Author + information
- Received June 28, 2010
- Revision received January 5, 2011
- Accepted January 7, 2011
- Published online May 1, 2011.
- Hélène Beaussier, PharmD, PhD⁎,†,
- Olivier Naggara, MD, PhD†,‡,
- David Calvet, MD‡,
- Robinson Joannides, MD, PhD§,
- Evelyne Guegan-Massardier, MD§,
- Emmanuel Gerardin, MD, PhD§,
- Michelle Iacob, MD§,
- Brigitte LaLoux, PhD⁎,†,
- Erwan Bozec, PhD⁎,†,
- Jérémy Bellien, PharmD, PhD§,
- Emmanuel Touze, MD, PhD†,‡,
- Ingrid Masson, PhD⁎∥,
- Christian Thuillez, MD, PhD§,
- Catherine Oppenheim, MD, PhD†,‡,
- Pierre Boutouyrie, MD, PhD⁎,† and
- Stéphane Laurent, MD, PhD⁎,†,⁎ ()
- ↵⁎Reprint requests and correspondence:
Dr. Stéphane Laurent, Department of Pharmacology and INSERM U970, Hôpital Européen Georges Pompidou, Assistance Publique, Hôpitaux de Paris, Université Paris Descartes, 20, rue Leblanc, Paris 75015, France
Objectives The purpose of this study was to correlate the arterial mechanics of carotid atherosclerotic plaques assessed from echotracking with their composition by high-resolution magnetic resonance imaging (HR-MRI).
Background Analysis of the relationship between mechanical parameters and structure of the plaque allows better understanding of the mechanisms leading to mechanical fatigue of plaque material, plaque rupture, and ischemic events. A specific longitudinal gradient of strain (reduced strain, i.e., lower radial strain at the plaque level than at the adjacent segment) has been shown in atherosclerotic plaques on the common carotid artery (CCA) in patients with hypertension, dyslipidemia, or type 2 diabetes mellitus. The structural abnormalities underlying this functional behavior have not been determined.
Methods Forty-six carotid plaques from 27 patients were evaluated; plaques were present at the site of the carotid bifurcation and extended to the CCA. Among the 27 patients, 9 had previous ischemic stroke ipsilateral to carotid stenosis (symptomatic) and 18 had not (asymptomatic). Mechanical parameters were measured at 128 sites on a 4-cm long CCA segment by noninvasive echotracking system, and strain gradient was calculated. Plaque composition was noninvasively determined by HR-MRI.
Results Complex plaques at HR-MRI (i.e., American Heart Association [AHA] stages IV to VIII) more often displayed a reduced strain than the simple plaques (i.e., AHA stages I to III; p = 0.046). HR-MRI verified complex plaques were associated with an outer remodeling upon echotracking, and had a lower distensibility than adjacent CCA (17.0 ± 5.0 MPa−1 vs. 21.7 ± 7.3 MPa−1; p = 0.007). An outer remodeling was observed in plaques with a lipid core at HR-MRI and was more frequent in symptomatic carotids.
Conclusions These findings indicate that the longitudinal mechanics of “complex” plaques follows a specific pattern of reduced strain. They also suggest that reduced strain, associated with an outer remodeling, may be a feature of high-risk plaques.
- arterial remodeling
- atherosclerotic plaque
- carotid artery
- high-resolution magnetic resonance imaging
Plaque rupture is a critical step in the evolution of atherosclerotic plaque, especially as clinically significant events occur in critical arteries (1). Although various animal models and post-mortem studies have improved our knowledge of the mechanics of plaque rupture and vulnerability of plaque (1,2), very few studies have clearly documented in vivo the mechanical characteristics of large arteries at the site of the plaque, and particularly at the level of the common carotid artery (CCA). It is generally accepted that mildly stenotic plaques with thin or ruptured fibrous cap, large lipid core, and hemorrhage are more susceptible to rupture than plaques with thick cap, high degree of fibrosis, and calcifications (1,2). Rupture mechanisms are complex processes that are dependent on plaque morphology, composition, and mechanical characteristics (1,2).
We have previously proposed that plaque rupture is influenced by repetitive strain of the arterial wall in the longitudinal direction. In previous communications, we took advantage of 128 radiofrequency (RF)-line high-resolution echotracking technology to characterize mechanical properties of various segments of a CCA bearing an atherosclerotic plaque in vivo (3,4). We calculated the strain gradient along the CCA, and determined 2 distinct patterns: a greater strain at the level of the plaque than at the adjacent CCA (pattern A) (3,4), and its opposite, a reduced strain at the level of the plaque (pattern B) (3,4). A reduced strain was more often observed in patients with hypertension, dyslipidemia, and type 2 diabetes mellitus than a greater strain (3,4).
An analysis of the relationship between mechanical parameters and structure of the plaque is mandatory to understand the mechanisms leading to mechanical fatigue of plaque material, and therefore plaque rupture and ischemic events (5). This characterization should be obtained noninvasively to be translated into clinical practice. Thus, high-resolution magnetic resonance imaging (HR-MRI) and echotracking are particularly well suited.
Our objectives were: 1) to determine, with echotracking and HR-MRI, the mechanics and composition of the arterial wall, respectively, at the site of the atherosclerotic plaque and in its vicinity; 2) to specify the relationship between strain gradient patterns and plaque composition; and 3) to determine whether mechanical behavior and structure of carotid plaque differ between patients with previous ischemic stroke and patients without stroke.
Twenty-seven patients were selected for the study (Fig. 1). Patients were eligible for the study if: 1) they were aged 50 to 80 years; and 2) they had at least an echotracking verified moderate atherosclerotic plaque at the site of the carotid bifurcation extending to the CCA. Noninclusion criteria were: 1) patients unable to tolerate HR-MRI; 2) HR-MRI contraindications; and 3) pregnancy (Fig. 1). The study was approved by the local ethics committee, and all patients signed an informed consent.
Symptomatic patients (n = 9) were defined by a previous ischemic stroke or transient ischemic attack (TIA) in the territory of the ipsilateral carotid artery stenosis without any other cause explaining stroke or TIA, and a stenosis of mild to moderate degree (20% to 40% NASCET [North American Symptomatic Carotid Endarterectomy Trial] criteria or 30% to 60% ECST [European Carotid Surgery Trial] criteria) (Fig. 1). Asymptomatic patients (n = 18) were defined as devoid of previous ischemic stroke or TIA, and having a stenosis of mild to moderate degree (20% to 60% NASCET criteria or 30% to 80% ECST criteria) at the site of the carotid bifurcation extending to the CCA. Patients were recruited from the departments of hypertension, neurology, vascular surgery, and vascular medicine.
Ultrasound examination and HR-MRI were performed within 1 month. The plaque at the level of the carotid bifurcation was defined according to the Mannheim consensus (6) as a focal structure that encroaches into the arterial lumen of at least 0.5 mm or 50% of the surrounding intima-media thickness (IMT) value or demonstrates a thickness of ≥1.5 mm as measured from the media-adventitia interface to the intima-lumen interface.
Essential hypertension was determined by a systolic blood pressure (SBP) ≥140 mm Hg or a diastolic blood pressure (DBP) ≥90 mm Hg or treatment with blood pressure (BP)-lowering drugs. Diabetes mellitus was indicated by abnormal fasting plasma glucose levels or the current use of insulin or an oral hypoglycemic agent. Dyslipidemia was defined as abnormal fasting plasma cholesterol levels (low-density lipoprotein cholesterol >3.0 mmol/l [115 mg/dl]) or the current use of lipid-lowering agents. Smoking status was defined as current or past use.
Data were recorded according to the requirements of the Commission Nationale Informatique et Liberté (i.e., the national committee for protection of individuals in computer use; authorization number 546379).
The ultrasonic noninvasive investigation was performed in a room dedicated to echography, after 15 min of recumbent rest. BP and arterial measurements were performed by pharmacologists (H.B., J.B., E.B., M.I.), trained and certified in vascular echography. Before the ultrasound examination, brachial BP measurements were taken using an oscillometric device (Colins, BP 8800, Colin Corporation Ayashi, Komaki, Japan) at 3-min intervals for 20 min, and the average was taken as the casual BP level. Mean blood pressure (MBP) was calculated as: MBP = DBP + [(SBP − DBP)/3]. Brachial pulse pressure (PP) was calculated as: PP = SBP − DBP. Aortic stiffness was measured using carotid-femoral pulse wave velocity (PWV) and was recorded along the descending thoracoabdominal aorta by the foot-to-foot method.
All patients underwent CCA measurements with a high-resolution echotracking system (ArtLab, Esaote, the Netherlands) including the use of a 128 RF lines linear array probe (3,4,7). This system determines arterial parameters with a very high precision and reproducibility (3,4,7) and takes into account the entire carotid segment in real time. Thus, we had access to all major mechanical parameters along a 4-cm arterial segment: IMT, internal and external diameters, and pulsatile strain in the radial direction (i.e., distension = Ds-Dd, where Ds is systolic internal diameter and Dd is diastolic internal diameter). External diameter corresponded to the adventitia-adventitia diameter. Internal diameter was obtained from adventitia-adventitia diameter minus twice the IMT at the posterior wall. We used a combination of: 1) a bidimensional acquisition mode (named B-mode; high spatial but low temporal resolutions, 128 RF lines) to determine IMT and diameter with a 17 μm and 35 μm resolution, respectively, at each of the 128 sites of a 4-cm long CCA segment (6,8); and 2) an arterial wall motion acquisition mode (named “fast B-mode”; high spatial and temporal resolution, 14 RF lines) to determine distension with a 1.7 μm spatial resolution at each of the 14 sites of a 2 cm long CCA segment (7).
Cross-sectional distensibility coefficient was calculated as: DC = ΔA / (A · ΔP), where A is the diastolic lumen area, ΔA is the stroke change in lumen area, and ΔP is local pulse pressure (PP) (9). Common carotid artery pressure waveform and local carotid PP values were recorded noninvasively with a pencil-type probe incorporating a high-fidelity Millar strain gauge transducer (SPT-301, Millar Instruments, Houston, Texas; Sphygmocor Millar Instruments, Nycomed, Paris France) as previously described (9).
Nineteen patients (12 asymptomatic and 7 symptomatic) among 27 had a plaque on both sides (Fig. 1). Echographic measurements were performed on the right and left CCAs, thus increasing the number of carotid plaques available for analysis. A total of 46 CCA plaques were taken into account to study structural and functional parameters (Fig. 1). In addition, 39 CCA plaques were analyzed to compare asymptomatic and symptomatic status.
Each carotid plaque was insonated in different axes to determine the position in which the major body of plaque could be best imaged. Measurements were performed several times, at different levels of the CCA, which was segmented into 8 adjacent zones to best match with the anatomical location of the plaque and its composition at HR-MRI. Data (IMT, diameter, distension, distensibility) were averaged within each of the 8 zones. Thus, it was possible to compare all arterial parameters between various locations (upstream of the plaque, on the plaque, and in some cases, downstream of the plaque) and to obtain longitudinal gradients of strain along the carotid segment bearing the plaque.
Strain was directly measured as the absolute change in diameter (distension), which required no mechanical assumption. Distension gradient was calculated as “strain at the adjacent CCA (upstream of the plaque)” minus “strain at the level of the plaque” (Fig. 2, Online Fig. 1) (3,4). Inner remodeling of the carotid plaque was characterized by a smaller internal diameter than in adjacent carotid, and a similar external diameter, thus a larger cross-sectional area (Online Fig. 2). Outer remodeling was characterized by a larger external diameter at the site of the plaque than in adjacent carotid and a similar internal diameter, thus a larger cross-sectional area (Online Fig. 2) (9–11).
Measurements were digitally stored for off-line analysis. The analysis of data acquired in bidimensional or arterial wall motion modes was performed using a custom written software platform (Matlab R2006a 7.2, MathWorks, Inc., Natick, Massachusetts).
All HR-MRI examinations were obtained on a 1.5-T Signa MR Unit (General Electric Healthcare, Milwaukee, Wisconsin), using a pair of 4-channel surface coils (Machnet, Eelde, the Netherlands) placed bilaterally around the neck. The standardized protocol combined 4 pulse sequences: T1-weighted imaging, proton density-weighted imaging, and T2-weighted imaging with fat suppression, and 3-dimensional time-of-flight, as previously detailed (12) (Fig. 3, Online Appendix). Total acquisition time was approximately 30 min. Images were provided in a DICOM format for analysis.
The MR images were read by consensus by 2 blinded readers (H.B., O.N.), on a dedicated workstation (Advantage Windows 4.3, General Electric Healthcare). They reviewed all qualifying carotid arteries using a standardized form adapted for the study to juxtapose echotracking data of the same zones of the qualifying carotid plaque. For each location, the 4 HR-MRI sequences were reviewed together. Image quality was assessed using a 3-point scale (excellent, good, poor) based on the quality of each image depending upon the overall signal-to-noise ratio, the presence of flow or motion artefacts, and conspicuousness of the vessel margins and wall architecture. This analysis was carried out on the whole image set for each sequence. In case of poor quality, images were excluded from the analysis. The qualitative assessment (tissue components) is described in the Online Appendix.
Finally, we classified each plaque into 8 categories (Table 1) according to the American Heart Association (AHA) recently adapted for MRI (13): types I and II have been combined because HR-MRI cannot differentiate between type I (discrete foam cells) and type II (multiple foam cells layers of the fatty streaks); types IV and V have been combined because HR-MRI cannot distinguish between proteoglycan composition of the type IV cap and the dense collagen of the type V cap. Quantitative measurements are described in the Online Appendix. The intensity of statin exposure, i.e., statin intensity, was calculated according to Jones et al. (14).
Nonparametric tests were used, and data are expressed as median (quartile 1; quartile 3). Quantitative variables were compared by means of an unpaired Student t test or a paired Student t test (at the site of plaque and at the surrounding normal adjacent CCA) or Wilcoxon rank-sum nonparametric test for difference in medians for each group. No corrections for correlated observations within subjects were made when plaque rather than subject was the unit of analysis. Categorical variables were compared by means of a chi-square test. A value of p < 0.05 was considered significant. Statistical analysis was performed using NCSS 2004 package software (Hintze JL, Kaysville, Utah).
Patients with simple (n = 15) and complex plaques (n = 12) were comparable for all variables (Fig. 1, Table 2)⇓ (14), as well as for asymptomatic (n = 18) and symptomatic (n = 9) patients (Online Table 1). The median delay from stroke at exploration was 3 ± 3 months in symptomatic patients.
HR-MRI parameters according to ultrasonic strain gradient
Carotid plaques with greater strain (n = 20) did not differ from carotid plaques with reduced strain (n = 26), as far as the various HR-MRI parameters were concerned (Table 2). However, when plaques were described according to the AHA modified classification (15), carotid plaques with reduced strain were more often characterized by an advanced AHA stage (IV to VIII), thus were a complex feature versus carotid plaques with greater strain (p = 0.046) (Table 1). In other words, complex plaques at MRI more often displayed reduced strain at echotracking than simple plaques.
Ultrasonographic parameters according to simple or complex plaque at HR-MRI
The geometrical and mechanical carotid parameters were compared between plaque and adjacent CCA, both within a group of 32 simple (AHA stages I to III) carotid plaques, and within a group of 14 complex (AHA stages IV to VIII) carotid plaques at HR-MRI (Table 4). The 14 complex plaques had a higher external diameter on the plaque (+5.8%, p < 0.003) than on the adjacent CCA, whereas internal diameter was not significantly different, indicating an outer remodeling. The opposite was observed among the 32 simple plaques, which had a lower (−6%; p < 0.006) internal diameter on the plaque and a similar external diameter, indicating an inner remodeling. Complex plaques also had a significantly lower (−23%; p = 0.007) distensibility on the plaque than on the adjacent CCA.
Ultrasonographic parameters and HR-MRI according to lipid core or previous stroke/TIA
The 14 carotid plaques without lipid core at HR-MRI presented with an inner remodeling (Table 5), whereas the opposite was observed among the 32 carotid plaques with lipid core (more details are given in the Online Appendix). The carotid of symptomatic patients presented with an outer remodeling. No significant difference was observed between asymptomatic and symptomatic carotids as far as functional parameters (Table 6) and HR-MRI parameters (data not shown) were concerned (more details are given in the Online Appendix).
The present study is, to our knowledge, the first to combine ultrasonographic and HR-MRI to better understand the structural basis of the mechanical properties of the carotid plaque.
Consideration of methods and limitations
The matching of the functional characteristics of the plaque at echographic examination with its composition characterized by HR-MRI was done by precisely overlapping the anatomical location of the plaque zones during both investigations. All measurements were performed on the CCA, on carotid plaques displaying a mild to moderate degree of stenosis, located on the bifurcation and extending to the CCA.
Only 31 patients were included in the study, and 27 were analyzed, because of stringent inclusion criteria and long duration measurement sessions. Although the limited sample size may increase the likelihood of finding negative results, it should be noted that the high number of analyses, either at echotracking (128 RF lines) or HR-MRI, limits the variability of measurements for each carotid. However, regardless of precision provided by the imaging techniques, the findings observed in this highly selected cohort may not be extrapolated to other plaques and clinical settings. We defined inner and outer remodeling by comparing the internal diameter at the level of the plaque with the internal diameter in adjacent carotid, in line with the intravascular echotracking definition of remodeling (an arterial area at the reference site different from the lesion site) (11).
Nineteen patients had a plaque on both right and left CCA (Fig. 1). We considered each carotid (right and left in the same subject) as an independent artery for studying the relationship between HR-MRI and echotracking. Thus, we compared 32 plaques with lipid core to 14 plaques without. However, when we studied plaques in the 9 patients with previous stroke or TIA, namely, symptomatic plaques, we included only plaques ipsilateral to stroke or TIA, and compared them to the 30 asymptomatic plaques. The word “symptomatic” should not be misleading. In the present study, it referred to the patient and not to the etiology of stroke. Indeed, it is likely that the culprit lesion was located downstream of the CCA, at the bifurcation. In none of the symptomatic patients was stroke attributed to the studied CCA plaque. The geometrical changes observed at the site of the CCA plaque in symptomatic patients cannot indicate, but only suggest, that similar changes occurred downstream, at the site of the culprit lesion.
Interpretation of findings
A major result of the present study is that plaques with reduced strain were more often associated with a complex (i.e., AHA stages IV to VIII) structure at HR-MRI than were plaques with greater strain (Table 1). That none of the individual components of the AHA score (collagen, lipid, calcium, or fibrous cap) was significantly different between simple and complex plaques (Table 3), and only score was different, was not surprising. This underlines the usefulness of combining quantitative changes into a score. Altogether, these results indicate that the arterial wall material of complex atherosclerotic plaque with a lipid core, surface defect, hemorrhage, or thrombus (AHA stages IV to VIII), is less extensible than upstream, and that carotid is less strained in the zone affected by plaque.
Several findings suggest that reduced strain is a feature of “vulnerable plaques” (1). First, vulnerable plaques are defined as complex plaques (1), which is consistent with the association between AHA stages IV to VIII and reduced strain. Second, vulnerable plaques display an outer remodeling (1), a feature that we previously reported to be strongly associated with reduced strain (3,4). Third, vulnerable plaques are more frequent in patients with hypertension, dyslipidemia, and diabetes, in whom reduced strain was more frequently observed than greater strain (3,4).
The mechanisms through which the structural modifications associated with vulnerable plaque lead to increased wall stiffness, and thus limit the strain of the whole carotid wall induced by pulsatile pressure, are unknown. Various mechanisms can be suggested, including a disorganization of the “musculoelastic complex” described by Glagov (10); mechanical fatigue (i.e., repeated cycles of distensions and elastic recoils of the arterial wall) of wall components, inducing repeated microfissuring and repairs with fibrous material; accumulation of inflammatory cells, over-expression of matrix metalloproteinase, degradation of extracellular matrix and reduction of smooth muscle cell number—all features of vulnerable plaque; and outer remodeling.
The relationship between longitudinal strain gradient and plaque rupture is not unequivocal. Plaque mechanics is generally analyzed in a cross-sectional way, and circumferential stress concentrations were found to be maximum at the shoulder of the fibrous cap of an asymmetrical plaque with increased lumen convexity (1,2,15). Heterogeneity of arterial mechanics should also be analyzed along the longitudinal axis, because it may generate stress concentrations, thus fatigue of biomaterials (15), at the junction of distensible and stiff areas. Whether stress concentrations and fatigue generated by reduced strain expose the plaque to a greater risk of rupture than stress concentrations and fatigue generated by greater strain remains to be determined.
In conclusion, an HR-MRI analysis of carotid plaque composition, combined with an ultrasonic investigation of the functional pattern of the plaque along the longitudinal axis, indicates that the longitudinal mechanics of complex plaques follows a specific pattern of reduced strain. Our findings also suggest that reduced strain, associated with an outer remodeling, is a feature of vulnerable plaques.
List of investigators: Paris (Hôpital Européen Georges Pompidou): Hélène Beaussier, Stéphane Laurent, and Pierre Boutouyrie (pharmacologists), Erwan Bozec and Brigitte Laloux (engineers), and Ingrid Masson (bioengineer); Paris (Sainte-Anne): David Calvet and Emmanuel Touzé (neurologists), Olivier Naggara and Catherine Oppenheim (radiologists); Rouen: Jérémy Bellien, Robinson Joannides, Michelle Iacob, and Christian Thuillez (pharmacologists); Aude Triquenot-Bagan and Evelyne Guégan-Massardier (neurologists), and Emmanuel Gérardin (radiologist).
The authors wish to acknowledge Jean-François Toussaint for his contribution to the design of the study.
This study was funded by an unrestricted grant from Astra-Zeneca; by INSERM (PNR 2005; C05-18/DGS 2006/0042), University Paris Descartes, and Assistance Publique-Hôpitaux de Paris; and by Association Charles Nicolle, CHU de Rouen, and Hôpitaux de Rouen. The authors have reported that they have no relationships to disclose.
- Abbreviations and Acronyms
- American Heart Association
- blood pressure
- common carotid artery
- diastolic blood pressure
- high-resolution magnetic resonance imaging
- intima-media thickness
- pulse pressure
- pulse wave velocity
- systolic blood pressure
- transient ischemic attack
- Received June 28, 2010.
- Revision received January 5, 2011.
- Accepted January 7, 2011.
- American College of Cardiology Foundation
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