Contrast-enhanced high resolution MRI for atherosclerotic carotid artery tissue characterization

PLACD-7T Study: Atherosclerotic Carotid Plaque Components Correlated with Cerebral Damage at 7 Tesla Magnetic Resonance Imaging

Introduction:

In patients with carotid artery stenosis histological plaque composition is associated with plaque stability and with presenting symptomatology. Preferentially, plaque vulnerability should be taken into account in pre-operative work-up of patients with severe carotid artery stenosis. However, currently no appropriate and conclusive (non-) invasive technique to differentiate between the high and low risk carotid artery plaque in vivo is available. We propose that 7 Tesla human high resolution MRI scanning will visualize carotid plaque characteristics more precisely and will enable correlation of these specific components with cerebral damage.

Study objective:

The aim of the PlaCD-7T study is 1: to correlate 7T imaging with carotid plaque histology (gold standard); and 2: to correlate plaque characteristics with cerebral damage ((clinically silent) cerebral (micro) infarcts or bleeds) on 7 Tesla high resolution (HR) MRI.

Design:

We propose a single center prospective study for either symptomatic or asymptomatic patients with haemodynamic significant (70%) stenosis of at least one of the carotid arteries. The Athero-Express (AE) biobank histological analysis will be derived according to standard protocol. Patients included in the AE and our prospective study will undergo a pre-operative 7 Tesla HR-MRI scan of both the head and neck area.

Discussion:

We hypothesize that the 7 Tesla MRI scanner will allow early identification of high risk carotid plaques being associated with micro infarcted cerebral areas, and will thus be able to identify patients with a high risk of periprocedural stroke, by identification of surrogate measures of increased cardiovascular risk.

Clinical and histological significance of gadolinium enhancement in carotid atherosclerotic plaque

BACKGROUND AND PURPOSE

Although the ability of MRI to investigate carotid plaque composition is well established, the mechanism and the significance of plaque gadolinium (Gd) enhancement remain unknown. We evaluated clinical and histological significance of Gd enhancement of carotid plaque in patients undergoing endarterectomy for carotid stenosis.

METHODS

Sixty-nine patients scheduled for a carotid endarterectomy prospectively underwent a 3-T MRI. Carotid plaque enhancement was assessed on T1-weighted images performed before and 5 minutes after Gd injection. Enhancement was recorded according to its localization. Histological analysis was performed of the entire plaque and of the area with matched contrast enhancement on MR images.

RESULTS

Gd enhancement was observed in 59% patients. Three types of carotid plaques were identified depending on enhancement location (shoulder region, shoulder and fibrous cap, and central in the plaque). Fibrous cap rupture, intraplaque hemorrhage, and plaque Gd enhancement was significantly more frequent in symptomatic than in asymptomatic patients (P=0.043, P<0.0001, and P=0.034, respectively). After histological analysis, Gd enhancement was significantly associated with vulnerable plaque (American Heart Association VI, P=0.006), neovascularization (P<0.0001), macrophages (P=0.030), and loose fibrosis (P<0.0001). Prevalence of neovessels, macrophages, and loose fibrosis in the area of Gd enhancement was 97%, 87%, and 80%, respectively, and was different depending on the enhancement location in the plaque. Fibrous cap status and composition were different depending on the type of plaque.

CONCLUSIONS

Gd enhancement of carotid plaque is associated with vulnerable plaque phenotypes and related to an inflammatory process.

Contrast-enhanced 3T high-resolution MR imaging in symptomatic atherosclerotic basilar artery stenosis

BACKGROUND AND PURPOSE

Contrast-enhanced 3T high-resolution MR imaging can be used to determine the wall enhancement pattern of the basilar artery in symptomatic atherosclerotic stenosis. We used this method to explore the relationship between wall enhancement and both recent infarction in the territory of the stenotic BA and subsequent ischemic events associated with the stenotic BA.

MATERIALS AND METHODS

Sixty patients with symptomatic atherosclerotic BA stenosis ≥70% were enrolled consecutively. HR-MRI of cross-sectional BAs was obtained before and after contrast media injection, and wall enhancement indices were calculated for sections proximal to, at, and distal to the site of maximal luminal narrowing. DWI of the brain was performed to determine the presence of recent infarction.

RESULTS

Images from 56 patients were suitable for analysis. Thirty-three patients underwent stent placement for the stenotic BA, and 23 patients underwent conservative medical treatment with antiplatelet agents and risk-factor control. All 23 patients with medical treatment had a 12-month follow-up. Greater wall enhancement was seen in the section proximal to the MLN section in both patients with recent infarction (74 ± 65% versus 44 ± 44%; P = .046) and in patients with subsequent ischemic events (100 ± 57% versus 44 ± 44%; P = .014).

CONCLUSIONS

Greater wall enhancement proximal to the MLN site correlates with recent infarction in the territory of the stenotic BA and subsequent ischemic events associated with the stenotic BA. Contrast-enhanced HR-MRI may serve as a noninvasive tool for risk stratification of BA atherosclerosis.

Noninvasive magnetic resonance imaging evaluation of endothelial permeability in murine atherosclerosis using an albumin-binding contrast agent

BACKGROUND

Endothelial dysfunction promotes atherosclerosis and precedes acute cardiovascular events. We investigated whether in vivo magnetic resonance imaging with the use of an albumin-binding contrast agent, gadofosveset, could detect endothelial damage associated with atherosclerosis in apolipoprotein E-deficient (ApoE(-/-)) mice. Furthermore, we tested whether magnetic resonance imaging could noninvasively assess endothelial function by measuring the endothelial-dependent vasodilation in response to acetylcholine.

METHODS AND RESULTS

ApoE(-/-) mice were imaged at 4, 8, and 12 weeks after commencement of a high-fat diet. Statin-treated ApoE(-/-) mice were scanned after 12 weeks of a high-fat diet. Wild-type mice were imaged before and 48 hours after injection of Russell's viper venom, an endothelial toxin. Delayed enhancement magnetic resonance imaging and T1 mapping of the brachiocephalic artery, 30 minutes after injection of gadofosveset, showed increased vessel wall enhancement and relaxation rate (R(1)) with progression of atherosclerosis in ApoE(-/-)(R(1) [s(-1)]: R(4 weeks) 2.42±0.35, R(8 weeks) 3.45±0.54, R(12 weeks) 3.83±0.52) and Russell's viper venom-injected wild-type mice (R(1)=4.57±0.86). Conversely, wild-type (R(1)=2.15±0.34) and statin-treated ApoE(-/-) (R(1)=3.0±0.65) mice showed less enhancement. Uptake of gadofosveset correlated with Evans blue staining, morphological changes of endothelial cells, and widening of the cell-cell junctions, suggesting that uptake occurs in regions of increased vascular permeability. Endothelial-dependent vasomotor responses showed vasoconstriction of the arteries of the ApoE(-/-) (-22.22±7.95%) and Russell's viper venom-injected (-10.37±17.60%) mice compared with wild-type mice (32.45±12.35%). Statin treatment improved endothelium morphology and function (-8.12±8.22%).

CONCLUSIONS

We demonstrate the noninvasive assessment of endothelial permeability and function with the use of an albumin-binding magnetic resonance contrast agent. Blood albumin leakage could be a surrogate marker for the in vivo evaluation of interventions that aim to restore the endothelium.

Clinical risk factors and CT imaging features of carotid atherosclerotic plaques as predictors of new incident carotid ischemic stroke: a retrospective cohort study

BACKGROUND AND PURPOSE

Parameters other than luminal narrowing are needed to predict the risk of stroke more reliably, particularly in patients with <70% stenosis. The goal of our study was to identify clinical risk factors and CT features of carotid atherosclerotic plaques, in a retrospective cohort of patients free of stroke at baseline, that are independent predictors of incident stroke on follow-up.

MATERIALS AND METHODS

We identified a retrospective cohort of patients admitted to our emergency department with suspected stroke between 2001-2007 who underwent a stroke work-up including a CTA of the carotid arteries that was subsequently negative for acute stroke. All patients also had to receive a follow-up brain study at least 2 weeks later. From a random sample, we reviewed charts and imaging studies of patients with subsequent new stroke on follow-up as well as those who remained stroke-free. All patients were classified either as "new carotid infarct patients" or "no-new carotid infarct patients" based on the Causative Classification for Stroke. Independently, the baseline CTA studies were processed using a custom, CT-based automated computer classifier algorithm that quantitatively assesses a set of carotid CT features (wall thickness, plaque ulcerations, fibrous cap thickness, lipid-rich necrotic core, and calcifications). Univariate and multivariate statistical analyses were used to identify any significant differences in CT features between the patient groups in the sample. Subsequent ROC analysis allowed comparison to the classic NASCET stenosis rule in identifying patients with incident stroke on follow-up.

RESULTS

We identified a total of 315 patients without a new carotid stroke between baseline and follow-up, and 14 with a new carotid stroke between baseline and follow-up, creating the main comparison groups for the study. Statistical analysis showed age and use of antihypertensive drugs to be the most significant clinical variables, and maximal carotid wall thickness was the most relevant imaging variable. The use of age ≥ 75 years, antihypertensive medication use, and a maximal carotid wall thickness of at least 4 mm was able to successfully identify 10 of the 14 patients who developed a new incident infarct on follow-up. ROC analysis showed an area under the ROC curve of 0.706 for prediction of new stroke with this new model.

CONCLUSIONS

Our new paradigm of using age ≥ 75 years, history of hypertension, and carotid maximal wall thickness of >4 mm identified most of the patients with subsequent new carotid stroke in our study. It is simple and may help clinicians choose the patients at greatest risk of developing a carotid infarct, warranting validation with a prospective observational study.

Advances in noninvasive imaging for evaluating clinical risk and guiding therapy in carotid atherosclerosis

Managing asymptomatic carotid atherosclerosis with a view to preventing ischemic stroke is a challenging task. As the annual risk of stroke in untreated asymptomatic patients on average is less than the risk of surgical intervention, the key question is how to identify those asymptomatic individuals whose risk of stroke is elevated and who would benefit from surgery, while sparing low-risk asymptomatic patients from the risks of surgical intervention. The advent of a multitude of noninvasive carotid imaging techniques offers an opportunity to improve risk stratification in patients and to monitor the response to medical therapies; assessing efficacy at individual and population levels. As part of this, plaque measurement techniques (using ultrasound, computed tomography or MRI) may be employed in monitoring plaque/component regression and progression. Novel imaging applications targeted to plaque characteristics, inflammation and neovascularization, including contrast-enhanced ultrasound and MRI, dynamic contrast-enhanced MRI, and fluorodeoxyglucose-PET, are also being explored. Ultimately, noninvasive imaging and other advances in risk stratification aim to improve and individualize the management of patients with carotid atherosclerosis.

Vascular wall imaging of vulnerable atherosclerotic carotid plaques: current state of the art and potential future of endovascular optical coherence tomography

As stroke is one of the leading causes of death and long-term morbidity worldwide, the research community has studied cardiac embolic sources, as well as vessel wall pathologies. For the latter, attention has been focused on defining morphologic tissue features associated with catastrophic events stemming from the carotid artery. Multiple noninvasive imaging modalities are currently being used to image and classify carotid atherosclerotic plaques, such as MR imaging, CT, and sonography, in an effort to provide clinically relevant predictive metrics for use in patient risk stratification and to define appropriate treatment options. This article compares and contrasts these existing clinical imaging modalities along with discussion of a new endovascular technique originally developed for cardiology, OCT, with which 3D comprehensive high-resolution images of the arterial wall can be acquired.

Parametric subharmonic imaging using a commercial intravascular ultrasound scanner: an in vivo feasibility study

OBJECTIVES

The feasibility of visualizing atherosclerotic plaque using parametric subharmonic intravascular ultrasound (IVUS) was investigated in vivo.

METHODS

Atherosclerosis was induced in the aorta of 2 rabbits. Following injection of Definity (Lantheus Medical Imaging, North Billerica, MA), radiofrequency IVUS signals were acquired at 40 MHz with a Galaxy IVUS scanner (Boston Scientific/Scimed, Natick, MA). Subharmonic imaging (SHI; receiving at 20 MHz) was performed offline by applying an 8-order equalization filter. Contrast-to-tissue ratios (CTRs) were computed for the vessel relative to the plaque area over 4 time points. Contrast-to-tissue ratios were also calculated for the plaque-tissue and vessel-tissue from 4 tissue regions of interest at 4 time points. Finally, parametric images showing the cumulative maximum intensity (CMI), time to peak, perfusion (PER), and time-integrated intensity (TII) were generated for the fundamental and subharmonic data sets, and CTR measurements were repeated.

RESULTS

Injection of the contrast agent resulted in improved delineation between plaque and the vessel lumen. Subharmonic imaging resulted in noticeable tissue suppression, although the intensity from the contrast agent was reduced. No significant improvement in the plaque to vessel lumen CTR was observed between the subharmonic and fundamental IVUS (2.1 ± 3.64 versus 2.2 ± 4.20; P = .5). However, the CTR for plaque-tissue was improved (11.8 ± 7.32 versus 9.9 ± 7.06; P < .0001) for SHI relative to fundamental imaging. Cumulative-maximum-intensity and TII maps of both fundamental and subharmonic data provided increased CTRs relative to nonparametric data sets (P < .002). Additionally, the CMI, PER, and TII of SHI IVUS showed significantly improved vessel-plaque CTRs for SHI relative to the fundamental (P < .04).

CONCLUSIONS

Parametric SHI IVUS of atherosclerotic plaque is feasible and improves the visualization of the plaque.

Assessment of Carotid Plaque Stability Based on the Dynamic Enhancement Pattern in Plaque Components With Multidetector CT Angiography

Background and Purpose—

Recent studies have investigated plaque morphology to determine patients who are at high risk of carotid atherosclerosis. In this study, we investigated whether a difference in dynamic enhancement pattern in plaque components could be useful to assess plaque stability with multidetector CT angiography.

Methods—

Fifty-nine lesions with moderate to severe carotid atherosclerosis in 51 patients (33 symptomatic, 18 asymptomatic) were consecutively included. Early- and delayed-phase images were obtained in 3 equivalent axial slices with multidetector CT angiography. Hounsfield units (HU) in the early phase were subtracted from those in the delayed phase in plaques (ΔHU) and compared with clinical features, MRI-based plaque characteristics, and histological findings with 20 surgical specimens acquired from carotid endarterectomy.

Results—

The ΔHU was significantly higher in asymptomatic than that in symptomatic presentation ( P =0.02). With MRI, a higher ΔHU was negatively correlated with signal intensity on T1-weighted imaging ( r =−0.56, P <0.0001). Histology confirmed that ΔHU was positively correlated with fibrous tissue ( r =0.67, P =0.001) and negatively correlated with a lipid-rich necrotic core with hemorrhage ( r =−0.70, P <0.001). Moreover, less neovascularization and inflammation was found in plaques with a higher ΔHU.

Conclusions—

Delayed-phase images provide information regarding the dynamic change in contrast media from the early arterial phase. An increase in HU from the early phase on multidetector CT angiography indicates plaque stability with more fibrous tissue and a less lipid-rich necrotic core, intraplaque hemorrhage, and neovascularization.

Symptomatic Low-Grade Carotid Stenosis With Intraplaque Hemorrhage and Expansive Arterial Remodeling Is Associated With a High Relapse Rate Refractory to Medical Treatment

BACKGROUND:

Carotid plaque characteristics influence future risk of stroke considerably. However, the severity of stenosis does not accurately reflect plaque burden in patients with expansive arterial remodeling.

OBJECTIVE:

To determine the therapeutic outcome of symptomatic carotid low-grade stenosis with vulnerable plaque based on magnetic resonance imaging (MRI) characterization.

METHODS:

We studied 25 (male, n = 23; age, 74.2 ± 5.6 years) of 29 consecutive patients with symptomatic carotid low-grade stenosis (&lt;50%) and both high-signal plaque and expansive remodeling on T1-weighted MRIs. The remaining 4 were excluded because of impending stroke. A single antithrombotic and statin were administered, and recurrent ischemic stroke was treated with dual antithrombotics. We considered carotid endarterectomy when recurrence was refractory to aggressive medical treatment.

RESULTS:

During a 31.3 ± 16.4-month follow-up, 11 of the 25 patients developed a total of 30 recurrent ischemic events (46.0% per patient-year). The patients' characteristics did not differ significantly between the groups with and without recurrence (n = 11 and n = 14, respectively). Seven of 11 patients in the recurrence group treated with carotid endarterectomy remained free of ischemic events during a postoperative follow-up of 19.1 ± 14.6 months.

CONCLUSION:

Symptomatic low-grade carotid stenosis with vulnerable plaque confirmed by MRI was associated with a high rate of stroke recurrence that was refractory to aggressive medical treatment. However, carotid endarterectomy was safe and effective for such patients. Plaque characterization by MRI has the potential for more accurate stroke risk stratification in the management of carotid low-grade stenosis.

Molecular MRI of Inflammation in Atherosclerosis

Inflammatory activity in atherosclerotic plaque is a risk factor for plaque rupture and atherothrombosis and may direct interventional therapy. Inflammatory activity can be evaluated at the (sub)cellular level using in vivo molecular MRI. This paper reviews recent progress in contrast-enhanced molecular MRI to visualize atherosclerotic plaque inflammation. Various MRI contrast agents, among others ultra-small particles of iron oxide, low-molecular-weight Gd-chelates, micelles, liposomes, and perfluorocarbon emulsions, have been used for in vivo visualization of various inflammation-related targets, such as macrophages, oxidized LDL, endothelial cell expression, plaque neovasculature, MMPs, apoptosis, and activated platelets/thrombus. An enzyme-activatable magnetic resonance contrast agent has been developed to study myeloperoxidase activity in inflamed plaques. Agents creating contrast based on the chemical exchange saturation transfer mechanism were used for thrombus imaging. Transfer of these molecular MRI techniques to the clinic will critically depend on the safety profiles of these newly developed magnetic resonance contrast agents.

Quantifying the evolution of vascular barrier disruption in advanced atherosclerosis with semipermeant nanoparticle contrast agents

Rationale

Acute atherothrombotic occlusion in heart attack and stroke implies disruption of the vascular endothelial barrier that exposes a highly procoagulant intimal milieu. However, the evolution, severity, and pathophysiological consequences of vascular barrier damage in atherosclerotic plaque remain unknown, in part because quantifiable methods and experimental models are lacking for its in vivo assessment.

Objective

To develop quantitative nondestructive methodologies and models for detecting vascular barrier disruption in advanced plaques.

Methods and Results

Sustained hypercholesterolemia in New Zealand White (NZW) rabbits for >7–14 months engendered endothelial barrier disruption that was evident from massive and rapid passive penetration and intimal trapping of perfluorocarbon-core nanoparticles (PFC-NP: ∼250 nm diameter) after in vivo circulation for as little as 1 hour. Only older plaques (>7 mo), but not younger plaques (<3 mo) demonstrated the marked enhancement of endothelial permeability to these particles. Electron microscopy revealed a complex of subintimal spongiform channels associated with endothelial apoptosis, superficial erosions, and surface-penetrating cholesterol crystals. Fluorine (¹⁹F) magnetic resonance imaging and spectroscopy (MRI/MRS) enabled absolute quantification (in nanoMolar) of the passive permeation of PFC-NP into the disrupted vascular lesions by sensing the unique spectral signatures from the fluorine core of plaque-bound PFC-NP.

Conclusions

The application of semipermeant nanoparticles reveals the presence of profound barrier disruption in later stage plaques and focuses attention on the disrupted endothelium as a potential contributor to plaque vulnerability. The response to sustained high cholesterol levels yields a progressive deterioration of the vascular barrier that can be quantified with fluorine MRI/MRS of passively permeable nanostructures. The possibility of plaque classification based on the metric of endothelial permeability to nanoparticles is suggested.

Evaluation of multicontrast MRI including fat suppression and inversion recovery spin echo for identification of intra-plaque hemorrhage and lipid core in human carotid plaque using the mahalanobis distance measure

Intra-plaque hemorrhage (IPH) and lipid core, characteristics of rupture prone carotid plaques, are often visualized in vivo with MRI using T1 weighted gradient and spin echo, respectively. Increasing magnetic field strength may help to identify IPH and lipid core better. As a proof of concept, automatic segmentation of plaque components was performed with the Mahalanobis distance (MD) measure derived from image contrast from multicontrast MR images including inversion recovery spin echo and T1 weighted gradient echo with fat suppression. After MRI of nine formaldehyde-fixated autopsy specimens, the MDs and Euclidean Distances between plaque component intensities were calculated for each MR weighting. The distances from the carotid bifurcation and the size and shape of calcification spots were used as landmarks for coregistration of MRI and histology. MD between collagen/cell-rich area and IPH was largest with inversion recovery spin echo (4.2/9.3, respectively), between collagen/cell-rich area/foam cells and lipid core with T1 weighted gradient echo with fat suppression (26.9/38.2/4.6, respectively). The accuracy of detection of IPH, cell-rich area, and collagen increased when the MD classifier was used compared with the Euclidean Distance classifier. The enhanced conspicuity of lipid core and IPH in human carotid artery plaque, using ex vivo T1 weighted gradient echo with fat suppression and inversion recovery spin echo MRI and MD classifiers, demands further in vivo evaluation in patients.

MR angiography and imaging for the evaluation of middle cerebral artery atherosclerotic disease

Intracranial atherosclerotic disease may constitute the most common cause of ischemic stroke worldwide; yet, in the developed world, imaging research has largely focused on extracranial atherosclerosis. Many studies in populations of Asian, African, and Hispanic descent demonstrate the preponderance of intracranial stenosis compared with carotid stenosis. This review examines the clinical presentations of MCA atherosclerosis and stenosis and the use of noninvasive MR imaging in the assessment of intracranial vasculature. MRA is a well-validated technique that offers great advantage over traditional angiography. Advances in high-resolution MR imaging of MCA stenosis have the potential to yield excellent visualization of plaque. Future developments in high-resolution MR imaging to depict intracranial atherosclerosis are explored in this review; these advances will guide endovascular therapy and the comparison of novel interventions.

Carotid plaque enhancement and symptom correlations: an evaluation by using multidetector row CT angiography

BACKGROUND AND PURPOSE

The identification of plaque characteristics that determine its vulnerability is extremely important. The purpose of this work was to evaluate CPE after administration of contrast material and to assess whether there is a statistical association between CPE and cerebrovascular symptoms.

MATERIALS AND METHODS

Ninety-seven consecutive patients (69 men, 28 women; mean age, 62 years; age range, 39-82 years), studied by using an MDCT scanner, were retrospectively analyzed. Examinations were performed before and after administration of contrast medium. Plaque enhancement was analyzed, and the obtained data were compared with the patient's symptoms. Patients were classified as symptomatic (TIA or stroke with a temporal window of 6 months) or asymptomatic according to neurologic assessment and the TOAST criteria. The ROC curve and Az were calculated, and multiple logistic regression analysis was performed.

RESULTS

Thirty-nine patients were excluded because they had calcified plaques (40.2%). CPE was observed in 74% of the remaining 58 patients. A statistically significant difference was observed between symptomatic and asymptomatic patients for the presence of CPE (P = .0013; OR = 7.5). Moreover, we observed that CPE was higher in fatty plaques (P = .035) than in mixed ones and more frequent in the former (P = .0119). The ROC curve demonstrated that a threshold of 15 HU is associated with a specificity and sensitivity of 83.33% and 76.47%, respectively. Multiple logistic regression showed that CPE and symptoms are associated (P = .0315).

CONCLUSIONS

The results of our study suggest that for noncalcified carotid plaques, the presence of CPE is associated with cerebrovascular symptoms. Fatty plaques are more likely to have CPE compared with mixed plaques.

Study of carotid arterial plaque stress for symptomatic and asymptomatic patients

Stroke is one of the leading causes of death in the world, resulting mostly from the sudden ruptures of atherosclerosis carotid plaques. Until now, the exact plaque rupture mechanism has not been fully understood, and also the plaque rupture risk stratification. The advanced multi-spectral magnetic resonance imaging (MRI) has allowed the plaque components to be visualized in-vivo and reconstructed by computational modeling. In the study, plaque stress analysis using fully coupled fluid structure interaction was applied to 20 patients (12 symptomatic and 8 asymptomatic) reconstructed from in-vivo MRI, followed by a detailed biomechanics analysis, and morphological feature study. The locally extreme stress conditions can be found in the fibrous cap region, 85% at the plaque shoulder based on the present study cases. Local maximum stress values predicted in the plaque region were found to be significantly higher in symptomatic patients than that in asymptomatic patients (200 ± 43 kPa vs. 127 ± 37 kPa, p=0.001). Plaque stress level, defined by excluding 5% highest stress nodes in the fibrous cap region based on the accumulative histogram of stress experienced on the computational nodes in the fibrous cap, was also significantly higher in symptomatic patients than that in asymptomatic patients (154 ± 32 kPa vs. 111 ± 23 kPa, p<0.05). Although there was no significant difference in lipid core size between the two patient groups, symptomatic group normally had a larger lipid core and a significantly thinner fibrous cap based on the reconstructed plaques using 3D interpolation from stacks of 2D contours. Plaques with a higher stenosis were more likely to have extreme stress conditions upstream of plaque throat. The combined analyses of plaque MR image and plaque stress will advance our understanding of plaque rupture, and provide a useful tool on assessing plaque rupture risk.

Rationale and design of dal-PLAQUE: a study assessing efficacy and safety of dalcetrapib on progression or regression of atherosclerosis using magnetic resonance imaging and 18F-fluorodeoxyglucose positron emission tomography/computed tomography

dal-PLAQUE is a placebo-controlled multicenter study designed to assess the effect of dalcetrapib on imaging measures of plaque inflammation and plaque burden. dal-PLAQUE is a multimodality imaging study in the context of the large dal-HEART Program. Decreased high-density lipoprotein cholesterol is linked to increased risk of coronary heart disease (CHD). Dalcetrapib, a compound that increases high-density lipoprotein cholesterol by modulating cholesteryl ester transfer protein, is being studied to assess if it can reduce the progression of atherosclerotic disease and thereby decrease cardiovascular morbidity and mortality. Patients with CHD or CHD-risk equivalents were randomized to receive 600 mg dalcetrapib or placebo daily for 24 months, in addition to conventional lipid-lowering medication and other medications for cardiovascular risk factors. The primary outcomes are the effect of dalcetrapib on 18F-fluorodeoxyglucose positron emission tomography target-to-background ratio after 6 months and magnetic resonance imaging (MRI) plaque burden (wall area, wall thickness, total vessel area, and wall area/total vessel area ratio) after 12 months. Secondary objectives include positron emission tomography target-to-background ratio at 3 months and MRI plaque burden at 6 and 24 months; plaque composition at 6, 12, and 24 months; and aortic compliance at 6 months. A tertiary objective is to examine the dynamic contrast-enhanced MRI parameters of plaque neovascularization. In total, 189 subjects entered screening, and 130 were randomized. dal-PLAQUE will provide important information on the effects of dalcetrapib on markers of inflammation and atherosclerotic plaque burden and, thereby, on the safety of cholesteryl ester transfer protein modulation with dalcetrapib. Results are expected in 2011.

High-resolution MRI of carotid plaque with a neurovascular coil and contrast-enhanced MR angiography: one-stop shopping for the comprehensive assessment of carotid atherosclerosis

OBJECTIVE

The objective of our study was to assess a protocol of study of carotid atherosclerosis coupling vascular wall imaging and luminal imaging in the same examination and to evaluate the accuracy of high-resolution MRI with a neurovascular coil in carotid plaque characterization.

SUBJECTS AND METHODS

Thirty-two consecutive patients with 34 carotid artery stenoses were prospectively enrolled. MRI was performed on a 1.5-T unit. Plaque assessment was performed starting with a diffusion-weighted sequence and followed by a fat-suppressed T1-weighted sequence; after contrast-enhanced MR angiography (CE-MRA), all patients were evaluated with a T1-weighted 3D high-resolution sequence. Carotid plaques were classified as type A, having a large lipid-necrotic core; type B, being a complex fibrotic-calcified plaque with soft content (mixed plaque); or type C, being a fibrotic-calcified plaque (hard). Additional features indicative of vulnerable plaque such as intraplaque hemorrhage (IPH), ulceration, and severe stenosis were registered. MR findings were compared with surgical specimens.

RESULTS

MRI correctly identified 11 of 13 type A, eight of 11 type B, and eight of 10 type C plaques (sensitivity, 84.6%, 72.7%, and 80%, respectively). In the identification of lipid-necrotic core plaque, MRI showed a sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of 84.6%, 100%, 100%, and 91.3%, respectively (κ = 0.87). For reordering all plaques in two groups (i.e., soft vs nonsoft) in the identification of soft plaques, MRI had a sensitivity, specificity, PPV, and NPV of 83.3%, 80%, 90.9%, and 66.7%, respectively (κ = 0.59). IPH, ulcers, and severe stenosis were detected in eight of eight, 11 of 13, and 25 of 25 cases, respectively.

CONCLUSION

In patients with carotid atherosclerosis, ongoing CE-MRA with a neurovascular coil for the simultaneous detection of unstable plaques is feasible. Our MR protocol accurately identifies the major features of vulnerable plaque.

Noninvasive carotid artery imaging with a focus on the vulnerable plaque

Currently carotid imaging has 2 main focuses: assessment of luminal stenosis and classification of atherosclerotic plaque characteristics. Measurement of the degree of stenosis is the main assessment used for current treatment decision making, but an evolving idea that is now driving imaging is the concept of vulnerable plaque, which is where plaque components are identified and used to define which plaques are at high risk of causing symptoms compared with those at low risk. This review article covers the methods used for noninvasive assessment of carotid luminal stenosis and the options available for plaque imaging.

Imaging of atherosclerosis: magnetic resonance imaging

Atherosclerosis and its thrombotic complications are the major cause of morbidity and mortality in the industrialized countries. Despite advances in our understanding of the pathophysiology, pathogenesis, and new treatment modalities, the absence of an adequate non-invasive imaging tool for early detection limits both the prevention and treatment of patients with various degrees and anatomical localizations of atherothrombotic disease. An ideal clinical imaging modality for atherosclerotic vascular disease should be safe, inexpensive, non-invasive or minimally invasive, accurate, and reproducible, and the results should correlate with the extent of atherosclerotic disease and have high predictive values for future clinical events. High-resolution magnetic resonance imaging (MRI) has emerged as the most promising technique for studying atherothrombotic disease in humans in vivo. Most importantly, MRI allows for the characterization of plaque composition, i.e. the discrimination of lipid core, fibrosis, calcification, and intraplaque haemorrhage deposits. Magnetic resonance imaging also allows for the detection of arterial thrombi and in defining thrombus age. Magnetic resonance imaging has been used to monitor plaque progression and regression in several animal models of atherosclerosis and in humans. Emerging MRI techniques capable of imaging biological processes, including inflammation, neovascularization, and mechanical forces, may aid in advancing our understanding of the atherothrombotic disease. Advances in diagnosis do prosper provided they march hand-in-hand with advances in treatment. We stand at the threshold of accurate non-invasive assessment of atherosclerosis. Thus, MRI opens new strategies ranging from screening of high-risk patients for early detection and treatment as well as monitoring of the target lesions for pharmacological intervention. Identification of subclinical atherosclerosis and early treatment initiation has the potential to surpass conventional risk factor assessment and management in terms of overall impact on cardiovascular morbidity and mortality. Such strategy is currently under clinical investigation.

Carotid MRI: a tool for monitoring individual response to cardiovascular therapy?

Stroke remains a leading cause of morbidity and mortality. While stroke-related mortality has declined over the past four decades, data indicate that the mortality rate has begun to plateau. This change in trend may be attributable to variation in individual response to therapies that were derived from population-based studies. Further reductions in stroke mortality may require individualized care governed by directly monitoring the effects of cardiovascular therapy. In this article, carotid MRI is considered as a tool for monitoring in vivo carotid atherosclerotic disease, a principal etiology of stroke. Carotid MRI has been previously utilized to identify specific plaque features beyond luminal stenosis that are predictive of transient ischemic attack and stroke. To gain perspective on the possibility of monitoring plaque change within the individual, clinical trials and natural history studies that have used serial carotid MRI are considered. Data from these studies indicate that patients with a lipid-rich necrotic core with or without intraplaque hemorrhage may represent the desired phenotype for monitoring treatment effects in the individual. Advances in tissue-specific sequences, acquisition resolution, scan time, and techniques for monitoring inflammation and mechanical forces are expected to enable earlier detection of response to therapy. In so doing, cost-effective multicenter studies can be conducted to confirm the anticipated positive effects on outcomes of using carotid MRI for individualized care in patients with carotid atherosclerosis. In accordance, carotid MRI is poised to emerge as a powerful clinical tool for individualized management of carotid atherosclerotic disease to prevent stroke.

MRI of coronary wall remodeling in a swine model of coronary injury using an elastin-binding contrast agent

BACKGROUND

The extracellular matrix (ECM) plays an important role in the pathogenesis of atherosclerosis and in-stent restenosis. Elastin is an essential component of the ECM. ECM degradation can lead to plaque destabilization, whereas enhanced synthesis typically leads to vessel wall remodeling resulting in arterial stenosis or in-stent restenosis after stent implantation. The objective of this study was to demonstrate the feasibility of MRI of vascular remodeling using a novel elastin-binding contrast agent (BMS-753951).

METHODS AND RESULTS

Coronary injury was induced in 6 pigs by endothelial denudation and stent placement. At day 28, delayed-enhancement MRI coronary vessel wall imaging was performed before and after injection of gadolinium-diethylene triamine pentaacetic acid (Gd-DTPA). Two days later, DE-MRI was repeated after administration of BMS-753951. Contrast-to-noise-ratio and areas of enhancement were determined. Delayed-enhancement MRI with BMS-753951 caused strong enhancement of the aortic, pulmonary artery, and injured coronary artery walls, whereas Gd-DTPA did not. Delayed-enhancement MRI of the stented coronary artery with BMS-753951 yielded a 3-fold higher contrast-to-noise-ratio when compared with the balloon-injured and control coronary artery (21±6 versus 7±3 versus 6±4; P<0.001). The area of enhancement correlated well with the area of remodeling obtained from histological data (R(2)=0.86, P<0.05).

CONCLUSIONS

We demonstrate the noninvasive detection and quantification of vascular remodeling in an animal model of coronary vessel wall injury using an elastin-specific MR contrast agent. This novel approach may be useful for the assessment of coronary vessel wall remodeling in patients with suspected coronary artery disease. Further studies in atherosclerotic animal models and degenerative ECM disease are now warranted.

Evaluation of inflammatory status of atherosclerotic carotid plaque before thromboendarterectomy using delayed contrast-enhanced subtracted images after magnetic resonance angiography

OBJECTIVE

To investigate the correlation among carotid plaque contrast enhancement (CPCE) at MRI, inflammatory cell infiltration (ICI) at histopathology, and carotid stenosis degree.

MATERIALS AND METHODS

Twenty-eight patients (19 males; mean age 67±9 years) scheduled for thromboendarterectomy prospectively underwent 1.5-T MR imaging using: (a) axial T1-weighted gradient-echo (T1wGRE) sequence centered on carotid bifurcations; (b) contrast-enhanced MR angiography (CE-MRA) with 0.1 mmol/kg of gadobenate dimeglumine; (c) enhanced axial T1wGRE sequence as in (a), 3 min after contrast injection. A three-point score system (absent, focal, wide) was used to assess CPCE on native and subtracted MRI images (c minus a) and ICI at histopathology. Carotid stenosis degree was determined on CE-MRA.

RESULTS

Six CPCE studies were discarded due to patient movement. In the remaining 22 studies, CPCE was absent, focal and wide in 13, 6 and 3 cases, respectively; ICI was absent, focal and wide in 13, 7 and 2 cases, respectively (k=0.57). On CE-MRA 21/28 stenoses were severe and 7/28 moderate. There was no correlation either with ICI (p=1.000, n=28) or CPCE (p=0.747, n=22).

CONCLUSION

The correlation between CPCE and ICI suggests a role for CPCE as an independent marker of plaque inflammation.

Noninvasive imaging of the vulnerable atherosclerotic plaque

Atherosclerosis is an inflammatory disease, complicated by progressively increasing atherosclerotic plaques that eventually may rupture. Plaque rupture is a major cause of cardiovascular events, such as unstable angina, myocardial infarction, and stroke. A number of noninvasive imaging techniques have been developed to evaluate the vascular wall in an attempt to identify so-called vulnerable atherosclerotic plaques that are prone to rupture. The purpose of the present review is to systematically investigate the accuracy of noninvasive imaging techniques in the identification of plaque components and morphologic characteristics associated with plaque vulnerability, assessing their clinical and diagnostic value.

Assessment of atherosclerotic plaques in a rabbit model by delayed-phase contrast-enhanced CT angiography: comparison with histopathology

The aim of this study was to compare delayed-phase computed tomography angiography (CTA) attenuation values with histopathology, in ability to differentiate between fibrous and lipid-rich plaques in an experimental rabbit model. Twelve atherosclerotic rabbits underwent CTA of the abdominal aorta. The scan protocol included early-phase scans (EP), delayed scans at 90 s after contrast injection (DP(90s)), delayed scans at 10 min after contrast injection (DP(10min)), and delayed scan with saline infusion (DP(Saline)). Plaque composition was analyzed by histopathology (% of lipid-rich, fibrous and macrophage areas) and CT attenuation values in Hounsfield units. Using histopathology as the reference standard (n = 119), the overall sensitivity, specificity and accuracy of 64-slice CTA for the detection of plaques was 59, 100 and 79% for the EP scans; 88, 100 and 94% for the DP(90s) scans; 81, 100 and 90% for the DP(10min) scans; and 53, 100 and 76% for the DP(Saline) scans. CT density measurements showed a substantial overlap between fibrous and lipid-rich plaques, and poor correlations with the percentage of macrophage areas in both fibrous and lipid-rich plaques (r = 0.408, and r = 0.333). In delayed-phase 64-slice CTA, DP(90s) images have the best diagnostic performance for the detection of aortic plaques.

CMR assessment of endothelial damage and angiogenesis in porcine coronary arteries using gadofosveset

BACKGROUND

Endothelial damage and angiogenesis are essential for atherosclerotic plaque development and destabilization. We sought to examine whether contrast enhanced cardiovascular magnetic resonance (CMR) using gadofosveset could show endothelial damage and neovessel formation in balloon injured porcine coronary arteries.

METHODS AND RESULTS

Data were obtained from seven pigs that all underwent balloon injury of the left anterior descending coronary artery (LAD) to induce endothelial damage and angiogenesis. Between one - 12 days (average four) after balloon injury, in vivo and ex vivo T1-weighted coronary CMR was performed after intravenous injection of gadofosveset. Post contrast, CMR showed contrast enhancement of the coronary arteries with a selective and time-dependent average expansion of the injured LAD segment area of 45% (p = 0.04; CI95 = [15%-75%]), indicating local extravasation of gadofosveset. Vascular and perivascular extravasation of albumin (marker of endothelial leakiness) and gadofosveset was demonstrated with agreement between Evans blue staining and ex vivo CMR contrast enhancement (p = 0.026). Coronary MRI contrast enhancement and local microvessel density determined by microscopic examination correlated (ρ = 0.82, p < 0.001).

CONCLUSION

Contrast enhanced coronary CMR with gadofosveset can detect experimentally induced endothelial damage and angiogenesis in the porcine coronary artery wall.

Localized measurement of atherosclerotic plaque inflammatory burden with dynamic contrast-enhanced MRI

Inflammation plays an important role in progression and rupture of atherosclerotic plaque. Dynamic contrast-enhanced MRI has been proposed as a tool to evaluate inflammation in vivo by measuring the transfer constant and partial plasma volume, which are influenced by inflammation. This study sought to demonstrate the ability of dynamic contrast-enhanced MRI to provide localized measurements of transfer constant and partial plasma volume within plaque regions of different compositions. In order to do that, a highly automatic procedure for localized measurement of dynamic contrast-enhanced MRI parameters was developed. In 47 subjects, the average transfer constant and partial plasma volume were highest in loose matrix and fibrous tissue and substantially lower in intraplaque hemorrhage, lipid rich/necrotic core, and calcification. In addition, except for hemorrhage and calcification, statistically significant differences of transfer constant and partial plasma volume were observed for any pair of these components. This suggests that transfer constant and partial plasma volume could be helpful to differentiate different plaque components and that dynamic contrast-enhanced MRI has the potential to assess inflammatory burden in specific regions.

Imaging of atherosclerosis

It is now well recognized that the atherosclerotic plaques responsible for thrombus formation are not necessarily those that impinge most on the lumen of the vessel. Nevertheless, clinical investigations for atherosclerosis still focus on quantifying the degree of stenosis caused by plaques. Many of the features associated with a high-risk plaque, including a thin fibrous cap, large necrotic core, macrophage infiltration, neovascularization, and intraplaque hemorrhage, can now be probed by novel imaging techniques. Each technique has its own strengths and drawbacks. In this article, we review the various imaging modalities used for the evaluation and quantification of atherosclerosis.

[Significance of MR angiography for imaging diagnostics of carotid artery diseases]

During the last decade, magnetic resonance angiography (MRA) evolved to an essential method for radiological diagnostics of extracranial arteries' diseases. Contrast enhanced MRA enables meanwhile the acquisition of angiographic series in high diagnostic quality comparable to that originating from conventional DSA. Due to MRA, conventional DSA anymore plays a crucial role in the diagnostic assessment of carotid artery disease or highly vascularised tumors of the head and neck region. Besides reliable quantification of carotid stenoses, highly resolved MRI sequences provide a promising approach for characterization of plaque morphologies and thereby contribute to turn the risk for a stroke calculable. Furthermore, MRA has nearly replaced DSA in radiographics of carotid artery dissections whereas MRA has especially emerged as an appropriate method to visualize the intramural hematoma which is evidentiary for the diagnosis. However, not all methods of MRA are equivalent in respect to their diagnostic value. While CE MRA is able to completely substitute DSA in many clinical questions, applicability of flow-dependent Time-of-flight (TOF) or phase-contrast (PC) MRA is limited due to their distinct susceptibility to motion or flow-related artefacts.

Molecular imaging with targeted contrast agents

Molecular imaging with targeted contrast agents by magnetic resonance imaging (MRI) allows for the noninvasive detection and characterization of biological changes on a molecular level. In this article, the principles of molecular MRI and its applications in cardiovascular diseases are reviewed. First, basic properties of positive and negative contrast agents are introduced and their effect on signal generation in a magnetic field is described. In the next part, different types of MRI scanners and the influence of field strength on signal properties of contrast agents for molecular imaging are discussed. Additionally, the assessment, analysis, and quantification of the changes in T1 and T2* relaxation time induced by the different molecular contrast agents are reviewed. Finally, the basic mechanisms of targeting of imaging probes on a molecular level and recent applications of molecular MRI in cardiovascular disease are reviewed.

Current techniques for MR imaging of atherosclerosis

Vessel wall imaging of large vessels has the potential to identify culprit atherosclerotic plaques that lead to cardiovascular events. Comprehensive assessment of atherosclerotic plaque size, composition, and biological activity is possible with magnetic resonance imaging (MRI). Magnetic resonance imaging of the atherosclerotic plaque has demonstrated high accuracy and measurement reproducibility for plaque size. The accuracy of in vivo multicontrast MRI for identification of plaque composition has been validated against histological findings. Magnetic resonance imaging markers of plaque biological activity such as neovasculature and inflammation have been demonstrated. In contrast to other plaque imaging modalities, MRI can be used to study multiple vascular beds noninvasively over time. In this review, we compare the status of in vivo plaque imaging by MRI to competing imaging modalities. Recent MR technological improvements allow fast, accurate, and reproducible plaque imaging. An overview of current MRI techniques required for carotid plaque imaging including hardware, specialized pulse sequences, and processing algorithms are presented. In addition, the application of these techniques to coronary, aortic, and peripheral vascular beds is reviewed.

Monitoring of arterial wall remodelling in atherosclerotic rabbits with a magnetic resonance imaging contrast agent binding to matrix metalloproteinases

AIMS

P947 is a gadolinium-based magnetic resonance imaging (MRI) contrast agent with high affinity for several matrix metalloproteinases (MMPs) involved in arterial wall remodelling. We tested whether the intensity of enhancement detected in vivo in the arterial wall with P947 and MRI correlates with actual tissue MMP-related enzymatic activity measured in a rabbit atherosclerotic model subjected to dietary manipulations.

METHODS AND RESULTS

Aortas of 15 rabbits in which atherosclerotic lesions were induced by balloon angioplasty and 4 months of hypercholesterolaemic diet were imaged at 'baseline' with P947-enhanced MRI. Atherosclerotic rabbits were divided into three groups: five rabbits were sacrificed ('baseline' group); five rabbits continued to be fed a lipid-supplemented diet ('high-fat' group); and five rabbits were switched from atherogenic to a purified chow diet ('low-fat' group). Four months later, a second P947-enhanced MRI was acquired in the 10 remaining rabbits. A significantly lower signal was detected in the aortic wall of rabbits from the 'low-fat' group as compared with rabbits from the 'high-fat' group (21 ± 6 vs. 46 ± 3%, respectively; P = 0.04). Such differences were not detected with the contrast agent P1135, which lacks the MMP-specific peptide sequence. In addition, the intensity of aortic wall enhancement detected with MRI after injection of P947 strongly correlated with actual MMP-2 gelatinolytic activity measured in corresponding aortic segments using zymography (r = 0.87).

CONCLUSION

P947-enhanced MRI can distinguish dietary-induced variations in MMP-related enzymatic activity within plaques in an experimental atherosclerotic model, supporting its utility as a clinical imaging tool for in vivo detection of arterial wall remodelling.

Magnetic resonance imaging of carotid atherosclerotic plaque in clinically suspected acute transient ischemic attack and acute ischemic stroke

BACKGROUND

Carotid atherosclerotic plaque rupture is thought to cause transient ischemic attack (TIA) and ischemic stroke (IS). Pathological hallmarks of these plaques have been identified through observational studies. Although generally accepted, the relationship between cerebral thromboembolism and in situ atherosclerotic plaque morphology has never been directly observed noninvasively in the acute setting.

METHODS AND RESULTS

Consecutive acutely symptomatic patients referred for stroke protocol magnetic resonance imaging/angiography underwent additional T1- and T2-weighted carotid bifurcation imaging with the use of a 3-dimensional technique with blood signal suppression. Two blinded reviewers performed plaque gradings according to the American Heart Association classification system. Discharge outcomes and brain magnetic resonance imaging results were obtained. Image quality for plaque characterization was adequate in 86 of 106 patients (81%). Eight TIA/IS patients with noncarotid pathogenesis were excluded, yielding 78 study patients (38 men and 40 women with a mean age of 64.3 years, SD 14.7) with 156 paired watershed vessel/cerebral hemisphere observations. Thirty-seven patients had 40 TIA/IS events. There was a significant association between type VI plaque (demonstrating cap rupture, hemorrhage, and/or thrombosis) and ipsilateral TIA/IS (P<0.001). A multiple logistic regression model including standard Framingham risk factors and type VI plaque was constructed. Type VI plaque was the dominant outcome-associated observation achieving significance (P<0.0001; odds ratio, 11.66; 95% confidence interval, 5.31 to 25.60).

CONCLUSIONS

In situ type VI carotid bifurcation region plaque identified by magnetic resonance imaging is associated with ipsilateral acute TIA/IS as an independent identifier of events, thereby supporting the dominant disease pathophysiology.

Gadolinium Enhanced MR Coronary Vessel Wall Imaging at 3.0 Tesla

Purpose. We evaluated the influence of the time between low-dose gadolinium (Gd) contrast administration and coronary vessel wall enhancement (LGE) detected by 3T magnetic resonance imaging (MRI) in healthy subjects and patients with coronary artery disease (CAD). Materials and Methods. Four healthy subjects (4 men, mean age 29 ± 3 years and eleven CAD patients (6 women, mean age 61 ± 10 years) were studied on a commercial 3.0 Tesla (T) whole-body MR imaging system (Achieva 3.0 T; Philips, Best, The Netherlands). T1-weighted inversion-recovery coronary magnetic resonance imaging (MRI) was repeated up to 75 minutes after administration of low-dose Gadolinium (Gd) (0.1 mmol/kg Gd-DTPA). Results. LGE was seen in none of the healthy subjects, however in all of the CAD patients. In CAD patients, fifty-six of 62 (90.3%) segments showed LGE of the coronary artery vessel wall at time-interval 1 after contrast. At time-interval 2, 34 of 42 (81.0%) and at time-interval 3, 29 of 39 evaluable segments (74.4%) were enhanced. Conclusion. In this work, we demonstrate LGE of the coronary artery vessel wall using 3.0 T MRI after a single, low-dose Gd contrast injection in CAD patients but not in healthy subjects. In the majority of the evaluated coronary segments in CAD patients, LGE of the coronary vessel wall was already detectable 30–45 minutes after administration of the contrast agent.

Contrast enhancement by differently sized paramagnetic MRI contrast agents in mice with two phenotypes of atherosclerotic plaque

Interest in the use of contrast-enhanced MRI to enable in vivo specific characterization of atherosclerotic plaques is increasing. In this study the intrinsic ability of three differently sized gadolinium-based contrast agents to permeate different mouse plaque phenotypes was evaluated with MRI. A tapered cast was implanted around the right carotid artery of apoE(-/-) mice to induce two different plaque phenotypes: a thin cap fibroatheroma (TCFA) and a non-TCFA lesion. Both plaques were allowed to develop over 6 and 9 weeks, leading to an intermediate and advanced lesion, respectively. Signal enhancement in the carotid artery wall, following intravenous injection of Gd-HP-DO3A as well as paramagnetic micelles and liposomes was evaluated. In vivo T(1) -weighted MRI plaque enhancement characteristics were complemented by fluorescence microscopy and correlated to lesion phenotype. The two smallest contrast agents, i.e. Gd-HP-DO3A and micelles, were found to enhance contrast in T(1) -weighted MR images of all investigated plaque phenotypes. Maximum contrast enhancement ranged between 53 and 70% at 6 min after injection of Gd-HP-DO3A with highest enhancement and longest retention in the non-TCFA lesion. Twenty-four hours after injection of micelles maximum contrast enhancement ranged between 24 and 35% in all plaque phenotypes. Administration of the larger liposomes did not cause significant contrast enhancement in the atherosclerotic plaques. Confocal fluorescence microscopy confirmed the MRI-based differences in plaque permeation between micelles and liposomes. Plaque permeation of contrast agents was strongly dependent on size. Our results implicate that, when equipped with targeting ligands, liposomes are most suitable for the imaging of plaque-associated endothelial markers due to low background enhancement, whereas micelles, which accumulate extravascularly on a long timescale, are suited for imaging of less abundant markers inside plaques. Low molecular weight compounds may be employed for target-specific imaging of highly abundant extravascular plaque-associated targets.

Imaging biomarkers of cardiovascular disease

Cardiovascular disease (CVD) is a leading cause of morbidity and mortality worldwide. Current clinical techniques that rely on stenosis measurement alone appear to be insufficient for risk prediction in atherosclerosis patients. Many novel imaging methods have been developed to study atherosclerosis progression and to identify new features that can predict future clinical risk. MRI of atherosclerotic vessel walls is one such method. It has the ability to noninvasively evaluate multiple biomarkers of the disease such as luminal stenosis, plaque burden, tissue composition and plaque activity. In addition, the accuracy of in vivo MRI has been validated against histology with high reproducibility, thus paving the way for application to epidemiological studies of disease pathogenesis and, by serial MRI, in monitoring the efficacy of therapeutic intervention. In this review, we describe the various MR techniques used to evaluate aspects of plaque progression, discuss imaging-based measurements (imaging biomarkers), and also detail their validation. The application of plaque MRI in clinical trials as well as emerging imaging techniques used to evaluate plaque compositional features and biological activities are also discussed.

Stress analysis of carotid atheroma in a transient ischaemic attack patient using the MRI-based fluid-structure interaction method

Rupture of atherosclerotic plaque is a major cause of mortality. Plaque stress analysis, based on patient-specific multisequence in vivo MRI, can provide critical information for the understanding of plaque rupture and could eventually lead to plaque rupture prediction. However, the direct link between stress and plaque rupture is not fully understood. In the present study, the plaque from a patient who recently experienced a transient ischaemic attack (TIA) was studied using a fluid-structure interaction method to quantify stress distribution in the plaque region based on in vivo MR images. The results showed that wall shear stress is generally low in the artery with a slight increase at the plaque throat owing to minor luminal narrowing. The oscillatory shear index is much higher in the proximal part of the plaque. Both local wall stress concentrations and the relative stress variation distribution during a cardiac cycle indicate that the actual plaque rupture site is collocated with the highest rupture risk region in the studied patient.