stress outperforms the classification capacity of the combined geometric markers only (0.82 vs 0.51 area under curve) when accounting for uncertainty in material parameters. Furthermore, the results suggest that the combination of the peak plaque structural stress at diastolic and systolic pressures with the maximum plaque structural stress difference between systolic and diastolic pressures together with the systolic pressure and the diastolic to systolic pressure gradient is a robust classifier for coronary plaques when the intrinsic variability in material parameters is considered (area under curve equal to [0.91-0.93]).

CONCLUSION

In summary, our results emphasize that peak plaque structural stress in combination with the patient's luminal pressure is a potential classifier of plaque vulnerability as it independently considers stress in all directions and incorporates total geometric and compositional features of atherosclerotic plaques.

Comparison of Biomechanical and Microstructural Properties of Aortic Graft Materials in Aortic Repair Surgeries

Mechanical mismatch between native aortas and aortic grafts can induce graft failure. This study aims to compare the mechanical and microstructural properties of different graft materials used in aortic repair surgeries with those of normal and dissected human ascending aortas. Five types of materials including normal aorta (n = 10), dissected aorta (n = 6), human pericardium (n = 8), bovine pericardium (n = 8) and Dacron graft (n = 5) were collected to perform uniaxial tensile testing to determine their material stiffness, and ultimate strength/stretch. The elastin and collagen contents in four tissue groups except for Dacron were quantified by histological examinations, while the material ultrastructure of five material groups was visualized by scanning electron microscope. Statistical results showed that three graft materials including Dacron, human pericardium and bovine pericardium had significantly higher ultimate strength and stiffness than both normal and dissected aortas. Human and bovine pericardia had significantly lower ultimate stretch than native aortas. Histological examinations revealed that normal and diseased aortic tissues had a significantly higher content of elastic fiber than two pericardial tissues, but less collagen fiber content. All four tissue groups exhibited lamellar fiber ultrastructure, with aortic tissues possessing thinner lamella. Dacron was composed of densely coalesced polyethylene terephthalate fibers in thick bundles. Aortic graft materials with denser fiber ultrastructure and/or higher content of collagen fiber than native aortic tissues, exhibited higher ultimate strength and stiffness. This information provides a basis to understand the mechanical failure of aortic grafts, and inspire the design of biomimetic aortic grafts.

Matrix stiffness regulates macrophage polarisation via the Piezo1-YAP signalling axis

Macrophages play a pivotal role in the immunological cascade activated in response to biomedical implants, which predetermine acceptance or rejection of implants by the host via pro- and anti-inflammatory polarisation states. The role of chemical signals in macrophage polarisation is well-established, but how physical cues regulate macrophage function that may play a fundamental role in implant-bone interface, remains poorly understood. Here we find that bone marrow-derived macrophages (BMDM) cultured on polyacrylamide gels of varying stiffness exhibit different polarisation states. BMDM are 'primed' to a pro-inflammatory M1 phenotype on stiff substrates, while to an anti-inflammatory M2 phenotype on soft and medium stiffness substrates. It is further observed that matrix stiffening increases Piezo1 expression, as well as leads to subsequent activation of the mechanotransduction signalling effector YAP, thus favouring M1 polarisation whilst suppressing M2 polarisation. Moreover, upon treatment with YAP inhibitor, we successfully induce macrophage re-polarisation to the M2 state within the implant site microenvironment, which in turn promotes implant osseointegration. Collectively, our present study thus characterises the critical role of the Piezo1-YAP signalling axis in macrophage mechanosensing and stiffness-mediated macrophage polarisation and provides cues for the design of immuno-modulatory biomaterials that can regulate the macrophage phenotype.

Hemodynamic Impairment of Blood Pressure and Stroke Mechanisms in Symptomatic Intracranial Atherosclerotic Stenosis

BACKGROUND

Hemodynamic impairment of blood pressure may play a crucial role in determining the mechanisms of stroke in symptomatic intracranial atherosclerotic stenosis). We aimed to elucidate this issue and assess the impacts of modifications to blood pressure on hemodynamic impairment.

METHODS

From the Third China National Stroke Registry III, computed fluid dynamics modeling was performed using the Newton-Krylov-Schwarz method in 339 patients with symptomatic intracranial atherosclerotic stenosis during 2015 to 2018. The major exposures were translesional systolic blood pressure (SBP) drop and poststenotic mean arterial pressure (MAP), and the major study outcomes were cortex-involved infarcts and borderzone-involved infarcts, respectively. Multivariate logistic regression models and the bootstrap resampling method were utilized, adjusting for demographics and medical histories.

RESULTS

In all, 184 (54.3%) cortex-involved infarcts and 70 (20.6%) borderzone-involved infarcts were identified. In multivariate logistic model, the upper quartile of SBP drop correlated with increased cortex-involved infarcts (odds ratio, 1.92 [95% CI, 1.03-3.57]; bootstrap analysis odds ratio, 2.07 [95% CI, 1.09-3.93]), and the lower quartile of poststenotic MAP may correlate with increased borderzone-involved infarcts (odds ratio, 2.07 [95% CI, 0.95-4.51]; bootstrap analysis odds ratio, 2.38 [95% CI, 1.04-5.45]). Restricted cubic spline analysis revealed a consistent upward trajectory of the relationship between translesional SBP drop and cortex-involved infarcts, while a downward trajectory between poststenotic MAP and borderzone-involved infarcts. SBP drop correlated with poststenotic MAP negatively (rs=-0.765; P<0.001). In generating hemodynamic impairment, simulating blood pressure modifications suggested that ensuring adequate blood pressure to maintain sufficient poststenotic MAP appears preferable to the reverse approach, due to the prolonged plateau period in the association between the translesional SBP drop and cortex-involved infarcts and the relatively short plateau period characterizing the correlation between poststenotic MAP and borderzone-involved infarcts.

CONCLUSIONS

This research elucidates the role of hemodynamic impairment of blood pressure in symptomatic intracranial atherosclerotic stenosis-related stroke mechanisms, underscoring the necessity to conduct hemodynamic assessments when managing blood pressure in symptomatic intracranial atherosclerotic stenosis.

Early matrix softening contributes to vascular smooth muscle cell phenotype switching and aortic dissection through down-regulation of microRNA-143/145

Thoracic aortic dissection (TAD) is characterized by extracellular matrix (ECM) dysregulation. Aberrations in the ECM stiffness can lead to changes in cellular functions. However, the mechanism by which ECM softening regulates vascular smooth muscle cell (VSMCs) phenotype switching remains unclear. To understand this mechanism, we cultured VSMCs in a soft extracellular matrix and discovered that the expression of microRNA (miR)-143/145, mediated by activation of the AKT signalling pathway, decreased significantly. Furthermore, overexpression of miR-143/145 reduced BAPN-induced aortic softening, switching the VSMC synthetic phenotype and the incidence of TAD in mice. Additionally, high-throughput sequencing of immunoprecipitated RNA indicated that the TEA domain transcription factor 1 (TEAD1) is a common target gene of miR-143/145, which was subsequently verified using a luciferase reporter assay. TEAD1 is upregulated in soft ECM hydrogels in vitro, whereas the switch to a synthetic phenotype in VSMCs decreases after TEAD1 knockdown. Finally, we verified that miR-143/145 levels are associated with disease severity and prognosis in patients with thoracic aortic dissection. ECM softening, as a result of promoting the VSMCs switch to a synthetic phenotype by downregulating miR-143/145, is an early trigger of TAD and provides a therapeutic target for this fatal disease. miR-143/145 plays a role in the early detection of aortic dissection and its severity and prognosis, which can offer information for future risk stratification of patients with dissection.

Computed tomography defined femoral artery plaque composition predicts vascular complications during transcatheter aortic valve implantation

OBJECTIVE

Vascular and bleeding complications after transcatheter aortic valve implantation (TAVI) are common and lead to increased morbidity and mortality. Analysis of plaque at the arterial access site may improve prediction of complications.

METHODS

We investigated the association between demographic and procedural risk factors for Valve Academic Research Consortium (VARC-3) vascular complications in patients undergoing transfemoral TAVI with use of a vascular closure device (ProGlide® or MANTA®) in this retrospective cohort study. The ability of pre-procedure femoral CT angiography to predict complications was investigated including a novel method of quantifying plaque composition of the common femoral artery using plaque maps created with patient specific X-ray attenuation cut-offs.

RESULTS

23 vascular complications occurred in the 299 patients in the study group (7.7%). There were no demographic risk factors associated with vascular complications and no statistical difference between use of closure device (ProGlide® vs MANTA®) and vascular complications. Vascular complications after TAVI were associated with sheath size (OR 1.36, 95% CI 1.08-1.76, P 0.01) and strongly associated with CT-derived necrotic core volume in the common femoral artery of the procedural side (OR 17.49, 95% CI 1.21-226.60, P 0.03).

CONCLUSION

Plaque map analysis of the common femoral artery by CT angiography reveals patients with greater necrotic core are at increased risk of VARC-3 vascular complications.

ADVANCES IN KNOWLEDGE

The novel measurement of necrotic core volume in the common femoral artery on the procedural side by CT analysis was associated with post-TAVI vascular complications, which can be used to highlight increased risk.

Biomechanical characterization of normal and pathological human ascending aortic tissues via biaxial testing Experiment, constitutive modeling and finite element analysis

BACKGROUND

Aortic dissection and atherosclerosis are two common pathological conditions affecting the aorta. Aortic biomechanics are believed to be closely associated with the pathological development of these diseases. However, the biomechanical environment that predisposes the aortic wall to these pathological conditions remains unclear.

METHODS

Sixteen ascending aortic specimens were harvested from 16 human subjects and further categorized into three groups according to their disease states: aortic dissection group, aortic dissection with accompanied atherosclerosis group and healthy group. Experimental stress-strain data from biaxial tensile testing were used to fit the anisotropic Mooney-Rivlin model to determine material parameters. Computed tomography images or transesophageal echocardiography images were collected to construct computational models to simulate the stress/strain distributions in aortas at the pre-dissection state. Statistical analyses were performed to identify the biomechanical factors to distinguish three groups of aortic tissues.

RESULTS

Material parameters of anisotropic Mooney-Rivlin model were fitted with average R2 value 0.9749. The aortic diameter showed no significant difference among three groups. Changes of maximum and average stress values from minimum pressure to maximum pressure (△MaxStress and △AveStress) had significantly difference between dissection group and dissection with accompanied atherosclerosis group (p = 0.0201 and 0.0102). Changes of maximum and average strain values from minimum pressure to maximum pressure (△MaxStrain and △AveStrain) from dissection group were significant different from healthy group (p = 0.0171 and 0.0281).

CONCLUSION

Changes of stress and strain values during the cardiac cycle are good biomechanical factors for predicting potential aortic dissection and aortic dissection accompanied with atherosclerosis.

Comparison of multilayer and single-layer coronary plaque models on stress/strain calculations based on optical coherence tomography images

Mechanical stress and strain conditions are closely related to atherosclerotic plaque progression and rupture and have been under intensive investigations in recent years. It is well known that arteries have a three-layer structure: intima, media and adventitia. However, in vivo image-based multilayer plaque models are not available in the current literature due to lack of multilayer image segmentation data. A multilayer segmentation and repairing technique was introduced to segment coronary plaque optical coherence tomography (OCT) image to obtain its three-layer vessel structure. A total of 200 OCT slices from 20 patients (13 male; 7 female) were used to construct multilayer and single-layer 3D thin-slice models to calculate plaque stress and strain and compare model differences. Our results indicated that the average maximum plaque stress values of 20 patients from multilayer and single-layer models were 385.13 ± 110.09 kPa and 270.91 ± 95.86 kPa, respectively. The relative difference was 42.2%, with single-layer stress serving as the base value. The average mean plaque stress values from multilayer and single-layer models were 129.59 ± 32.77 kPa and 93.27 ± 18.20 kPa, respectively, with a relative difference of 38.9%. The maximum and mean plaque strain values obtained from the multilayer models were 11.6% and 19.0% higher than those from the single-layer models. Similarly, the maximum and mean cap strains showed increases of 9.6% and 12.9% over those from the single-layer models. These findings suggest that use of multilayer models could improve plaque stress and strain calculation accuracy and may have large impact on plaque progression and vulnerability investigation and potential clinical applications. Further large-scale studies are needed to validate our findings.

A new approach of using organ-on-a-chip and fluid-structure interaction modeling to investigate biomechanical characteristics in tissue-engineered blood vessels

The tissue-engineered blood vessel (TEBV) has been developed and used in cardiovascular disease modeling, preclinical drug screening, and for replacement of native diseased arteries. Increasing attention has been paid to biomechanical cues in TEBV and other tissue-engineered organs to better recapitulate the functional properties of the native organs. Currently, computational fluid dynamics models were employed to reveal the hydrodynamics in TEBV-on-a-chip. However, the biomechanical wall stress/strain conditions in the TEBV wall have never been investigated. In this paper, a straight cylindrical TEBV was placed into a polydimethylsiloxane-made microfluidic device to construct the TEBV-on-a-chip. The chip was then perfused with cell culture media flow driven by a peristaltic pump. A three-dimensional fluid-structure interaction (FSI) model was generated to simulate the biomechanical conditions in TEBV and mimic both the dynamic TEBV movement and pulsatile fluid flow. The material stiffness of the TEBV wall was determined by uniaxial tensile testing, while the viscosity of cell culture media was measured using a rheometer. Comparison analysis between the perfusion experiment and FSI model results showed that the average relative error in diameter expansion of TEBV from both approaches was 10.0% in one period. For fluid flow, the average flow velocity over a period was 2.52 cm/s from the FSI model, 10.5% higher than the average velocity of the observed cell clusters (2.28 mm/s) in the experiment. These results demonstrated the facility to apply the FSI modeling approach in TEBV to obtain more comprehensive biomechanical results for investigating mechanical mechanisms of cardiovascular disease development.

A spatiotemporal analysis of the left coronary artery biomechanics using fluid-structure interaction models

Biomechanics plays a critical role in coronary artery disease development. FSI simulation is commonly used to understand the hemodynamics and mechanical environment associated with atherosclerosis pathology. To provide a comprehensive characterization of patient-specific coronary biomechanics, an analysis of FSI simulation in the spatial and temporal domains was performed. In the current study, a three-dimensional FSI model of the LAD coronary artery was built based on a patient-specific geometry using COMSOL Multiphysics. The effect of myocardial bridging was simulated. Wall shear stress and its derivatives including time-averaged wall shear stress, wall shear stress gradient, and OSI were calculated across the cardiac cycle in multiple locations. Arterial wall strain (radial, circumferential, and longitudinal) and von Mises stress were calculated. To assess perfusion, vFFR was calculated. The results demonstrated the FSI model could identify regional and transient differences in biomechanical parameters within the coronary artery. The addition of myocardial bridging caused a notable change in von Mises stress and an increase in arterial strain during systole. The analysis performed in this manner takes greater advantage of the information provided in the space and time domains and can potentially assist clinical evaluation.

Measurement of Layer-Specific Mechanical Properties of Intact Blood Vessels Based on Intravascular Optical Coherence Tomography

PURPOSE

The biomechanical analysis of stress and strain state of multilayered blood vessels has shown great importance in vascular pathology and physiology. However, there is a lack of method in measuring the mechanical property of each layer of a vascular sample without splitting up the wall.

METHODS

Here we develop a vascular inflation test method based on intravascular optical coherence tomography (IVOCT) imaging and inverse parametric estimation. We propose a three-step inverse parametric estimation method to solve the six constitutive parameters of the GOH models for the intima-media and adventitia of the coronaries simultaneously. A bilayer silicone vascular phantom inflation test and a virtual deformation test using finite element simulated data are conducted to evaluate the accuracy of the proposed method.

RESULTS

The virtual deformation test demonstrates that the errors of the constitutive constants are less than 2.56% determined by the proposed inverse parametric estimation method. The stress-strain curves of a bilayer silicone vascular phantom obtained based on the parameters determined by the proposed method match well with those obtained by the uniaxial test.

CONCLUSION

The proposed layer-specific vascular mechanical property measurement method provides a new experimental method for mechanical properties characterization of blood vessels. It also has the potential to be used for patient-specific mechanical properties estimation with IVOCT imaging in vivo.

Human Coronary Plaque Optical Coherence Tomography Image Repairing, Multilayer Segmentation and Impact on Plaque Stress/Strain Calculations

Coronary vessel layer structure may have a considerable impact on plaque stress/strain calculations. Most current plaque models use single-layer vessel structures due to the lack of available multilayer segmentation techniques. In this paper, an automatic multilayer segmentation and repair method was developed to segment coronary optical coherence tomography (OCT) images to obtain multilayer vessel geometries for biomechanical model construction. Intravascular OCT data were acquired from six patients (one male; mean age: 70.0) using a protocol approved by the local institutional review board with informed consent obtained. A total of 436 OCT slices were selected in this study. Manually segmented data were used as the gold standard for method development and validation. The edge detection method and cubic spline surface fitting were applied to detect and repair the internal elastic membrane (IEM), external elastic membrane (EEM) and adventitia-periadventitia interface (ADV). The mean errors of automatic contours compared to manually segmented contours were 1.40%, 4.34% and 6.97%, respectively. The single-layer mean plaque stress value from lumen was 117.91 kPa, 10.79% lower than that from three-layer models (132.33 kPa). On the adventitia, the single-layer mean plaque stress value was 50.46 kPa, 156.28% higher than that from three-layer models (19.74 kPa). The proposed segmentation technique may have wide applications in vulnerable plaque research.

Association of Collagen, Elastin, Glycosaminoglycans, and Macrophages With Tissue Ultimate Material Strength and Stretch in Human Thoracic Aortic Aneurysms: A Uniaxial Tension Study

Fiber structures and pathological features, e.g., inflammation and glycosaminoglycan (GAG) deposition, are the primary determinants of aortic mechanical properties which are associated with the development of an aneurysm. This study is designed to quantify the association of tissue ultimate strength and extensibility with the structural percentage of different components, in particular, GAG, and local fiber orientation. Thoracic aortic aneurysm (TAA) tissues from eight patients were collected. Ninety-six tissue strips of thickened intima, media, and adventitia were prepared for uni-extension tests and histopathological examination. Area ratios of collagen, elastin, macrophage and GAG, and collagen fiber dispersion were quantified. Collagen, elastin, and GAG were layer-dependent and the inflammatory burden in all layers was low. The local GAG ratio was negatively associated with the collagen ratio (r2 = 0.173, p < 0.05), but positively with elastin (r2 = 0.037, p < 0.05). Higher GAG deposition resulted in larger local collagen fiber dispersion in the media and adventitia, but not in the intima. The ultimate stretch in both axial and circumferential directions was exclusively associated with elastin ratio (axial: r2 = 0.186, p = 0.04; circumferential: r2 = 0.175, p = 0.04). Multivariate analysis showed that collagen and GAG contents were both associated with ultimate strength in the circumferential direction, but not with the axial direction (collagen: slope = 27.3, GAG: slope = -18.4, r2 = 0.438, p = 0.002). GAG may play important roles in TAA material strength. Their deposition was found to be associated positively with the local collagen fiber dispersion and negatively with ultimate strength in the circumferential direction.

Image-Based Finite Element Modeling Approach for Characterizing In Vivo Mechanical Properties of Human Arteries

Mechanical properties of the arterial walls could provide meaningful information for the diagnosis, management and treatment of cardiovascular diseases. Classically, various experimental approaches were conducted on dissected arterial tissues to obtain their stress-stretch relationship, which has limited value clinically. Therefore, there is a pressing need to obtain biomechanical behaviors of these vascular tissues in vivo for personalized treatment. This paper reviews the methods to quantify arterial mechanical properties in vivo. Among these methods, we emphasize a novel approach using image-based finite element models to iteratively determine the material properties of the arterial tissues. This approach has been successfully applied to arterial walls in various vascular beds. The mechanical properties obtained from the in vivo approach were compared to those from ex vivo experimental studies to investigate whether any discrepancy in material properties exists for both approaches. Arterial tissue stiffness values from in vivo studies generally were in the same magnitude as those from ex vivo studies, but with lower average values. Some methodological issues, including solution uniqueness and robustness; method validation; and model assumptions and limitations were discussed. Clinical applications of this approach were also addressed to highlight their potential in translation from research tools to cardiovascular disease management.

Plaque Structural Stress: Detection, Determinants and Role in Atherosclerotic Plaque Rupture and Progression

Atherosclerosis remains a major cause of death worldwide, with most myocardial infarctions being due to rupture or erosion of coronary plaques. Although several imaging modalities can identify features that confer risk, major adverse cardiovascular event (MACE) rates attributable to each plaque are low, such that additional biomarkers are required to improve risk stratification at plaque and patient level. Coronary arteries are exposed to continual mechanical forces, and plaque rupture occurs when plaque structural stress (PSS) exceeds its mechanical strength. Prospective studies have shown that peak PSS is correlated with acute coronary syndrome (ACS) presentation, plaque rupture, and MACE, and provides additional prognostic information to imaging. In addition, PSS incorporates multiple variables, including plaque architecture, plaque material properties, and haemodynamic data into a defined solution, providing a more detailed overview of higher-risk lesions. We review the methods for calculation and determinants of PSS, imaging modalities used for modeling PSS, and idealized models that explore structural and geometric components that affect PSS. We also discuss current experimental and clinical data linking PSS to the natural history of coronary artery disease, and explore potential for refining treatment options and predicting future events.

Mechanical and histological characteristics of aortic dissection tissues

AIMS

This study investigated the association between the macroscopic mechanical response of aortic dissection (AoD) flap, its fibre features, and patient physiological features and clinical presentations.

METHODS

Uniaxial test was performed with tissue strips in both circumferential and longitudinal directions from 35 patients with (AoD:CC) and without (AoD:w/oCC) cerebral/coronary complications, and 19 patients with rheumatic or valve-related heart diseases (RH). A Bayesian inference framework was used to estimate the expectation of material constants (C₁, D₁, and D₂) of the modified Mooney-Rivlin strain energy density function. Histological examination was used to visualise the elastin and collagen in the tissue strips and image processing was performed to quantify their area percentages, fibre misalignment and waviness.

RESULTS

The elastin area percentage was negatively associated with age (p = 0.008), while collagen increased about 6% from age 40 to 70 (p = 0.03). Elastin fibre was less dispersed and wavier in old patients and no significant association was found between patient age and collagen fibre dispersion or waviness. Features of fibrous microstructures, either elastin or collagen, were comparable between AoD:CC and AoD:w/oCC group. Elastin and collagen area percentages were positively correlated with C₁ and D₂, respectively, while the elastin and collagen waviness were negatively correlated with C₁ and D₂, respectively. Elastin dispersion was negatively correlated to D₂. Multivariate analysis showed that D₂ was an effective parameter which could differentiate patient groups with different age and clinical presentations, as well as the direction of tissue strip.

CONCLUSION

Fibre dispersion and waviness in the aortic dissection flap changed with patient age and clinical presentations, and these can be captured by the material constants in the strain energy density function.

STATEMENT OF SIGNIFICANCE

Aortic dissection (AoD) is a severe cardiovascular disease. Understanding the mechanical property of intimal flap is essential for its risk evaluation. In this study, mechanical testing and histology examination were combined to quantify the relationship between mechanical presentations and microstructure features. A Bayesian inference framework was employed to estimate the expectation of the material constants in the modified Mooney-Rivlin constitutive equation. It was found that fibre dispersion and waviness in the AoD flap changed with patient age and clinical presentations, and these could be captured by the material constants. This study firstly demonstrated that the expectation of material constants can be used to characterise tissue microstructures and differentiate patients with different clinical presentations.

A Powerful Paradigm for Cardiovascular Risk Stratification Using Multiclass, Multi-Label, and Ensemble-Based Machine Learning Paradigms: A Narrative Review

Background and Motivation: Cardiovascular disease (CVD) causes the highest mortality globally. With escalating healthcare costs, early non-invasive CVD risk assessment is vital. Conventional methods have shown poor performance compared to more recent and fast-evolving Artificial Intelligence (AI) methods. The proposed study reviews the three most recent paradigms for CVD risk assessment, namely multiclass, multi-label, and ensemble-based methods in (i) office-based and (ii) stress-test laboratories. Methods: A total of 265 CVD-based studies were selected using the preferred reporting items for systematic reviews and meta-analyses (PRISMA) model. Due to its popularity and recent development, the study analyzed the above three paradigms using machine learning (ML) frameworks. We review comprehensively these three methods using attributes, such as architecture, applications, pro-and-cons, scientific validation, clinical evaluation, and AI risk-of-bias (RoB) in the CVD framework. These ML techniques were then extended under mobile and cloud-based infrastructure. Findings: Most popular biomarkers used were office-based, laboratory-based, image-based phenotypes, and medication usage. Surrogate carotid scanning for coronary artery risk prediction had shown promising results. Ground truth (GT) selection for AI-based training along with scientific and clinical validation is very important for CVD stratification to avoid RoB. It was observed that the most popular classification paradigm is multiclass followed by the ensemble, and multi-label. The use of deep learning techniques in CVD risk stratification is in a very early stage of development. Mobile and cloud-based AI technologies are more likely to be the future. Conclusions: AI-based methods for CVD risk assessment are most promising and successful. Choice of GT is most vital in AI-based models to prevent the RoB. The amalgamation of image-based strategies with conventional risk factors provides the highest stability when using the three CVD paradigms in non-cloud and cloud-based frameworks.

Estimation of the zero-pressure computational start shape of atherosclerotic plaques: Improving the backward displacement method with deformation gradient tensor

Advances in medical imaging have enabled patient-specific biomechanical modelling of arterial lesions such as atherosclerosis and aneurysm. Geometry acquired from in-vivo imaging is already pressurized and a zero-pressure computational start shape needs to be identified. The backward displacement algorithm was proposed to solve this inverse problem, utilizing fixed-point iterations to gradually approach the start shape. However, classical fixed-point implementations were reported with suboptimal convergence properties under large deformations. In this paper, a dynamic learning rate guided by the deformation gradient tensor was introduced to control the geometry update. The effectiveness of this new algorithm was demonstrated for both idealized and patient-specific models. The proposed algorithm led to faster convergence by accelerating the initial steps and helped to avoid the non-convergence in large-deformation problems.

Constitutive models and failure properties of fibrous tissues of carotid artery atheroma based on their uniaxial testing

To obtain an experimental background for the description of mechanical properties of fibrous tissues of carotid atheroma, a cohort of 141 specimens harvested from 44 patients during endarterectomies, were tested. Uniaxial stress-strain curves and ultimate stress and strain at rupture were recorded. With this cohort, the impact of the direction of load, presence of calcifications, specimen location, patient's age and sex were investigated. A significant impact of sex was revealed for the stress-strain curves and ultimate strains. The response was significantly stiffer for females than for males but, in contrast to ultimate strain, the strength was not significantly different. The differences in strength between calcified and non-calcified atheromas have reached statistical significance in the female group. At most of the analysed stress levels, the loading direction was found significant for the male cohort which was also confirmed by large differences in ultimate strains. The representative uniaxial stress-strain curves (given by median values of strains at chosen stress levels) were fitted with an isotropic hyperelastic model for different groups specified by the investigated factors while the observed differences between circumferential and longitudinal direction were captured by an anisotropic hyperelastic model. The obtained results should be valid also for the tissue of the fibrous cap, the rupture of which is to be predicted in clinics using computational modelling because it may induce arterial thrombosis and consequently a brain stroke.

Multicomponent material property characterization of atherosclerotic human carotid arteries through a Bayesian Optimization based inverse finite element approach

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.

Quantifying Patient-Specific in vivo Coronary Plaque Material Properties for Accurate Stress/Strain Calculations: An IVUS-Based Multi-Patient Study

Introduction: Mechanical forces are closely associated with plaque progression and rupture. Precise quantifications of biomechanical conditions using in vivo image-based computational models depend heavily on the accurate estimation of patient-specific plaque mechanical properties. Currently, mechanical experiments are commonly performed on ex vivo cardiovascular tissues to determine plaque material properties. Patient-specific in vivo coronary material properties are scarce in the existing literature.

Methods:In vivo Cine intravascular ultrasound and virtual histology intravascular ultrasound (IVUS) slices were acquired at 20 plaque sites from 13 patients. A three-dimensional thin-slice structure-only model was constructed for each slice to obtain patient-specific in vivo material parameter values following an iterative scheme. Effective Young's modulus (YM) was calculated to indicate plaque stiffness for easy comparison purposes. IVUS-based 3D thin-slice models using in vivo and ex vivo material properties were constructed to investigate their impacts on plaque wall stress/strain (PWS/PWSn) calculations.

Results: The average YM values in the axial and circumferential directions for the 20 plaque slices were 599.5 and 1,042.8 kPa, respectively, 36.1% lower than those from published ex vivo data. The YM values in the circumferential direction of the softest and stiffest plaques were 103.4 and 2,317.3 kPa, respectively. The relative difference of mean PWSn on lumen using the in vivo and ex vivo material properties could be as high as 431%, while the relative difference of mean PWS was much lower, about 3.07% on average.

Conclusion: There is a large inter-patient and intra-patient variability in the in vivo plaque material properties. In vivo material properties have a great impact on plaque stress/strain calculations. In vivo plaque material properties have a greater impact on strain calculations. Large-scale-patient studies are needed to further verify our findings.

Interpretation of Experimental Data is Substantial for Constitutive Characterization of Arterial Tissue

The paper aims at evaluation of mechanical tests of soft tissues and creation of their representative stress-strain responses and respective constitutive models. Interpretation of sets of experimental results depends highly on the approach to the data analysis. Their common representation through mean and standard deviation may be misleading and give nonrealistic results. In the paper, raw data of seven studies consisting of 11 experimental data sets (concerning carotid wall and atheroma tissues) are re-analyzed to show the importance of their rigorous analysis. The sets of individual uniaxial stress-stretch curves are evaluated using three different protocols: stress-based, stretch-based, and constant-based, and the population-representative response is created by their mean or median values. Except for nearly linear responses, there are substantial differences between the resulting curves, being mostly the highest for constant-based evaluation. But also the stretch-based evaluation may change the character of the response significantly. Finally, medians of the stress-based responses are recommended as the most rigorous approach for arterial and other soft tissues with significant strain stiffening.

Stress distribution in the walls of major arteries: implications for atherogenesis

Background

There is a correlation between the sites of atheroma development and stress points in the arterial system. Generally, pulse pressure results in stresses acting on the vascular vessel, including longitudinal stress, radial or normal stress, tangential stress or hoop stress and shear stress. This paper explores the relationship between arterial wall shear stress and pulsatile blood pressure with the aim of furthering the understanding of atherogenesis and plaque progression.

Methods

We computed the magnitude of the shear stresses within the carotid bifurcation geometry of a patient and calculated the increase in shear stress levels that would occur when the blood pressure and pulse pressures rise during exertion. We also determined in which layer of the artery wall the maximum shear stress is located, and computed the shear stress at different levels within the media. We used the theory of laminate analysis, (Classical Laminate Plate Theory), to analyse the stress distribution on the carotid artery wall. Computational Fluid Dynamics (CFD) analysis was used on anatomy based on a CT angiogram of the carotid bifurcation of a patient with a 90% stenosis on the right side and 10% on the left. The pulsatile non-Newtonian blood flow with a resting blood pressure of 120/80 mmHg and an exertion pressure of 200/100 mmHg was simulated and the resultant forces were transferred to an ANSYS Composite PrepPost (ACP) model for wall shear stress analysis. A multilayer elastic, anisotropic, and inhomogeneous arterial wall (intima, internal elastic lamina, media, external elastic lamina, and adventitial layers) was modelled and the shear stress magnitudes and change over time between the layers was calculated.

Results

Shear stress in the individual composite layers is far greater than that acting on the endothelium (less than 5 Pa). At rest, the maximum variation of shear stress in the arterial wall occurs in the intima (138 Pa) and adventitia (135 Pa). The medial layer has the lowest variation of shear stress. Under severe exertion, the maximum shear stress magnitude in the intimal layer and the adjacent medial layer is near the ultimate stress level. The maximum/minimum shear stress ratios during the cardiac cycle vary most widely in the innermost part of the media, adjacent to the intima, with a four-fold ratio increase. This compares with a less than two-fold increase in all the other layers including the intima and adventitia, making the inner media the most vulnerable layer to mechanical injury.

Conclusions

This study showed that the magnitude of exertion-induced shear stress approaches the ultimate stress limit in the intima and the immediate adjacent medial layer. The variation in stress is maximal in the inner layer of the media. These findings correlate the site of atheroma development with the most vulnerable site for injury in the media and emphasise the impact of pulse pressure. Further biological studies are required to ascertain whether this leads to injury that initiates atheroma that then precipitates an injury/healing cycle.

The composition of vulnerable plaque and its effect on arterial waveforms

Carotid plaque composition is a key factor of plaque stability and it carries significant prognostic information. The carotid unstable plaques are characterized by a thin fibrous cap (FC) ≤65μm with large lipid core (LC), while stable plaques have a thicker FC and less LC. Identifying the percentage of plaque compositions could help surgeons to make a precise decision for their patients' treatment protocol. This study aims to distinguish between stable and unstable plaque by defining the relationship between plaque composition and arterial waveform non-invasively. An in-vitro arterial system, composed of a Harvard pulsatile flow pump and artificial circulation system, was used to investigate the effect of the plaque compositions on the pulsatile arterial waveforms. Five types of arterial plaques, composed of the LC, FC, Collagen (Col) and Calcium (Ca), were implemented into the artificial carotid artery to represent the diseased arterial system with 30% of blockage. The pulsatile pressure, velocity and arterial wall movement were measured simultaneously at the site proximal to the plaque. Non-invasive wave intensity analysis (Non-WIA) was used to separate the waves into forward and backward components. The correlation between the plaque compositions and the reflected waveforms was quantitatively analysed. The experimental results indicate that the reflected waveforms are strongly correlated with the plaque compositions, where the percentages of the Col are linearly correlated with the amplitude of the backward diameter (correlation coefficient, r = 0.74) and the lipid content has a strong negative correlation with the backward diameter (r = 0.82). A slight weak correlation exists between the reflected waveform and the percentage of Ca. The strong correlation between the compositions of the plaques with the backward waveforms observed in this study demonstrates that the components of the arterial plaques could be distinguished by the arterial waveforms. This finding might lead to a potential novel non-invasive clinical tool to determine the composition of the plaques and distinguish between stable and vulnerable arterial plaques at the early stage.

Studying the Factors of Human Carotid Atherosclerotic Plaque Rupture, by Calculating Stress/Strain in the Plaque, Based on CEUS Images: A Numerical Study

Carotid plaque neovascularization is one of the major factors for the classification of vulnerable plaque, but the axial force effects of the pulsatile blood flow on the plaque with neovessel and intraplaque hemorrhage was unclear. Together with the severity of stenosis, the fibrous cap thickness, large lipid core, and the neovascularization followed by intraplaque hemorrhage (IPH) have been regarded as high-risk features of plaque rupture. In this work, the effects of these factors were evaluated on the progression and rupture of the carotid atherosclerotic plaques. Five geometries of carotid artery plaque were developed based on contrast-enhanced ultrasound (CEUS) images, which contain two types of neovessel and IPH, and geometry without neovessel and IPH. A one-way fluid-structure interaction model was applied to compute the maximum principal stress and strain in the plaque. For that hyper-elastic and non-linear material, Yeoh 3rd Order strain energy density function was used for components of the plaque. The simulation results indicated that the maximum principal stress of plaque in the carotid artery was higher when the degree of the luminal stenosis increased and the thickness of the fibrous cap decreased. The neovessels within the plaque could introduce a 2.5% increments of deformation in the plaque under the pulsatile blood flow pressure. The IPH also contributed to the increased risk of plaque rupture that a gain of stress was 8.983, 14.526, and 34.47 kPa for the plaque with 50, 65, and 75%, respectively, when comparing stress in the plaque with IPH distributed at the middle to the shoulder of the plaque. In conclusion, neovascularization in the plaque could reduce the stability of the plaque by increasing the stress within the plaque. Also, the risk of plaque rupture increased when large luminal stenosis, thin fibrous cap, and IPH were observed.

It Takes Two to Tango: Endothelial TGFβ/BMP Signaling Crosstalk with Mechanobiology

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta (TGFβ) superfamily of cytokines. While some ligand members are potent inducers of angiogenesis, others promote vascular homeostasis. However, the precise understanding of the molecular mechanisms underlying these functions is still a growing research field. In bone, the tissue in which BMPs were first discovered, crosstalk of TGFβ/BMP signaling with mechanobiology is well understood. Likewise, the endothelium represents a tissue that is constantly exposed to multiple mechanical triggers, such as wall shear stress, elicited by blood flow or strain, and tension from the surrounding cells and to the extracellular matrix. To integrate mechanical stimuli, the cytoskeleton plays a pivotal role in the transduction of these forces in endothelial cells. Importantly, mechanical forces integrate on several levels of the TGFβ/BMP pathway, such as receptors and SMADs, but also global cell-architecture and nuclear chromatin re-organization. Here, we summarize the current literature on crosstalk mechanisms between biochemical cues elicited by TGFβ/BMP growth factors and mechanical cues, as shear stress or matrix stiffness that collectively orchestrate endothelial function. We focus on the different subcellular compartments in which the forces are sensed and integrated into the TGFβ/BMP growth factor signaling.

Effects of Extracellular Matrix Softening on Vascular Smooth Muscle Cell Dysfunction

Vascular smooth muscle cells (VSMCs) shift from a physiological contractile phenotype to an adverse proliferative or synthetic state, which is a major event leading to aortic disease. VSMCs are exposed to multiple mechanical signals from their microenvironment including vascular extracellular matrix (ECM) stiffness and stretch which regulate VSMC contraction. How ECM stiffness regulates the function and phenotype of VSMCs is not well understood. In this study, we introduce in vitro and in vivo models to evaluate the impact of ECM stiffnesses on VSMC function. Through unbiased transcriptome sequencing analysis, we detected upregulation of synthetic phenotype-related genes including osteopontin, matrix metalloproteinases, and inflammatory cytokines in VSMCs cultured using soft matrix hydrogels in vitro, suggesting VSMC dedifferentiation toward a synthetic phenotype upon ECM softening. For the in vivo model, the lysyl oxidase inhibitor β-aminopropionitrile monofumarate (BAPN) was administrated to disrupt the cross-linking of collagen to induce ECM softening. Consistently, decreased ECM stiffnesses promoted VSMC phenotypic switching to a synthetic phenotype as evidenced by upregulation of synthetic phenotype-related genes in the aortas of mice following BAPN treatment. Finally, BAPN-treated mice showed severe expansion and developed aortic dissection. Our study reveals the pivotal role of ECM softening in regulating the VSMC phenotype switch and provides a potential target for treating VSMC dysfunction and aortic dissection disease.

IDENTIFICATION OF UNIAXIAL DEFORMATION BEHAVIOR AND ITS INITIAL TANGENT MODULUS FOR ATHEROMATOUS INTIMA IN THE HUMAN CAROTID ARTERY AND THORACIC AORTA USING THREE-PARAMETER ISOTROPIC HYPERELASTIC MODELS

Uniaxial stretching tests are used for mechanical identification of small fibrous regions of atheromatous arteries. Material constants in isotropic hyperelastic models are determined to minimize the fitting error for the stress–strain curve. We developed a novel method to better characterize the material constants in typical forms of Yeoh, Ogden, Chuong–Fung (CF) and Gasser–Ogden–Holzapfel (GOH) isotropic hyperelastic models for fibrous caps and normal intimal layers from human carotid artery and thoracic aorta by incorporating Young’s modulus, i.e., the initial tangent modulus of uniaxial stress–strain relationships, as one of three material constants. We derived a unified, isotropic form for the anisotropic exponential-type strain energy density functions of CF and GOH models. The uniaxial stress–strain relationship equations were expanded to Maclaurin series to identify Young’s modulus as a coefficient of the linear term of the strain and to examine the roles of the material constants in the nonlinear function. The remaining two material constants were determined by curvefitting. The incorporation of Young’s modulus into the CF and GOH models gave reasonable curvefitting, with errors [Formula: see text], whereas large errors ([Formula: see text]) were observed in one case for the Yeoh model and in two cases for the Ogden model.

Optical Coherence Tomography-Based Modeling of Stent Deployment in Heavily Calcified Coronary Lesion

In this work, a heavily calcified coronary artery model was reconstructed from optical coherence tomography (OCT) images to investigate the impact of calcification characteristics on stenting outcomes. The calcification was quantified at various cross sections in terms of angle, maximum thickness, and area. The stent deployment procedure, including the crimping, expansion, and recoil, was implemented. The influence of calcification characteristics on stent expansion, malapposition, and lesion mechanics was characterized. Results have shown that the minimal lumen area following stenting occurred at the cross section with the greatest calcification angle. The calcification angle constricted the stretchability of the lesion and thus resulted in a small lumen area. The maximum principal strain and von Mises stress distribution patterns in both the fibrotic tissue and artery were consistent with the calcification profiles. The radially projected region of the calcification tends to have less strain and stress. The peak strain and stress of the fibrotic tissue occurred near the interface with the calcification. It is also the region with a high risk of tissue dissection and strut malapposition. In addition, the superficial calcification with a large angle aggregated the malapposition at the middle of the calcification arc. These detailed mechanistic quantifications could be used to provide a fundamental understanding of the role of calcification in stent expansions, as well as to exploit their potential for enhanced pre- and post-stenting strategies.

Stress-Relaxation and Cyclic Behavior of Human Carotid Plaque Tissue

Atherosclerotic plaque rupture is a catastrophic event that contributes to mortality and long-term disability. A better understanding of the plaque mechanical behavior is essential for the identification of vulnerable plaques pre-rupture. Plaque is subjected to a natural dynamic mechanical environment under hemodynamic loading. Therefore, it is important to understand the mechanical response of plaque tissue under cyclic loading conditions. Moreover, experimental data of such mechanical properties are fundamental for more clinically relevant biomechanical modeling and numerical simulations for risk stratification. This study aims to experimentally and numerically characterize the stress-relaxation and cyclic mechanical behavior of carotid plaque tissue. Instron microtester equipped with a custom-developed setup was used for the experiments. Carotid plaque samples excised at endarterectomy were subjected to uniaxial tensile, stress-relaxation, and cyclic loading protocols. Thirty percent of the underlying load level obtained from the uniaxial tensile test results was used to determine the change in mechanical properties of the tissue over time under a controlled testing environment (Control tests). The stress-relaxation test data was used to calibrate the hyperelastic (neo-Hookean, Ogden, Yeoh) and linear viscoelastic (Prony series) material parameters. The normalized relaxation force increased initially and slowly stabilized toward the end of relaxation phase, highlighting the viscoelastic behavior. During the cyclic tests, there was a decrease in the peak force as a function of the cycle number indicating mechanical distension due to repeated loading that varied with different frequencies. The material also accumulated residual deformation, which increased with the cycle number. This trend showed softening behavior of the samples. The results of this preliminary study provide an enhanced understanding of in vivo stress-relaxation and cyclic behavior of the human atherosclerotic plaque tissue.

Adventitial Collagen Crosslink Reduces Intimal Hyperplasia in a Rabbit Arteriovenous Graft Model

BACKGROUND

Intimal hyperplasia (IH) is the initial lesion of vein graft failure after coronary artery bypass grafting. The weak venous wall is likely one of the primary reasons for IH after exposure to the arterial environment. We investigate whether adventitial collagen cross-link by glutaraldehyde (GA) reinforces the venous wall and then reduces IH.

MATERIALS AND METHODS

Adventitial collagen cross-link by 0.3% GA was performed on the rabbit jugular veins. The degree of cross-link was accessed by tensile test. The jugular vein with or without cross-link was implanted into the carotid artery of rabbit. Vein dilatation at the immediate anastomosis and pathological remodeling of vein graft after 4 wk was assessed.

RESULTS

Tensile test indicated that the mechanical property of 3-min cross-linked veins more closely resembled that of the carotid artery. In rabbit arteriovenous graft models, 3-min adventitial collagen cross-link limited overdistension (diameter: 3.24 mm versus 4.65 mm, P < 0.01) at the immediate anastomosis and reduced IH (intima thickness: 78.83 μm versus 140.19 μm, P < 0.01) of vein grafts 4 wk after implantation in the cross-link group as compared with the graft group (without cross-link). Compared with the cross-link group, the expression of proliferating cell nuclear antigen and vascular cell adhesion molecule-1 increased significantly at both the mRNA and protein levels within the graft group (P < 0.01), but the expression of smooth muscle-22α decreased significantly (P < 0.01).

CONCLUSIONS

Adventitial collagen cross-link by GA increased the vessel stiffness and remarkably reduced IH in a rabbit arteriovenous graft model.

Down-regulated of SREBP-1 in circulating leukocyte is a risk factor for atherosclerosis: a case control study

BACKGROUND

Sterol regulatory-element binding proteins (SREBPs) and mir-33 (miR-33a, miR-33b), which are encoded by the introns of SREBPs, are key factors in the lipid metabolism pathway. SREBPs mRNA in circulating leucocyte and carotid plaques, along with various risk factors that associated with Coronary Atherosclerotic Disease (CAD) were investigated in a central Chinese cohort.

METHODS

A total of 218 coronary atherosclerotic disease (CAD) patients, and 178 non-CAD controls, were recruited to collect leukocytes. Carotid plaques and peripheral blood were obtained from CAD patients undergoing carotid endarterectomy (CEA) (n = 12) while THP-1 and peripheral blood mononuclear cells (PBMCs) were stimulated with Oxidized low-density lipoprotein (ox-LDL) to establish an in vitro foam cell formation model. SREBPs and miR-33 levels were quantified by qPCR. Routine biochemical markers were measured using standard procedures.

RESULTS

SREBP-1 mRNA level of circulating leucocytes in CAD patients were significantly lower than in non-CAD controls (p = 0.005). After stratification coronary artery atherosclerotic complexity, we detected a significant reduction of SREBP-1 in high-risk complexity CAD patients (SYNTAX score > 23) (p = 0.001). Logistic regression analysis indicated that decreased expression of SREBP-1 was a risk factor of CAD (odds ratio (OR) =0.48, 95% confidence interval (CI) = 0.30~0.76, p = 0.002) after adjusting clinical confounders; the mRNA levels of SREBPs in carotid plaques correlated with the corresponding value in circulating leukocytes (SREBP-1 r = 0.717, p = 0.010; SREBP-2 r = 0.612, p = 0.034). Finally, there was no significant difference in serum miR-33 levels between CAD patients and controls.

CONCLUSIONS

Our finding suggesting a potential role in the adjustment of established CAD risk. The future clarification of how SREBP-1 influence the pathogenesis of CAD might pave the way for the development of novel therapeutic methods.

Sensitivity of Arterial Hyperelastic Models to Uncertainties in Stress-Free Measurements

Despite major advances made in modeling vascular tissue biomechanics, the predictive power of constitutive models is still limited by uncertainty of the input data. Specifically, key measurements, like the geometry of the stress-free (SF) state, involve a definite, sometimes non-negligible, degree of uncertainty. Here, we introduce a new approach for sensitivity analysis of vascular hyperelastic constitutive models to uncertainty in SF measurements. We have considered two vascular hyperelastic models: the phenomenological Fung model and the structure-motivated Holzapfel-Gasser-Ogden (HGO) model. Our results indicate up to 160% errors in the identified constitutive parameters for a 5% measurement uncertainty in the SF data. Relative margins of errors of up to 30% in the luminal pressure, 36% in the axial force, and over 200% in the stress predictions were recorded for 10% uncertainties. These findings are relevant to the large body of studies involving experimentally based modeling and analysis of vascular tissues. The impact of uncertainties on calibrated constitutive parameters is significant in context of studies that use constitutive parameters to draw conclusions about the underlying microstructure of vascular tissues, their growth and remodeling processes, and aging and disease states. The propagation of uncertainties into the predictions of biophysical parameters, e.g., force, luminal pressure, and wall stresses, is of practical importance in the design and execution of clinical devices and interventions. Furthermore, insights provided by the present findings may lead to more robust parameters identification techniques, and serve as selection criteria in the trade-off between model complexity and sensitivity.

A Multi-Modality Image-Based FSI Modeling Approach for Prediction of Coronary Plaque Progression Using IVUS and OCT Data with Follow-Up

Medical image resolution has been a serious limitation in plaque progression research. A modeling approach combining intravascular ultrasound (IVUS) and optical coherence tomography (OCT) was introduced and patient follow-up IVUS and OCT data were acquired to construct 3D coronary models for plaque progression investigations. Baseline and follow-up in vivo IVUS and OCT coronary plaque data were acquired from one patient with 105 matched slices selected for model construction. 3D FSI models based on IVUS and OCT data (denoted as IVUS+OCT model) were constructed to obtain stress/strain and wall shear stress (WSS) for plaque progression prediction. IVUS-based IVUS50 and IVUS200 models were constructed for comparison with cap thickness set as 50 and 200 microns, respectively. Lumen area increase (LAI), plaque area increase (PAI) and plaque burden increase (PBI) were chosen to measure plaque progression. The least squares support vector machine method was employed for plaque progression prediction using 19 risk factors. For IVUS+OCT model with LAI, PAI and PBI, the best single predictor was plaque strain, local plaque stress, and minimal cap thickness, with prediction accuracy as 0.766, 0.838 and 0.890, respectively; The prediction accuracy using best combinations of 19 factors was 0.911, 0.881 and 0.905, respectively. Compared to IVUS+OCT model, IVUS50 and IVUS200 models had errors ranging from 1% to 66.5% in quantifying cap thickness, stress, strain and prediction accuracies. WSS showed relatively lower prediction accuracy compared to other predictors in all 9 prediction studies.

Vulnerability analysis on the interaction between Asymmetric stent and arterial layer

The utilization of Asymmetric stent for recovering atherosclerotic diseases, particularly non-symmetric obstruction, is a quite challenging breakthrough treatment. In terms of eccentric plaque, the non-uniform stiffness of arterial layer causes the increasingly complex issues of vulnerability. This study investigated the vulnerability of the interaction between the Asymmetric stent and the surrounding arterial layer using structural transient dynamic analysis in ANSYS. Four combinations of stent deployment, i.e. the Sinusoidal stent expanded by the offset balloon, the Sinusoidal stent expanded by the ordinary cylindrical balloon, the Asymmetric stent expanded by the offset balloon, and the Asymmetric stent expanded by the ordinary cylindrical balloon, are generated for this comparative study. Multilayer material properties from recent in vitro experiments are adopted for the surrounding arterial layer, such as a fibrous cap, lipid core, diseased-healthy intima, and diseased-healthy media. In order to address plaque vulnerability, the Cauchy stresses and Hencky strains are used for stress measure because of convenience in comparison with the uniaxial/biaxial tension test data. The location-specific threshold value from the diseased human carotid artery is adopted for rupture criteria. The simulation indicated that as regards the eccentric plaque, the plaque vulnerability is caused by the plaque shape and components rather than caused by the geometrical structure of the stent or balloon expansion method. Nevertheless, the non-symmetric inflation of balloon, which leads against the plaque, contributed to an increase in the vulnerability of fibrous cap of fibroatheroma plaque.

Material stiffness influences the polarization state, function and migration mode of macrophages

Biomaterial implantation is followed by an inflammatory cascade dominated by macrophages, which determine implant acceptance or rejection through pro- and anti-inflammatory polarization states (Anderson et al., 2008; Brown and Badylak, 2013). It is known that chemical signals such as bacterial endotoxins and cytokines (IL4) can direct macrophage polarization (Mantovani et al., 2004); however, recent evidence implicates biophysical cues in this process (McWhorter et al., 2015; Patel et al., 2012). Here we report that THP-1 derived macrophages cultured on collagen-coated polyacrylamide gels of varying stiffness adapt their polarization state, functional roles and migration mode according to the stiffness of the underlying substrate. Through gene expression and protein secretion analysis, we show that stiff polyacrylamide gels (323 kPa) prime macrophages towards a pro-inflammatory phenotype with impaired phagocytosis in macrophages, while soft (11 kPa) and medium (88 kPa) stiffness gels prime cells towards an anti-inflammatory, highly phagocytic phenotype. Furthermore, we show that stiffness dictates the migration mode of macrophages; on soft and medium stiffness gels, cells display Rho-A kinase (ROCK)-dependent, podosome-independent fast amoeboid migration and on stiff gels they adopt a ROCK-independent, podosome-dependent slow mesenchymal migration mode. We also provide a mechanistic insight into this process by showing that the anti-inflammatory property of macrophages on soft and medium gels is ROCK-dependent and independent of the ligand presented to them. Together, our results demonstrate that macrophages adapt their polarization, function and migration mode in response to the stiffness of the underlying substrate and suggest that biomaterial stiffness is capable of directing macrophage behaviour independent of the biochemical cues being presented to them. The results from this study establish an important role for substrate stiffness in directing macrophage behaviour, and will lead to the design of immuno-informed biomaterials that are capable of modulating the macrophage response after implantation. STATEMENT OF SIGNIFICANCE: Biomaterial implantation is followed by an inflammatory cascade dominated by macrophages, which determine implant acceptance or rejection through pro- and anti-inflammatory polarization states. It is known that chemical signals can direct macrophage polarization; however, recent evidence implicates biophysical cues in this process. Here we report that macrophages cultured on gels of varying stiffness adapt their polarization state, functional roles and migration mode according to the stiffness of the underlying substrate. The results from this study establish an important role for substrate stiffness in directing macrophage behaviour, and will lead to the design of immuno-informed biomaterials that are capable of modulating the macrophage response after implantation.

Bayesian Inference-Based Estimation of Normal Aortic, Aneurysmal and Atherosclerotic Tissue Mechanical Properties: From Material Testing, Modeling and Histology

OBJECTIVE

Mechanical properties of healthy, aneurysmal, and atherosclerotic arterial tissues are essential for assessing the risk of lesion development and rupture. Strain energy density function (SEDF) has been widely used to describe these properties, where material constants of the SEDF are traditionally determined using the ordinary least square (OLS) method. However, the material constants derived using OLS are usually dependent on initial guesses.

METHODS

To avoid such dependencies, Bayesian inference-based estimation was used to fit experimental stress-stretch curves of 312 tissue strips from 8 normal aortas, 19 aortic aneurysms, and 21 carotid atherosclerotic plaques to determine the constants, C₁, D₁, and D₂ of the modified Mooney-Rivlin SEDF.

RESULTS

Compared with OLS, material constants varied much less with prior in the Bayesian inference-based estimation. Moreover, fitted material constants differed amongst distinct tissue types. Atherosclerotic tissues associated with the biggest D₂, an indicator of the rate of increase in stress during stretching, followed by aneurysmal tissues and those from normal aortas. Histological analyses showed that C₁ and D₂ were associated with elastin content and details of the collagen configuration, specifically, waviness and dispersion, in the structure.

CONCLUSION

Bayesian inference-based estimation robustly determines material constants in the modified Mooney-Rivlin SEDF and these constants can reflect the inherent physiological and pathological features of the tissue structure.

SIGNIFICANCE

This study suggested a robust procedure to determine the material constants in SEDF and demonstrated that the obtained constants can be used to characterize tissues from different types of lesions, while associating with their inherent microstructures.

Virtual stenting with simplex mesh and mechanical contact analysis for real-time planning of thoracic endovascular aortic repair

In aortic endovascular repair, the prediction of stented vessel remodeling informs treatment plans and risk evaluation; however, there are no highly accurate and efficient methods to quantitatively simulate stented vessels. This study developed a fast virtual stenting algorithm to simulate stent-induced aortic remodeling to assist in real-time thoracic endovascular aortic repair planning.

Methods: The virtual stenting algorithm was established based on simplex deformable mesh and mechanical contact analysis. The key parameters of the mechanical contact analysis were derived from mechanical tests on aortic tissue (n=40) and commonly used stent-grafts (n=6). Genetic algorithm was applied to select weighting parameters. Testing and validation of the algorithm were performed using pre- and post-treatment computed tomography angiography datasets of type-B aortic dissection cases (n=66).

Results: The algorithm was efficient in simulating stent-induced aortic deformation (mean computing time on a single processor: 13.78±2.80s) and accurate at the morphological (curvature difference: 1.57±0.57%; cross-sectional area difference: 4.11±0.85%) and hemodynamic (similarity of wall shear stress-derived parameters: 90.16-90.94%) levels. Stent-induced wall deformation was higher (p<0.05) in distal stent-induced new entry cases than in successfully treated cases, and this deformation did not differ significantly among the different stent groups. Additionally, the high stent-induced wall deformation regions and the new-entry sites overlapped, indicating the usefulness of wall deformation to evaluate the risks of device-induced complications.

Conclusion: The novel algorithm provided fast real-time and accurate predictions of stent-graft deployment with luminal deformation tracking, thereby potentially informing individualized stenting planning and improving endovascular aortic repair outcomes. Large, multicenter studies are warranted to extend the algorithm validation and determine stress-induced wall deformation cutoff values for the risk stratification of particular complications.

Progressive changes of elastic moduli of arterial wall and atherosclerotic plaque components during plaque development in human coronary arteries

Stiffness of the arterial wall and atherosclerotic plaque components is a determinant of the stress field within plaques, which has been suggested to be an indicator of plaque vulnerability. The diversity and inhomogeneous structure of atherosclerotic lesions complicate the characterization of plaque components. In the present study, stiffness of the arterial wall and atherosclerotic plaque components in human coronary arteries was examined in early and developed atherosclerotic lesions. The force-spectroscopy mode of the atomic force microscope and histological examination were used for determination of elastic moduli at specified locations within samples. Fibrous cap (E = 14.1 ± 3.8 kPa) showed lower stiffness than the fibrous tissue beneath the lipid pool (E = 17.6 ± 3.2 kPa). Calcification zones (E = 96.1 ± 18.8 kPa) and lipid pools (E = 2.7 ± 1.8 kPa) were the stiffest and softest components of atherosclerotic lesions, respectively. The increase of media stiffness (%44.8) and reduction of the elastic modulus of the internal elastic lamina (%28.9) was observed in coronary arteries. Moreover, significant differences were observed between the stiffness of medial layer in diseased parts and free-plaque segments in incomplete plaques of coronary arteries. Our results can be used for better understanding of remodeling mechanisms of the arterial wall with plaque development. Graphical abstract Stiffness alteration of the arterial wall and atherosclerotic plaque components with plaque development in coronary arteries.

Relating the mechanical properties of atherosclerotic calcification to radiographic density: A nanoindentation approach

Calcification morphology can determine atherosclerotic plaque stability and is associated with increased failures rates for endovascular interventions. Computational efforts have sought to elucidate the relationship between calcification and plaque rupture in addition to predicting tissue response during aggressive revascularisation techniques. However, calcified material properties are currently estimated and may not reflect real tissue conditions. The objective of this study is to correlate calcification mechanical properties with three radiographic density groups obtained from corresponding Computed Tomography (CT) images. Seventeen human plaques extracted from carotid (n = 10) and peripheral lower limb (n = 7) arteries were examined using micro-computed tomography (µCT), simultaneously locating the calcified deposits within their internal structure and quantifying their densities. Three radiographic density groups were defined based on the sample density distribution: (A) 130-299.99 Hounsfield Units (HU), (B) 300-449.99 HU and (C) >450 HU. Nanoindentation was employed to determine the Elastic Modulus (E) and Hardness (H) values within the three density groups. Results reveal a clear distinction between mechanical properties with respect to radiographic density groups (p < 0.0005). No significant differences exist in the density-specific behaviours observed between carotid and peripheral samples. Previously defined calcification classifications indicate an association with specific radiographic density patterns. Scanning Electron Microscopy (SEM) examination revealed that density group A regions consist of both calcified and non-calcified tissues. Further research is required to define the radiographic thresholds which identify varying degrees of tissue calcification. This study demonstrates that the mechanical properties of fully mineralised atherosclerotic calcification emulate that of bone tissues (17-25 GPa), affording computational models with accurate material parameters. STATEMENT OF SIGNIFICANCE: Global mechanical characterisation techniques disregard the heterogeneous nature of atherosclerotic lesions. Previous nanoindentation results for carotid calcifications have displayed a wide range. This study evaluates calcification properties with respect to radiographic density obtained from Micro-CT images. This is the first work to characterise calcifications from peripheral lower limb arteries using nanoindentation. Results demonstrate a strong positive correlation between radiographic density and calcification mechanical properties. Characterising calcifications using their density values provides clarity on the variation in published properties for calcified tissues. Furthermore, this study confirms the hypothesis that fully calcified plaque tissue behaviour similar to that of bone. Appropriate material parameters for calcified tissues can now be employed in computational simulations.

Mechanical Characterization of the Lamellar Structure of Human Abdominal Aorta in the Development of Atherosclerosis: An Atomic Force Microscopy Study

Atherosclerosis is a major risk factor for cardiovascular disease. However, mechanisms of interaction of atherosclerotic plaque development and local stiffness of the lamellar structure of the arterial wall are not well established. In the current study, the local Young's modulus of the wall and plaque components were determined for three different groups of healthy, mildly diseased and advanced atherosclerotic human abdominal aortas. Histological staining was performed to highlight the atherosclerotic plaque components and lamellar structure of the aortic media, consisting of concentric layers of elastin and interlamellar zones. The force spectroscopy mode of the atomic force microscopy was utilized to determine Young's moduli of aortic wall lamellae and plaque components at the micron level. The high variability of Young's moduli (E) at different locations of the atherosclerotic plaque such as the fibrous cap (E = 15.5± 2.6 kPa), calcification zone (E = 103.7±19.5 kPa), and lipid pool (E = 3.5±1.2 kPa) were observed. Reduction of elastin lamellae stiffness (18.6%), as well as stiffening of interlamellar zones (50%), were detected in the diseased portion of the medial layer of abdominal aortic wall compared to the healthy artery. Additionally, significant differences in the stiffness of both elastin lamellae and interlamellar zones were observed between the diseased wall and disease-free wall in incomplete plaques. Our results elucidate the alternation of the stiffness of different lamellae in the human abdominal aortic wall with atherosclerotic plaque development and may provide new insight on the remodeling of the aortic wall during the progression of atherosclerosis.