Inflammation in carotid atherosclerotic plaque: a dynamic contrast-enhanced MR imaging study

Imaging of the carotid artery

In the study of carotid arteries, modern techniques of imaging allow to analyze various alterations beyond simple luminal narrowing, including the morphology of atherosclerotic plaques, the arterial wall and the surrounding structures. By using CTA and MRI it is possible to obtain three-dimensional rendering of anatomic structures with excellent detail for treatment planning. This paper will detail the role of various imaging methods for the assessment of carotid artery pathology with emphasis on the detection, analysis and characterization of carotid atherosclerosis.

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.

Carotid artery atherosclerosis: effect of intensive lipid therapy on the vasa vasorum--evaluation by using dynamic contrast-enhanced MR imaging

PURPOSE

To investigate whether short-term, intensive lipid therapy leads to changes in microvascular characteristics, as measured by using dynamic contrast material-enhanced (DCE) magnetic resonance (MR) imaging.

MATERIALS AND METHODS

Institutional review board approval and informed consent were obtained for this HIPAA-compatible study. Subjects with established coronary artery disease or carotid artery stenosis of 15% or greater determined by using ultrasonography and with levels of apolipoprotein B of 120 mg/dL (1.2 g/L) or greater were enrolled in an ongoing study (clinical trial NCT00715273). All received intensive lipid therapy to achieve targeted high- and low-density lipoprotein cholesterol levels and underwent serial serum monitoring including high-sensitivity C-reactive protein (HsCRP) level measurements. Carotid artery MR imaging examinations including morphologic and DCE MR images were obtained at baseline and 1 year after treatment. In subjects with advanced lesions (>2 mm thick), MR image analysis was performed, including measurement of lipid-rich necrotic core size and kinetic modeling of DCE MR images to assess changes in the transfer constant (K(trans)). The differences in K(trans) between baseline and 1-year follow-up were compared by using the Wilcoxon signed rank test, and associations were assessed by using the Spearman rank correlation coefficient.

RESULTS

Twenty-eight subjects with interpretable DCE MR imaging results at both baseline and 1-year follow-up were included. After 1 year of treatment, a significant reduction was found in mean K(trans) (0.085 min(-1) ± 0.037 [standard deviation] to 0.067 min(-1) ± 0.028, P = .02). Reduction in K(trans) was not significantly correlated with observed reductions in lipid-rich necrotic core size or reductions in HsCRP level.

CONCLUSION

These findings suggest that DCE MR imaging may be a useful imaging method for the assessment of the therapeutic response of the vasa vasorum in patients with atherosclerotic plaque.

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.

Advances in imaging angiogenesis and inflammation in atherosclerosis

Advances in imaging technology have provided powerful tools for dissecting the angiogenic and inflammatory aspects of atherosclerosis. Improved technology along with multi-modal approaches has expanded the utilisation of imaging. Recent advances provide the ability to better define structure and development of angiogenic vessels, identify relationships between inflammatory mediators and the vessel wall, validate biological effects of anti-inflammatory and anti-angiogenic drugs, delivery and/or targeting specific molecules to inflammatory regions of atherosclerotic plaques.

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.

A comparative analysis of double inversion recovery TFE and TSE sequences on carotid artery wall imaging

OBJECTIVE

To analyze the characteristics of double inversion recovery (DIR) turbo field echo (TFE) and turbo spin echo (TSE) sequences and explore the value of double inversion recovery TFE sequence on carotid artery wall imaging.

PATIENTS AND METHODS

56 patients (32 males and 24 females, aged 31-76 years with a mean age of 53 years) were performed with DIR TFE and DIR TSE T1 weighted imaging (T1WI) sequences on carotid artery bifurcations. Image quality acquired by different techniques were evaluated and scored by two physicians. Whether there is significant difference is determined by SPSS 11.0 software. Paired-samples t test was used for statistics.

RESULTS

There was no significant difference in the image quality scores between two sequences (t = 0.880, P = 0.383 > 0.05).

CONCLUSIONS

DIR TFE sequence has short scanning time and high spatial resolution. DIR TFE sequence can be used as the preferred sequence for screening carotid atherosclerotic plaque compared with DIR TSE sequence.

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.

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.

Characterization of carotid artery plaques with USPIO-enhanced MRI: assessment of inflammation and vascularity as in vivo imaging biomarkers for plaque vulnerability

To evaluate ultra small superparamagnetic iron oxide particles (USPIO) enhanced magnetic resonance (MR) imaging for characterization of atherosclerotic carotid plaques by assessing vascularity and plaque inflammation, besides contrast-enhanced MR angiography (CE-MRA) of the carotid artery stenosis. Twelve patients with severe carotid artery stenosis, scheduled for endarterectomy, underwent MRI of the carotid artery bifurcation using SHU 555 C at a dose of 40 μmol Fe/kg BW. The MR imaging protocol comprised pre- and post-contrast T2*-w, a first-pass CE-MRA and dynamic T1-w sequences. For quantitative data analysis, the signal intensities (SI) were measured and SNR-data (SNR = SI(blood/plaque/bone marrow)/standard deviation(noise)) as well as ΔSI-data (SNR(post)-SNR(pre)) were calculated. In addition, two radiologists rated the diagnostic performance of first-pass MRA according to a four level decision scale. Staining of anti-dextran (SHU 555 C) and anti-CD68 (macrophages) was performed for immunohistological confirmation. Plaque sections with a T2*-w signal decline (intracellular USPIO accumulation in macrophages) showed significantly changes (mean -14%, 95% CI, -5 to -20%; P < 0.01) and corresponding plaque regions had significantly higher (15.15 ± 1.76 vs. 5.22 ± 1.50; P < 0.01) T1-w enhancement data (global estimation of vascularity). The first-pass MRA of the supra-aortal vessels provided images of diagnostic quality. Representative immunohistology sections revealed colocalization of dextran- and CD68-immunoreactive cells. USPIO-enhanced MRI is feasible for in vivo assessment of vascularity and macrophage content in atherosclerotic carotid plaques, determining an association of these potential imaging biomarkers of plaque vulnerability. Diagnostic MRA of the supra-aortal vessels can be imaged additionally with a single administration of SHU 555 C.

Reproducibility of black blood dynamic contrast-enhanced magnetic resonance imaging in aortic plaques of atherosclerotic rabbits

PURPOSE

To investigate the short-term reproducibility of black-blood dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) in atherosclerotic rabbits to evaluate the potential of this technique to be a reliable readout of plaque progression and/or regression upon therapeutic intervention.

MATERIALS AND METHODS

Atherosclerotic rabbits were imaged at baseline and 24 hours later with DCE-MRI on a 1.5T MRI system. DCE-MRI images were analyzed by calculating the area under the signal intensity versus time curve (AUC). Intraclass correlation coefficients (ICCs) were used to evaluate interscan, intraobserver, and interobserver reproducibility. In addition, the test-retest coefficient of variation (CoV) was evaluated.

RESULTS

Statistical analyses showed excellent interscan, intraobserver, and interobserver agreement. All ICCs were greater than 0.75, P < 0.01 indicating excellent agreement between measurements.

CONCLUSION

Experimental results show good interscan and excellent intra- and interobserver reproducibility, suggesting that DCE-MRI could be used in preclinical settings as a read-out for novel therapeutic interventions for atherosclerosis. This preliminary work encourages investigating the reproducibility of DCE-MRI also in clinical settings, where it could be used for monitoring high-risk patients and in longitudinal clinical drug trials.

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.

Gadofosveset-enhanced magnetic resonance imaging of human carotid atherosclerotic plaques: a proof-of-concept study

OBJECTIVE

To investigate the potential of gadofosveset-enhanced MR imaging for the characterization of human carotid atherosclerotic plaques.

MATERIALS AND METHODS

Sixteen (9 symptomatic, 7 asymptomatic) patients with 70% to 99% carotid stenosis (according to NASCET criteria) were included (13 men, 3 women, mean age 67.6 years). All patients underwent baseline precontrast MR imaging of the carotid plaque. Immediately after completion of the baseline examination, 0.03 mmol/kg gadofosveset was administered. At 24 hours postinjection, the acquisition was repeated. Twelve patients were scheduled for carotid endarterectomy. Carotid endarterectomy specimens were HE-, CD31-, CD68-, and albumin-stained to correlate signal enhancement with plaque composition, intraplaque microvessel density, and macrophage and albumin content. A random intercept model was used to compare signal enhancement between symptomatic and asymptomatic patients, adjusting for size of various plaque components. This study was approved by the institutional medical ethics committee. All participants gave written informed consent.

RESULTS

Signal enhancement (SE) of the plaque was significantly higher in symptomatic patients compared with asymptomatic patients (median log SE 0.182 vs. -0.109, respectively, P < 0.001). A positive association (as expressed by a regression coefficient beta = 0.0035) was found between signal enhancement on the log scale and intraplaque albumin content (P = 0.038). There was no association between signal enhancement and various other plaque components.

CONCLUSION

In this study, the potential of gadofosveset-enhanced human carotid plaque MR imaging for identification of high-risk plaques was demonstrated. Signal enhancement of the plaque after administration of gadofosveset was associated with differences in intraplaque albumin content. Although promising, we emphasize that these results are based on a small patient population. Larger prospective studies are warranted.

Inflammation within carotid atherosclerotic plaque: assessment with late-phase contrast-enhanced US

PURPOSE

To determine if the number of nontargeted microbubbles retained in human carotid plaque is sufficient to be detected with ultrasonography (US).

MATERIALS AND METHODS

The study protocol was approved by the local research ethics committee. Informed consent was obtained. A total of 37 subjects with carotid atherosclerosis (mean age, 69.9 years; age range, 49-86 years), of whom 27 (73%) were men (mean age, 69.7 years; age range, 58-86 years) and 10 (27%) were women (mean age, 70.3 years; age range, 49-86 years), were studied between December 2008 and May 2009 with late-phase (LP) contrast material-enhanced US by using flash imaging with a nonlinear mode at an intermediate mechanical index of 0.34 6 minutes after bolus contrast agent injection. Plaques were defined as symptomatic if symptoms consistent with stroke, transient ischemic attack, or amaurosis fugax had occurred in the neurovascular territory of the plaque studied within 12 months prior to entry into the study. Plaques were defined as asymptomatic if no such events had ever occurred within the neurovascular territory. Raw linear data were used to quantify echogenicity of the plaque, which was normalized to lumen echogenicity. Gray-scale median score was also calculated.

RESULTS

Of the 37 subjects, 16 (43%) had symptomatic plaques and 21 (57%) had asymptomatic plaques. All examinations yielded evaluable LP contrast-enhanced US data. Normalized LP plaque echogenicity was greater in the symptomatic group (0.39; 95% confidence interval: -0.11, 0.89) than in the asymptomatic group (0.69; 95% confidence interval: -1.04, -0.34) (P = .0005). There was a moderate (rho = -0.44, P = .016) inverse correlation between normalized LP plaque echogenicity and gray-scale median score.

CONCLUSION

By quantifying microbubble retention within the carotid plaque, LP contrast-enhanced US depicts clear differences between groups of subjects with plaque ipsilateral to symptoms and asymptomatic plaques. This technique has promise as a tissue-specific marker of inflammation and a potential role in the risk stratification of atherosclerotic carotid stenosis.

[State of the art: high resolution MR imaging of carotid atherosclerotic plaque]

A third of cerebrovascular accidents are a complication from carotid artery plaque. In addition to the degree of stenosis, plaque composition and morphology are key elements in determining the probability of complication from the atherosclerotic plaque. High resolution MRI can characterize plaque composition and morphology and therefore help identify unstable plaque. The purpose of this review is to summarize recent concepts on unstable plaque and underlying inflammation. The signal characteristics of the different components of plaque on high resolution MRI then be reviewed. Finally, current morphological and functional criteria for unstable plaque will be discussed.

Cardiovascular MRI in small animals

Imaging studies of cardiovascular disease in small rodents have become a prerequisite in preclinical cardiovascular research. Transgenic and gene-knockout models of cardiovascular diseases enables the investigation of the influence of single genes or groups of genes on disease pathogenesis. In addition, experimental and genetically altered models provide valuable in vivo platforms to investigate the efficacy of novel drugs and contrast agents. Owing to the excellent soft tissue contrast, high spatial and temporal resolution, as well as the tomographic nature of MRI, anatomy and function can be assessed with unique accuracy and reproducibility. Furthermore, using novel targeted MRI contrast agents, molecular changes associated with cardiovascular disease can be investigated in the same imaging session. This review focuses on recent advances in hardware, imaging sequences and probe design.

MR plaque imaging of the carotid artery

Atherosclerotic carotid plaque represents a major cause of cerebral ischemia. The detection of vulnerable plaque is important for preventing future cardiovascular events. The key factors in advanced plaque that are most likely to lead to patient complications are the condition of the fibrous cap, the size of the necrotic core and hemorrhage, and the extent of inflammatory activity within the plaque. Magnetic resonance (MR) imaging has excellent soft tissue contrast and can allow for a more accurate and objective estimation of carotid wall morphology and plaque composition. Recent advances in MR imaging techniques have permitted serial monitoring of atherosclerotic disease evolution and the identification of intraplaque risk factors for accelerated progression. The purpose of this review article is to review the current state of techniques of carotid wall MR imaging and the characterization of plaque components and surface morphology with MR imaging, and to describe the clinical practice of carotid wall MR imaging for the determination of treatment plan.

Atherosclerotic lesions rich in macrophages or smooth muscle cells discriminated in rabbit iliac arteries based on T1 relaxation time and lipid content

RATIONALE AND OBJECTIVES

Atherothrombosis usually occurs on macrophage- and lipid-rich unstable plaque, but rarely on smooth muscle cell (SMC)-rich stable plaque. Magnetic resonance imaging (MRI) has been extensively applied for noninvasive vascular imaging. We therefore investigated whether MRI provides valuable information about the characteristics of atherosclerotic vessels using rabbit models of macrophage-rich or SMC-rich atherosclerotic arteries.

MATERIALS AND METHODS

Rabbits were fed with a conventional (CD group, n = 3) or 0.5% cholesterol (ChD group, n = 3) diet for 1 week before and 3 weeks after balloon injury of the left iliac arteries. Three weeks later, these arteries were investigates by 1.5 T MRI and by conventional angiographic imaging, followed by histological and immunohistochemical analyses.

RESULTS

Three weeks after balloon injury, injured iliac arteries of both groups formed neointima with luminal stenosis. Conventional and MRI angiographic findings of the luminal diameter significantly and positively correlated. T1 relaxation time was significantly shorter and the lipid content was much higher in injured arteries from the ChD than from the CD group. The injured arteries from the ChD also contained more macrophages and less SMCs that those from the CD group. The T1 relaxation time and lipid content in injured arteries negatively and positively correlated with the degree of macrophage accumulation, respectively.

CONCLUSION

These results showed that MRI could provide valuable information about luminal stenosis and the characteristics of atherosclerotic vessels in rabbits.

MRI in the early identification and classification of high-risk atherosclerotic carotid plaques

Stroke is a leading cause of mortality and long-term morbidity. As a means for stroke prevention, an estimated 99,000 carotid endarterectomy procedures were performed in the USA in 2006. Traditionally, the degree of luminal stenosis has been used as a marker of the stage of atherosclerosis and as an indication for surgical intervention. However, prospective clinical trials have shown that the majority of patients with a history of recent transient ischemic attack or stroke have mild-to-moderate carotid stenosis. Using stenosis criteria, many of these symptomatic individuals would be considered to have early-stage carotid atherosclerosis. It is evident that improved criteria are needed for identifying the high-risk carotid plaque across a range of stenoses. Histological studies have led to the hypothesis that plaques with larger lipid-rich necrotic cores, thin fibrous cap rupture, intraplaque hemorrhage, plaque neovasculature and vessel wall inflammation are characteristics of the high-risk, 'vulnerable plaque'. Despite the widespread consensus on the importance of these plaque features, testing the vulnerable plaque hypothesis in prospective clinical studies has been hindered by the lack of reliable imaging tools for in vivo plaque characterization. MRI has been shown to accurately identify key carotid plaque features, including the fibrous cap, lipid-rich necrotic core, intraplaque hemorrhage, neovasculature and vascular wall inflammation. Thus, MRI is a histologically validated technique that will permit prospective testing of the vulnerable plaque hypothesis. This article will provide a summary of the histological validation of carotid MRI, and highlight its application in prospective clinical studies aimed at early identification of the high-risk atherosclerotic carotid plaque.

MRI of carotid atherosclerosis: clinical implications and future directions

Atherosclerosis is now widely recognized as a multifactorial disease with outcomes that arise from complex factors such as plaque components, blood flow, and inflammation. Despite recent advances in understanding of plaque biology, diagnosis, and treatment, atherosclerosis remains a leading cause of morbidity and mortality. Further research into the development and validation of reliable indicators of the high-risk individual is greatly needed. Carotid MRI is a histologically validated, noninvasive imaging method that can track disease progression and regression, and quantitatively evaluate a spectrum of parameters associated with in vivo plaque morphology and composition. Intraplaque hemorrhage and the lipid-rich necrotic core are the best indicators of lesion severity currently visualized by carotid MRI. However, MRI methods capable of imaging other important aspects of carotid atherosclerotic disease in vivo-including inflammation, neovascularization, and mechanical forces-are emerging and may aid in advancing our understanding of the pathophysiology of this multifactorial disease.

Diffusion-weighted imaging of human carotid artery using 2D single-shot interleaved multislice inner volume diffusion-weighted echo planar imaging (2D ss-IMIV-DWEPI) at 3T: diffusion measurement in atherosclerotic plaque

PURPOSE

To determine if 2D single-shot interleaved multislice inner volume diffusion-weighted echo planar imaging (ss-IMIV-DWEPI) can be used to obtain quantitative diffusion measurements that can assist in the identification of plaque components in the cervical carotid artery.

MATERIALS AND METHODS

The 2D ss-DWEPI sequence was combined with interleaved multislice inner volume region localization to obtain diffusion weighted images with 1 mm in-plane resolution and 2 mm slice thickness. Eleven subjects, six of whom have carotid plaque, were studied with this technique. The apparent diffusion coefficient (ADC) images were calculated using DW images with b = 10 s/mm(2) and b = 300 s/mm(2).

RESULTS

The mean ADC measurement in normal vessel wall of the 11 subjects was 1.28 +/- 0.09 x 10(-3) mm(2)/s. Six of the 11 subjects had carotid plaque and ADC measurements in plaque ranged from 0.29 to 0.87 x 10(-3) mm(2)/s. Of the 11 common carotid artery walls studied (33 images), at least partial visualization of the wall was obtained in all ADC images, more than 50% visualization in 82% (27/33 images), and full visualization in 18% (6/33 images).

CONCLUSION

2D ss-IMIV-DWEPI can perform diffusion-weighted carotid magnetic resonance imaging (MRI) in vivo with reasonably high spatial resolution (1 x 1 x 2 mm(3)). ADC values of the carotid wall and plaque are consistent with similar values obtained from ex vivo endarterectomy specimens. The spread in ADC values obtained from plaque indicate that this technique could form a basis for plaque component identification in conjunction with other MRI/MRA techniques.

Cardiovascular magnetic resonance in carotid atherosclerotic disease

Atherosclerosis is a chronic, progressive, inflammatory disease affecting many vascular beds. Disease progression leads to acute cardiovascular events such as myocardial infarction, stroke and death. The diseased carotid alone is responsible for one third of the 700,000 new or recurrent strokes occurring yearly in the United States. Imaging plays an important role in the management of atherosclerosis, and cardiovascular magnetic resonance (CMR) of the carotid vessel wall is one promising modality in the evaluation of patients with carotid atherosclerotic disease. Advances in carotid vessel wall CMR allow comprehensive assessment of morphology inside the wall, contributing substantial disease-specific information beyond luminal stenosis. Although carotid vessel wall CMR has not been widely used to screen for carotid atherosclerotic disease, many trials support its potential for this indication. This review summarizes the current state of knowledge regarding carotid vessel wall CMR and its potential clinical application for management of carotid atherosclerotic disease.

Magnetic [corrected] resonance imaging [corrected] features of the disruption-prone and the disrupted carotid plaque

Stroke is a leading cause of long-term disability and is the third most common cause of death in the U.S. and western countries. Twenty percent of strokes are thought to arise from the carotid artery. Histopathological studies have suggested that plaque disruption is a key factor in the etiology of carotid-related ischemic events. Features associated with plaque disruption include intraplaque hemorrhage, large necrotic cores with thin overlying fibrous caps, plaque neovasculature, and inflammatory cell infiltrate. In vivo high-spatial-resolution, multicontrast-weighted cardiac magnetic resonance (CMR) has been extensively evaluated using histology as the gold standard, and has documented reliability in the identification of these key carotid plaque features. This pictorial essay illustrates the capability of CMR for identifying features of disruption-prone and disrupted atherosclerotic carotid plaques.

Carotid magnetic resonance imaging. A window to study atherosclerosis and identify high-risk plaques

Despite recent advances in the understanding and etiology of cardiovascular disease, it remains the leading cause of morbidity and mortality worldwide. A great deal of research has been dedicated to investigating and identifying plaque instability: the so-called "vulnerable plaque". A reliable, in vivo, imaging method capable of identifying plaque characteristics associated with high-risk plaque will be immensely useful for evaluating plaque status and predicting future events. With excellent soft-tissue contrast and resolution, magnetic resonance imaging (MRI) has the ability to visualize features of vulnerable plaques, as well as perform longitudinal studies on the etiology, progression, and regression of atherosclerotic plaque. This review will cover the current state-of-the-art and new developments in carotid MRI to characterize atherosclerosis and its use in clinical diagnoses and longitudinal studies to understand mechanisms of lesion progression and regression.

Contrast-enhanced MRI of carotid atherosclerosis: dependence on contrast agent

PURPOSE

To investigate the dependence of contrast-enhanced magnetic resonance imaging (MRI) of carotid artery atherosclerotic plaque on the use of gadobenate dimeglumine versus gadodiamide.

MATERIALS AND METHODS

Fifteen subjects with carotid atherosclerotic plaque were imaged with 0.1 mmol/kg of each agent. For arteries with interpretable images, the areas of the lumen, wall, and necrotic core and overlying fibrous cap (when present) were measured, as were the percent enhancement and contrast-to-noise ratio (CNR). A kinetic model was applied to dynamic imaging results to determine the fractional plasma volume, v(p), and contrast agent transfer constant, K(trans).

RESULTS

For 12 subjects with interpretable images, the agent used did not significantly impact any area measurements or the presence or absence of necrotic core (P > 0.1 for all). However, the percent enhancement was greater for the fibrous cap (72% vs. 54%; P < 0.05) necrotic core (51% vs. 42%; P = 0.12), and lumen (42% vs. 63%; P < 0.05) when using gadobenate dimeglumine, although no apparent difference in CNR was found. Additionally, K(trans) was lower when using gadobenate dimeglumine (0.0846 min(-1) vs. 0.101 min(-1); P < 0.01), although v(p) showed no difference (9.5% vs. 10.1%; P = 0.39).

CONCLUSION

Plaque morphology measurements are similar with either contrast agent, but quantitative enhancement characteristics, such as percent enhancement and K(trans), differ.

Advanced techniques for MRI of atherosclerotic plaque

This review examines the state of the art in vessel wall imaging by magnetic resonance imaging (MRI) with an emphasis on the biomechanical assessment of atherosclerotic plaque. Three areas of advanced techniques are discussed. First, alternative contrast mechanisms, including susceptibility, magnetization transfer, diffusion, and perfusion, are presented as to how they facilitate accurate determination of plaque constituents underlying biomechanics. Second, imaging technologies including hardware and sequences, are reviewed as to how they provide the resolution and signal-to-noise ratio necessary for determining plaque structure. Finally, techniques for combining MRI data into an overall assessment of plaque biomechanical properties, including wall shear stress and internal plaque strain, are presented. The paper closes with a discussion of the extent to which these techniques have been applied to different arteries commonly targeted by vessel wall MRI.

Serial contrast-enhanced cardiac magnetic resonance imaging demonstrates regression of hyperenhancement within the coronary artery wall in patients after acute myocardial infarction

OBJECTIVES

Our aim was to determine whether serial contrast-enhanced cardiac magnetic resonance (CE-CMR) is useful for the characterization of tissue signal changes within the coronary vessel wall in patients after acute myocardial infarction (AMI).

BACKGROUND

Inflammation plays a key role in the development of AMI. CE-CMR of the vessel wall has been found useful for the characterization of inflammatory tissue signal changes in patients with carotid artery stenosis, giant cell arteritis, or Takayasu's arteritis; however, it has never been serially performed in the coronary artery wall in patients with acute and chronic myocardial infarction using a gadolinium-based contrast medium and compared with systemic markers of inflammation.

METHODS

CE-CMR using a T1-weighted 3-dimensional gradient echo inversion recovery sequence of the coronary artery wall and 0.2 mmol/kg of gadolinium-diethylenetriaminepentaacetic acid was performed in 10 patients with AMI 6 days and 3 months after coronary intervention and in 9 subjects without coronary artery disease on invasive coronary angiography. Contrast-to-noise ratio (CNR) within the coronary artery wall was quantified in comparison with blood signal.

RESULTS

Patients with AMI demonstrated a significantly increased coronary vessel wall enhancement 6 days after infarction compared with normal subjects (CNR 7.8 +/- 4.4 vs. 5.3 +/- 3.2, p < 0.001). Three months after infarction, CNR decreased to 6.5 +/- 4.7 (p < 0.03). This decrease paralleled declines in C-reactive protein. Angiographically normal segments showed no contrast changes, but CNR significantly decreased in stenotic segments, from 10.9 +/- 3.8 to 6.8 +/- 5.0 (p < 0.002), resulting in a reduction of enhanced segments from 70% to 25% (p < 0.01).

CONCLUSIONS

Serial CE-CMR identified changes in spatial extent and intensity of coronary contrast enhancement in patients after AMI. This technique may be useful for the characterization of transient coronary tissue signal changes, which may represent edema or inflammation during the post-infarction phase. In addition, CE-CMR may offer the potential for visualization of inflammatory activity in atherosclerosis associated with acute coronary syndromes.

Noninvasive imaging of atheromatous carotid plaques

Atherothrombosis is a systemic disease of the arterial wall that affects the carotid, coronary, and peripheral vascular beds, and the aorta. This condition is associated with complications such as stroke, myocardial infarction, and peripheral vascular disease, which usually result from unstable atheromatous plaques. The study of atheromatous plaques can provide useful information about the natural history and progression of the disease, and aid in the selection of appropriate treatment. Plaque imaging can be crucial in achieving this goal. In this Review, we focus on the various noninvasive imaging techniques that are being used for morphological and functional assessment of carotid atheromatous plaques in the clinical setting.

Hemorrhage and large lipid-rich necrotic cores are independently associated with thin or ruptured fibrous caps: an in vivo 3T MRI study

OBJECTIVE

Histological studies suggest associations between hemorrhage and large lipid-rich/necrotic cores with thin or ruptured fibrous caps in advanced atherosclerosis. We investigated these associations in carotid arteries with mild to severe stenosis by in vivo 3T MRI.

METHODS AND RESULTS

Seventy-seven patients with >or=50% carotid stenosis in at least one side by duplex ultrasound underwent bilateral multi-contrast carotid MRI scans. Measurements for wall and lipid-rich/necrotic core sizes, presence of hemorrhage, and fibrous cap status (classified as intact thick, intact thin or ruptured) were recorded. Arteries with poor image quality, occlusion, or no detectable lipid-rich/necrotic core were excluded. For the 798 MRI slices included, multivariate ordinal regression analysis demonstrated larger %lipid-rich/necrotic core (odds ratio for 10% increase, 1.49; P=0.02) and presence of hemorrhage (odds ratio, 5.91; P<0.001) were independently associated with a worse (intact thin or ruptured) stage of fibrous cap status. For artery-based multivariate analysis, a larger maximum %lipid-rich/necrotic core and presence of hemorrhage independently associated with worse fibrous cap status (P<0.001, for both). No hemorrhage was detected in arteries with thick fibrous caps.

CONCLUSIONS

Hemorrhage and larger %lipid-rich/necrotic core were independently associated with a thin or ruptured fibrous cap status at an early to advanced stage of carotid atherosclerosis.

Coronary computed tomography and magnetic resonance imaging

Cardiac computed tomography and magnetic resonance are relatively new imaging modalities that can exceed the ability of established imaging modalities to detect present pathology or predict patient outcomes. Coronary calcium scoring may be useful in asymptomatic patients at intermediate risk. Computed tomographic coronary angiography is a first-line indication to evaluate congenitally abnormal coronary arteries and, along with stress magnetic resonance myocardial perfusion imaging, is useful in symptomatic patients with nondiagnostic conventional stress tests. Cardiac magnetic resonance is indicated for visualizing cardiac structure and function, and delayed enhancement magnetic resonance is a first-line indication for assessing myocardial viability. Imaging plaque and molecular mechanisms related to plaque rupture holds great promise for the presymptomatic detection of patients at risk for coronary events but is not yet suitable for routine clinical use.