Influence of lumen shape and vessel geometry on plaque stresses: possible role in the increased vulnerability of a remodelled vessel and the "shoulder" of a plaque

Associations of Culprit Vessel Size and Plaque Characteristics in Patients with ST-Segment Elevation Myocardial Infarction

Background

Small vessel disease (SVD) widely exists in patients with acute coronary syndrome. However, the plaque characteristic of SVD has not been investigated.

Methods

Optical coherence tomography (OCT) of culprit lesion was examined in 576 patients with ST-segment elevation myocardial infarction (STEMI) and finally 404 patients with qualified images were analysed of plaque phenotypes and microstructure. The cohort was divided into three groups according to vessel diameters of culprit lesion which were measured by OCT. Major adverse cardiac events (MACEs) were recorded of each patient and compared among patients with different vessel diameters and plaque phenotypes.

Results

Gender, age and body mass index (BMI) were significantly different among patients with different diameters of culprit vessels (98.4% vs. 85.7% vs.71.4%, p < 0.001; 40.0 ± 7.0 vs. 54.9 ± 6.6 vs. 68.9 ± 5.8, p < 0.001; 28.4 ± 4.0 vs. 25.8 ± 2.9 vs. 25.2 ± 3.0, p < 0.001, respectively). Moreover, patients with diameters of culprit lesion > 3 mm presented with more incidence of plaque rupture and macrophage (57.7% vs. 42.1% vs. 46.2%, p = 0.015, 55.1% vs. 41.0% vs. 36.9%, p = 0.010). Total MACE did not differ among groups of different vessel diameters and plaque phenotypes.

Conclusions

Vessel size of culprit lesion is significantly associated with plaque phenotype in patients with STEMI. However, patients with different diameters and plaque phenotypes showed no significant difference of clinical outcomes.

Clinical Trial Registration

NCT03593928.

Consistency in Geometry Among Coronary Atherosclerotic Plaques Extracted From Computed Tomography Angiography

Background: The three-dimensional (3D) geometry of coronary atherosclerotic plaques is associated with plaque growth and the occurrence of coronary artery disease. However, there is a lack of studies on the 3D geometric properties of coronary plaques. We aim to investigate if coronary plaques of different sizes are consistent in geometric properties.

Methods: Nineteen cases with symptomatic stenosis caused by atherosclerotic plaques in the left coronary artery were included. Based on attenuation values on computed tomography angiography images, coronary atherosclerotic plaques and calcifications were identified, 3D reconstructed, and manually revised. Multidimensional geometric parameters were measured on the 3D models of plaques and calcifications. Linear and non-linear (i.e., power function) fittings were used to investigate the relationship between multidimensional geometric parameters (length, surface area, volume, etc.). Pearson correlation coefficient (r), R-squared, and p-values were used to evaluate the significance of the relationship. The analysis was performed based on cases and plaques, respectively. Significant linear relationship was defined as R-squared > 0.25 and p < 0.05.

Results: In total, 49 atherosclerotic plaques and 56 calcifications were extracted. In the case-based analysis, significant linear relationships were found between number of plaques and number of calcifications (r = 0.650, p = 0.003) as well as total volume of plaques (r = 0.538, p = 0.018), between number of calcifications and total volume of plaques (r = 0.703, p = 0.001) as well as total volume of calcification (r = 0.646, p = 0.003), and between the total volumes of plaques and calcifications (r = 0.872, p < 0.001). In plaque-based analysis, the power function showed higher R-squared values than the linear function in fitting the relationships of multidimensional geometric parameters. Two presumptions of plaque geometry in different growth stages were proposed with simplified geometric models developed. In the proposed models, the exponents in the power functions of geometric parameters were in accordance with the fitted values.

Conclusion: In patients with coronary artery disease, coronary plaques and calcifications are positively related in number and volume. Different coronary plaques are consistent in the relationship between geometry parameters in different dimensions.

Visualizing Angle-Independent Principal Strains in the Longitudinal View of the Carotid Artery: Phantom and In Vivo Evaluation

Non-invasive vascular elastography can evaluate the stiffness of the carotid artery by visualizing the vascular strain distribution. Axial strain estimates of the longitudinal cross section of the carotid artery are sensitive to the angle between the artery and the transducer. Anatomical variations in branching and arching of the carotid artery can affect the assessment of arterial stiffness. In this study, we hypothesized that principal strain elastograms computed using compounded plane wave imaging can reliably visualize the strain distribution in the carotid artery, independent of the transducer angle. We corroborated this hypothesis by conducting phantom and in vivo studies using a commercial ultrasound scanner (Sonix RP, Ultrasonix Medical Corp., Richmond, BC, Canada). The phantom studies were conducted using a homogeneous cryogel vessel phantom. The goal of the phantom study was to assess the feasibility of visualizing the radial deformation in the longitudinal plane of the vessel phantom, independent of the transducer angle (±30°, ±20°, ±10° and 0°). The in vivo studies were conducted on 20 healthy human volunteers in the age group 50-60 y. All echo imaging was performed at a transmit frequency of 5 MHz and sampling frequency of 40 MHz. The elastograms obtained from the phantom study revealed that for straight vessels, which had their lumen parallel to the transducer, principal strains were similar to axial strains. At non-parallel configurations (angles ±30°, ±20° and ±10°), the magnitudes of the mean principal strains were within 2.5% of the parallel configuration (0° angle) estimates and, thus, were observed to be relatively unaffected by change in angle. However, in comparison, the magnitude of the axial strain decreased with increase in angle because of coordinate dependency. Further, the pilot in vivo study indicated that the principal and axial strain elastograms were similar for subjects with relatively straight arteries. However, for arteries with arched geometry, axial strains were significantly lower (p <0.01) than the corresponding principal vascular strains, which was consistent with the results obtained from the phantom study. In conclusion, the results of the phantom and in vivo studies revealed that principal strain elastograms computed using CPW imaging could reliably visualize angle-independent vascular strains in the longitudinal plane of the carotid artery.

Implications of the local hemodynamic forces on the formation and destabilization of neoatherosclerotic lesions

OBJECTIVE

To examine the implications of endothelial shear stress (ESS) distribution in the formation of neoatherosclerotic lesions.

METHODS

Thirty six patients with neoatherosclerotic lesions on optical coherence tomography (OCT) were included in this study. The OCT data were used to reconstruct coronary anatomy. Blood flow simulation was performed in the models reconstructed from the stent borders which it was assumed that represented the lumen surface at baseline, immediate after stent implantation, and the estimated ESS was associated with the neointima burden, neoatherosclerotic burden and neointima characteristics. In segments with neointima rupture blood flow simulation was also performed in the model representing the lumen surface before rupture and the ESS was estimated at the ruptured site.

RESULTS

An inverse association was noted between baseline ESS and the incidence and the burden of neoatherosclerotic (β = -0.60, P < 0.001, and β = -4.05, P < 0.001, respectively) and lipid-rich neoatherosclerotic tissue (β = -0.54, P < 0.001, and β = -3.60, P < 0.001, respectively). Segments exposed to low ESS (<1 Pa) were more likely to exhibit macrophages accumulation (28.2% vs 10.9%, P < 0.001), thrombus (11.0% vs 2.6%, P < 0.001) and evidence of neointima discontinuities (8.1% vs 0.9%, P < 0.001) compared to those exposed to normal or high ESS. In segments with neointima rupture the ESS was high at the rupture site compared to the average ESS over the culprit lesion (4.00 ± 3.65 Pa vs 3.14 ± 2.90 Pa, P < 0.001).

CONCLUSIONS

Local EES is associated with neoatherosclerotic lesion characteristics, which suggests involvement of ESS in the formation of vulnerable plaques in stented segments.

STUDY ON THE IMPACT OF STRAIGHT STENTS ON ARTERIES WITH DIFFERENT CURVATURES

Different stent structures lead to different deformations of blood vessels, such as different cross-sectional shapes, which further influence the blood flow patterns. In this paper, six non-commercial stents with different link structures called I-, C-, S-, U-, N- and W-types were considered. Their influences on arteries with five different curvatures (i.e., 0[Formula: see text], 15[Formula: see text], 30[Formula: see text], 45[Formula: see text] and 60[Formula: see text]) were studied using finite element method. Four indices including the maximum plastic strain of stents, the rate of expansion, the maximum von Mises stress and the ellipticity of arteries, were compared for all cases. The results showed that the S-type or U-type stents, with larger plastic strain and lower von Mises stress on the arteries, provided the optimal outcome. As the link structures became complex, the arterial expansion increased monotonically, while the ellipticity of arteries showed a decreasing tendency in the vessel models. The present study could be useful for the commercial design and clinic selection of a stent with different link structures for different curved arteries.

Anxiety and adverse health outcomes among cardiac patients: a biobehavioral model

BACKGROUND

Anxiety is a common experience among patients with acute coronary syndrome (ACS) that can have a negative impact on health outcomes. Nonetheless, the negative role of anxiety remains underappreciated, as reflected by clinicians' underrecognition and undertreatment of anxious hospitalized and nonhospitalized patients with ACS. Underappreciation of the role of anxiety is possibly related to inadequate understanding of the mechanisms whereby anxiety may adversely affect health outcomes.

PURPOSE

The aim of this study was to synthesize the evidence about potential mechanisms by which anxiety and adverse health outcomes are related.

CONCLUSIONS

A biobehavioral model links anxiety to the development of thrombogenic and arrhythmic events in patients with ACS. Biologically, anxiety may interfere with the immune system, lipid profile, automatic nervous system balance, and the coagulation cascade, whereas behaviorally, anxiety may adversely affect adoption of healthy habits and cardiac risk-reducing behaviors. The biological and behavioral pathways complement each other in the production of poor outcomes.

CLINICAL IMPLICATIONS

Anxiety requires more attention from clinical cardiology. The adverse impact of anxiety on health outcomes could be avoided by efficient assessment and treatment of anxiety.

Local anisotropic mechanical properties of human carotid atherosclerotic plaques - characterisation by micro-indentation and inverse finite element analysis

Biomechanical models have the potential to predict failure of atherosclerotic plaques and to improve the risk assessment of plaque rupture. The applicability of these models depends strongly on the used material models. Current biomechanical models employ isotropic material models, although it is generally accepted that plaque tissue behaves highly anisotropic. The aim of the present study is to determine the local anisotropic mechanical properties of human atherosclerotic plaque tissue by means of micro-indentation tests. The indentation was performed on top of an inverted confocal microscope allowing the visualisation and quantification of the collagen fibre deformations perpendicular to the indentation direction of the plaque. Based on this, the anisotropic properties of plaque tissue perpendicular to the indentation direction (middle of the fibrous cap, shoulder of the cap, remaining intima tissue) were derived. There were no significant differences between the different indentation locations for the fibre stiffness (total median 80.6kPa, 25th-75th percentile 17.7-157.0kPa), and fibre dispersion.

How do we prevent the vulnerable atherosclerotic plaque from rupturing? Insights from in vivo assessments of plaque, vascular remodeling, and local endothelial shear stress

Coronary atherosclerosis progresses both as slow, gradual enlargement of focal plaque and also as a more dynamic process with periodic abrupt changes in plaque geometry, size, and morphology. Systemic vasculoprotective therapies such as statins, angiotensin-converting enzyme inhibitors, and antiplatelet agents are the cornerstone of prevention of plaque rupture and new adverse clinical outcomes, but such systemic therapies are insufficient to prevent the majority of new cardiac events. Invasive imaging methods have been able to identify both the anatomic features of high-risk plaque and the ongoing pathobiological stimuli responsible for progressive plaque inflammation and instability and may provide sufficient information to formulate preventive local mechanical strategies (eg, preemptive percutaneous coronary interventions) to avert cardiac events. Local endothelial shear stress (ESS) triggers vascular phenomena that synergistically exacerbate atherosclerosis toward an unstable phenotype. Specifically, low ESS augments lipid uptake and catabolism, induces plaque inflammation and oxidation, downregulates the production, upregulates the degradation of extracellular matrix, and increases cellular apoptosis ultimately leading to thin-cap fibroatheromas and/or endothelial erosions. Increases in blood thrombogenicity that result from either high or low ESS also contribute to plaque destabilization. An understanding of the actively evolving vascular phenomena, as well as the development of in vivo imaging methodologies to identify the presence and severity of the different processes, may enable early identification of a coronary plaque destined to acquire a high-risk state and allow for highly selective, focal preventive interventions to avert the adverse natural history of that particular plaque. In this review, we focus on the role of ESS in the pathobiologic processes responsible for plaque destabilization, leading either to accelerated plaque growth or to acute coronary events, and emphasize the potential to utilize in vivo risk stratification of individual coronary plaques to optimize prevention strategies to preclude new cardiac events.

An analysis of the contact between the stent and the artery using tube hydroforming simulation

Stents for angioplasty have been extensively used to treat coronary diseases. The aim of this study is to analyze the expansion of the stent and the contact with the artery using tube hydroforming simulation. In the simulation, the contact stress and the final shape of the artery after stent expansion process using the Stampack (®;) software will be studied. A model of a commercial stent made of 316L stainless steel was modeled by using an elastic-plastic constitutive law with isotropic hardening. The artery was modeled as a cylinder and made of hyperelastic material. The stent model studied in this work presented a good performance according to the results obtained. After expansion, any region of the stent's structure with strong risk of wrinkling, thinning, or buckling was not observed. In the forming limit diagram, all points were far from the Keeler-Goodwin diagram. Furthermore, the expanded stent model has a good conformability. In conclusion, our data show that the proposed methodology is a useful tool to check if the stent model implanted in the artery may cause restenosis after angioplasty; thus, our tests provided a reliable tool to analyze this risk.

Does microcalcification increase the risk of rupture?

Rupture of atherosclerotic plaque, which is related to maximal stress conditions in the plaque among others, is a major cause of mortality. More careful examination of stress distributions in atherosclerotic plaques reports that it could be due to local stress behaviors at critical sites caused by cap thinning, inflammation, macroscopic heterogeneity, and recently, the presence of microcalcifications. However, the role of microcalcifications is not yet fully understood, and most finite element models of blood vessels with atheroma plaque ignore the heterogeneity of the plaque constituents at the microscale. The goal of this work is to investigate the effect of microcalcifications on the stress field of an atheroma plaque vessel section. This is achieved by performing a parametric finite element study, assuming a plane strain hypothesis, of a coronary artery section with eccentric atheroma plaque and one microcalcification incorporated. The geometrical parameters used to define and design the idealized coronary plaque anatomy and the microcalcification were the fibrous cap thickness and the microcalcification ratio, angle and eccentricity. We could conclude that microcalcifications should be considered in the modeling of this kind of problems since they cause a significant alteration of the vulnerable risk by increasing the maximum maximal principal stress up to 32%, although this increase of stress is not uniform (12% on average). The obtained results show that the fibrous cap thickness, the microcalcification ratio and the microcalcification eccentricity, in combination with the microcalcification angle, appear to be the key morphological parameters that play a determinant role in the maximal principal stress and accordingly in the rupture risk of the plaque.

Statins have therapeutic potential for the treatment of Alzheimer's disease, likely via protection of the neurovascular unit in the AD brain

Structural and functional abnormalities in the neurovascular unit (NVU) have been recently observed in Alzheimer's disease (AD). Statins, which are used clinically for reducing cholesterol levels, can also exert beneficial vascular actions, improve behavioral memory and reduce senile plaque (SP). Thus, we examined cognitive function, the serum level of lipids, senile plaque (SP), and the protective effects of statins on NVU disturbances in a mouse AD model. Amyloid precursor protein (APP) transgenic (Tg) mice were used as a model of AD. Atorvastatin (30 mg/kg/day, p.o.) or pitavastatin (3mg/kg/day, p.o.) were administered from 5 to 20 months of age. These 2 statins improved behavioral memory and reduced the numbers of SP at 15 and 20 M without affecting serum lipid levels. There was a reduction in immunopositive staining for N-acetyl glucosamine oligomer (NAGO) in the endothelium and in collagen IV in the APP vehicle (APP/Ve) group, with collagen IV staining most weakest near SP. There was also an increase in intensity and numbers of glial fibrillary acidic protein (GFAP) positive astrocytes, particularly around the SP, where MMP-9 was more strongly labeled. Double immunofluorescent analysis showed that astrocytic endfeet had detached from the capillary endothelium in the APP/Ve group. Overall, these data suggest that statins may have therapeutic potential for AD by protecting NVU.

Effects of intima stiffness and plaque morphology on peak cap stress

BACKGROUND

Rupture of the cap of a vulnerable plaque present in a coronary vessel may cause myocardial infarction and death. Cap rupture occurs when the peak cap stress exceeds the cap strength. The mechanical stress within a cap depends on the plaque morphology and the material characteristics of the plaque components. A parametric study was conducted to assess the effect of intima stiffness and plaque morphology on peak cap stress.

METHODS

Models with idealized geometries based on histology images of human coronary arteries were generated by varying geometric plaque features. The constructed multi-layer models contained adventitia, media, intima, and necrotic core sections. For adventitia and media layers, anisotropic hyperelastic material models were used. For necrotic core and intima sections, isotropic hyperelastic material models were employed. Three different intima stiffness values were used to cover the wide range reported in literature. According to the intima stiffness, the models were classified as stiff, intermediate and soft intima models. Finite element method was used to compute peak cap stress.

RESULTS

The intima stiffness was an essential determinant of cap stresses. The computed peak cap stresses for the soft intima models were much lower than for stiff and intermediate intima models. Intima stiffness also affected the influence of morphological parameters on cap stresses. For the stiff and intermediate intima models, the cap thickness and necrotic core thickness were the most important determinants of cap stresses. The peak cap stress increased three-fold when the cap thickness was reduced from 0.25 mm to 0.05 mm for both stiff and intermediate intima models. Doubling the thickness of the necrotic core elevated the peak cap stress by 60% for the stiff intima models and by 90% for the intermediate intima models. Two-fold increase in the intima thickness behind the necrotic core reduced the peak cap stress by approximately 25% for both intima models. For the soft intima models, cap thickness was less critical and changed the peak cap stress by 55%. However, the necrotic core thickness was more influential and changed the peak cap stress by 100%. The necrotic core angle emerged as a critical determinant of cap stresses where a larger angle lowered the cap stresses. Contrary to the stiff and intermediate intima models, a thicker intima behind the necrotic core increased the peak cap stress by approximately 25% for the soft intima models. Adventitia thickness and local media regression had limited effects for all three intima models.

CONCLUSIONS

For the stiff and intermediate intima models, the cap thickness was the most important morphological risk factor. However for soft intima models, the necrotic core thickness and necrotic core angle had a bigger impact on the peak cap stress. We therefore need to enhance our knowledge of intima material properties if we want to derive critical morphological plaque features for risk evaluation.

An angular compounding technique using displacement projection for noninvasive ultrasound strain imaging of vessel cross-sections

Strain is considered to be a useful indicator of atherosclerotic plaque vulnerability. This study introduces an alternative for a recently introduced strain imaging method that combined beam steered ultrasound acquisitions to construct radial strain images of transverse cross-sections of superficial arteries. In that study, axial strains were projected in the radial direction. Using the alternative method introduced in this study, axial displacements are projected radially, followed by a least squares estimation of radial strains. This enables the use of a larger projection angle. Consequently, fewer acquisitions at smaller beam steering angles are required to construct radial strain images. Simulated and experimentally obtained radio-frequency data of radially expanding vessel phantoms were used to compare the two methods. Using only three beam steering angles (-30°, 0° and 30°), the new method outperformed the older method that used seven different angles and up to 45° of beam steering: the root mean squared error was reduced by 38% and the elastographic signal- and contrast-to-noise ratios increased by 1.8 dB and 4.9 dB, respectively. The new method was also superior for homogeneous and heterogeneous phantoms with eccentric lumens. To conclude, an improved noninvasive method was developed for radial strain imaging in transverse cross-sections of superficial arteries.

Molecular imaging of atherosclerotic plaques targeted to oxidized LDL receptor LOX-1 by SPECT/CT and magnetic resonance

BACKGROUND

The oxidized low-density lipoprotein receptor (LDLR) LOX-1 plays a crucial role in atherosclerosis. We sought to detect and assess atherosclerotic plaque in vivo by using single-photon emission computed tomography/computed tomography and magnetic resonance imaging and a molecular probe targeted at LOX-1.

METHODS AND RESULTS

Apolipoprotein E(-/-) mice fed a Western diet and LDLR(-/-) and LDLR(-/-)/LOX-1(-/-) mice fed an atherogenic diet were used. Imaging probes consisted of liposomes decorated with anti-LOX-1 antibodies or nonspecific immunoglobulin G, (111)indium or gadolinium, and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine fluorescence markers. In vivo imaging was performed 24 hours after intravenous injection (150 microL) of LOX-1 or nonspecific immunoglobulin G probes labeled with either (111)indium (600 muCi) or gadolinium (0.075 mmol/kg), followed by aortic excision for phosphor imaging and Sudan IV staining, or fluorescence imaging and hematoxylin/eosin staining. The LOX-1 probe also colocalized with specific cell types, apoptosis, and matrix metalloproteinase-9 expression in frozen aortic sections. Single-photon emission computed tomography/computed tomography imaging of the LOX-1 probe showed aortic arch "hot spots" in apolipoprotein E(-/-) mice (n=8), confirmed by phosphor imaging. Magnetic resonance imaging showed significant Gd enhancement in atherosclerotic plaques in LDLR(-/-) mice with the LOX-1 (n=7) but not with the nonspecific immunoglobulin G (n=5) probe. No signal enhancement was observed in LDLR(-/-)/LOX-1(-/-) mice injected with the LOX-1 probe (n=5). These results were confirmed by ex vivo fluorescence imaging. The LOX-1 probe bound preferentially to the plaque shoulder, a region with vulnerable plaque features, including extensive LOX-1 expression, macrophage accumulation, apoptosis, and matrix metalloproteinase-9 expression.

CONCLUSIONS

LOX-1 can be used as a target for molecular imaging of atherosclerotic plaque in vivo. Furthermore, the LOX-1 imaging signal is associated with markers of rupture-prone atherosclerotic plaque.

Carotid atheroma rupture observed in vivo and FSI-predicted stress distribution based on pre-rupture imaging

Atherosclerosis at the carotid bifurcation is a major risk factor for stroke. As mechanical forces may impact lesion stability, finite element studies have been conducted on models of diseased vessels to elucidate the effects of lesion characteristics on the stresses within plaque materials. It is hoped that patient-specific biomechanical analyses may serve clinically to assess the rupture potential for any particular lesion, allowing better stratification of patients into the most appropriate treatments. Due to a sparsity of in vivo plaque rupture data, the relationship between various mechanical descriptors such as stresses or strains and rupture vulnerability is incompletely known, and the patient-specific utility of biomechanical analyses is unclear. In this article, we present a comparison between carotid atheroma rupture observed in vivo and the plaque stress distribution from fluid–structure interaction analysis based on pre-rupture medical imaging. The effects of image resolution are explored and the calculated stress fields are shown to vary by as much as 50% with sub-pixel geometric uncertainty. Within these bounds, we find a region of pronounced elevation in stress within the fibrous plaque layer of the lesion with a location and extent corresponding to that of the observed site of plaque rupture.