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

Quantifying Intracranial Aneurysm Wall Permeability for Risk Assessment Using Dynamic Contrast-Enhanced MRI: A Pilot Study

BACKGROUND AND PURPOSE

Pathological changes in the intracranial aneurysm wall may lead to increases in its permeability; however the clinical significance of such changes has not been explored. The purpose of this pilot study was to quantify intracranial aneurysm wall permeability (K(trans), VL) to contrast agent as a measure of aneurysm rupture risk and compare these parameters against other established measures of rupture risk. We hypothesized K(trans) would be associated with intracranial aneurysm rupture risk as defined by various anatomic, imaging, and clinical risk factors.

MATERIALS AND METHODS

Twenty-seven unruptured intracranial aneurysms in 23 patients were imaged with dynamic contrast-enhanced MR imaging, and wall permeability parameters (K(trans), VL) were measured in regions adjacent to the aneurysm wall and along the paired control MCA by 2 blinded observers. K(trans) and VL were evaluated as markers of rupture risk by comparing them against established clinical (symptomatic lesions) and anatomic (size, location, morphology, multiplicity) risk metrics.

RESULTS

Interobserver agreement was strong as shown in regression analysis (R(2) > 0.84) and intraclass correlation (intraclass correlation coefficient >0.92), indicating that the K(trans) can be reliably assessed clinically. All intracranial aneurysms had a pronounced increase in wall permeability compared with the paired healthy MCA (P < .001). Regression analysis demonstrated a significant trend toward an increased K(trans) with increasing aneurysm size (P < .001). Logistic regression showed that K(trans) also predicted risk in anatomic (P = .02) and combined anatomic/clinical (P = .03) groups independent of size.

CONCLUSIONS

We report the first evidence of dynamic contrast-enhanced MR imaging-modeled contrast permeability in intracranial aneurysms. We found that contrast agent permeability across the aneurysm wall correlated significantly with both aneurysm size and size-independent anatomic risk factors. In addition, K(trans) was a significant and size-independent predictor of morphologically and clinically defined high-risk aneurysms.

Gadolinium enhancement of atherosclerotic plaque in the middle cerebral artery: relation to symptoms and degree of stenosis

BACKGROUND AND PURPOSE

High-resolution MR imaging can depict intracranial arterial atherosclerotic plaques. Our aim was to evaluate the relationship between the degree of enhancement of MCA plaques on contrast-enhanced high-resolution MR imaging and ischemic stroke and stenosis severity.

MATERIALS AND METHODS

This study enrolled 36 patients diagnosed with moderate-to-severe atherosclerotic MCA stenosis. A contrast-enhanced T1-weighted volume isotropic turbo spin-echo acquisition sequence was acquired for assessing plaque enhancement. Plaque-to-CSF contrast ratio was calculated after the signal intensity of plaques at the stenotic segment was measured. Univariate comparison and multivariate logistic regression analyses were performed for symptomatic and asymptomatic groups to assess the relationship between symptomatic stenosis and independent variables, including plaque-to-CSF contrast ratio, degree of stenosis, and clinical risk factors. Plaque-to-CSF contrast ratio was compared between the moderate and severe stenosis groups.

RESULTS

Twenty-one patients had symptomatic MCA stenosis, and 15 had asymptomatic stenosis. The plaque-to-CSF contrast ratio was significantly higher in the symptomatic group than in the asymptomatic group (63.6 ± 10.6% versus 54.1 ± 13.5%, respectively; P < .05). The degree of stenosis also differed significantly between the 2 groups (P < .05). Multivariate analysis revealed that the degree of stenosis was the only independent predictor of ischemic stroke symptoms. The plaque-to-CSF contrast ratio of severe stenosis was significantly higher than that of moderate stenosis (66.8 ± 8.7% versus 55.9 ± 12.8%, respectively; P < .05).

CONCLUSIONS

Plaque enhancement was significantly higher in patients with symptomatic plaques and may have been affected by the degree of stenosis. A difference in plaque enhancement according to the degree of stenosis has implications for understanding the development of intracranial atherosclerotic plaques.

Intraluminal thrombus, intraplaque hemorrhage, plaque thickness, and current smoking optimally predict carotid stroke

BACKGROUND AND PURPOSE

Intraplaque hemorrhage (IPH) is associated with acute and future stroke. IPH is also associated with lumen markers of stroke risk including stenosis, plaque thickness, and ulceration. Whether IPH adds further predictive value to these other variables is unknown. The purpose of this study was to determine whether IPH improves carotid-source stroke prediction.

METHODS

In this retrospective cross-sectional study, patients undergoing stroke workup were imaged with MRI and IPH detection. Seven hundred twenty-six carotid-brain image pairs were analyzed after excluding vessels with noncarotid plaque stroke sources (420) and occlusions (7) or near-occlusions (3). Carotid imaging characteristics were recorded, including percent diameter and mm stenosis, plaque thickness, ulceration, intraluminal thrombus, and IPH. Clinical confounders were recorded, and a multivariable logistic regression model was fitted. Backward elimination was used to determine essential carotid-source stroke predictors with a threshold 2-sided P<0.10. Receiver operating characteristic analysis was performed to determine discriminatory value.

RESULTS

Significant predictors of carotid-source stroke included intraluminal thrombus (odds ratio=103.6; P<0.001), IPH (odds ratio=25.2; P<0.001), current smoking (odds ratio=2.78; P=0.004), and thickness (odds ratio=1.24; P=0.020). The final model discriminatory value was excellent (area under the curve=0.862). This was significantly higher than the final model without IPH (area under the curve=0.814), or models using only stenosis as a continuous variable (area under the curve=0.770) or cutoffs of 50% and 70% (area under the curve=0.669), P<0.001.

CONCLUSIONS

After excluding patients with noncarotid plaque stroke sources, optimal discrimination of carotid-source stroke was obtained with intraluminal thrombus, IPH, plaque thickness, and smoking history but not ulceration and stenosis.

Increased microvascularization and vessel permeability associate with active inflammation in human atheromata

BACKGROUND

Studies have shown the feasibility of imaging plaques with 2-deoxy-2-[(18)F]fluoroglucose (FDG) positron emission tomography and dynamic contrast-enhanced magnetic resonance imaging with inconsistent results. We sought to investigate the relationship between markers of inflammatory activation, plaque microvascularization, and vessel wall permeability in subjects with carotid plaques using a multimodality approach combining FDG positron emission tomography, dynamic contrast-enhanced magnetic resonance imaging, and histopathology.

METHODS AND RESULTS

Thirty-two subjects with carotid stenoses underwent noninvasive imaging with FDG positron emission tomography and dynamic contrast-enhanced magnetic resonance imaging, 46.9% (n=15) before carotid endarterectomy. We measured FDG uptake (target:background ratio [TBR]) by positron emission tomography and K(trans) (reflecting microvascular permeability and perfusion) by magnetic resonance imaging and correlated imaging with immunohistochemical markers of macrophage content (CD68), activated inflammatory cells (major histocompatibility complex class II), and microvessels (CD31) in plaque and control regions. TBR and K(trans) correlated significantly with tertiles of CD68(+) (P=0.009 and P=0.008, respectively), major histocompatibility complex class II(+) (P=0.003 and P<0.001, respectively), and CD31(+) (P=0.004 and P=0.008, respectively). Regions of plaques were associated with increased CD68(+) (P=0.002), major histocompatibility complex class II(+) (P=0.002), CD31(+) (P=0.02), TBR (P<0.0001), and K(trans) (P<0.0001), as compared with those without plaques. Microvascularization correlated with macrophage content (rs=0.52; P=0.007) and inflammatory activity (rs=0.68; P=0.0001) and TBR correlated with K(trans) (rs=0.53; P<0.0001). In multivariable mixed linear regression modeling, TBR remained independently associated with K(trans) (β[SE], 2.68[0.47]; P<0.0001).

CONCLUSIONS

Plaque regions with active inflammation, as determined by macrophage content and major histocompatibility complex class II expression, showed increased FDG uptake, which correlated with increased K(trans) and microvascularization. The correlation between K(trans) and TBR was moderate, direct, highly significant, and independent of clinical symptoms and plaque luminal severity.

Scan-rescan reproducibility of quantitative assessment of inflammatory carotid atherosclerotic plaque using dynamic contrast-enhanced 3T CMR in a multi-center study

BACKGROUND

The aim of this study is to investigate the inter-scan reproducibility of kinetic parameters in atherosclerotic plaque using dynamic contrast-enhanced (DCE) cardiovascular magnetic resonance (CMR) in a multi-center setting at 3T.

METHODS

Carotid arteries of 51 subjects from 15 sites were scanned twice within two weeks on 3T scanners using a previously described DCE-CMR protocol. Imaging data with protocol compliance and sufficient image quality were analyzed to generate kinetic parameters of vessel wall, expressed as transfer constant (K trans ) and plasma volume (v p ). The inter-scan reproducibility was evaluated using intra-class correlation coefficient (ICC) and coefficient of variation (CV). Power analysis was carried out to provide sample size estimations for future prospective study.

RESULTS

Ten (19.6%) subjects were found to suffer from protocol violation, and another 6 (11.8%) had poor image quality (n=6) in at least one scan. In the 35 (68.6%) subjects with complete data, the ICCs of K trans and v p were 0.65 and 0.28, respectively. The CVs were 25% and 62%, respectively. The ICC and CV for v p improved to 0.73 and 28% in larger lesions with analyzed area larger than 25 mm2. Power analysis based on the measured CV showed that 50 subjects per arm are sufficient to detect a 20% difference in change of K trans over time between treatment arms with 80% power without consideration of the dropout rate.

CONCLUSION

The result of this study indicates that quantitative measurement from DCE-CMR is feasible to detect changes with a relatively modest sample size in a prospective multi-center study despite the limitations. The relative high dropout rate suggested the critical needs for intensive operator training, optimized imaging protocol, and strict quality control in future studies.

Distribution of Intravascular and Extravascular Extracellular Volume Fractions by Total Area under Curve for Neovascularization Assessment by Dynamic Contrast-Enhanced Magnetic Resonance Imaging

In this paper, we propose and investigate distribution of intravascular and extravascular extracellular volume fractions (DIEEF) as a noninvasive biomarker for neovascularization assessment by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). A generalized two-compartment exchange model (G2CXM) that uniformly includes the Patlak model, Tofts model, extended Tofts model, and recent two-compartment exchange model as special instances is first presented. Based on the total area under curve of the G2CXM a method of DIEEF estimation without knowing the artery input function is proposed. The mean square error of DIEEF estimate in the presence of noise and with incomplete DCE-MRI data is analyzed. Simulation results demonstrate that DIEEF estimate is accurate when signal to noise ratio is only 5 dB in both cases of tracer infusion and bolus injection, and slightly favors the bolus injection. Tested on a model of atherosclerotic rabbits, the DIEEF of aorta plaques is positively correlated with the histological neovessel count with correlation coefficient of 0.940 and P = 0.017, and outperforms six semiquantitative parameters in the literature. DIEEF might be useful as a biomarker for noninvasive neovascularization assessment by DCE-MRI.

Dynamic contrast-enhanced magnetic resonance imaging: fundamentals and application to the evaluation of the peripheral perfusion

INTRODUCTION

The ability to ascertain information pertaining to peripheral perfusion through the analysis of tissues' temporal reaction to the inflow of contrast agent (CA) was first recognized in the early 1990's. Similar to other functional magnetic resonance imaging (MRI) techniques such as arterial spin labeling (ASL) and blood oxygen level-dependent (BOLD) MRI, dynamic contrast-enhanced MRI (DCE-MRI) was at first restricted to studies of the brain. Over the last two decades the spectrum of ailments, which have been studied with DCE-MRI, has been extensively broadened and has come to include pathologies of the heart notably infarction, stroke and further cerebral afflictions, a wide range of neoplasms with an emphasis on antiangiogenic treatment and early detection, as well as investigations of the peripheral vascular and musculoskeletal systems.

APPLICATIONS TO PERIPHERAL PERFUSION

DCE-MRI possesses an unparalleled capacity to quantitatively measure not only perfusion but also other diverse microvascular parameters such as vessel permeability and fluid volume fractions. More over the method is capable of not only assessing blood flowing through an organ, but in contrast to other noninvasive methods, the actual tissue perfusion. These unique features have recently found growing application in the study of the peripheral vascular system and most notably in the diagnosis and treatment of peripheral arterial occlusive disease (PAOD).

REVIEW OUTLINE

The first part of this review will elucidate the fundamentals of data acquisition and interpretation of DCE-MRI, two areas that often remain baffling to the clinical and investigating physician because of their complexity. The second part will discuss developments and exciting perspectives of DCE-MRI regarding the assessment of perfusion in the extremities. Emerging clinical applications of DCE-MRI will be reviewed with a special focus on investigation of physiology and pathophysiology of the microvascular and vascular systems of the extremities.

Varying Correlation Between 18 F-Fluorodeoxyglucose Positron Emission Tomography and Dynamic Contrast-Enhanced MRI in Carotid Atherosclerosis

Background and Purpose—

18 F-fluorodeoxyglucose positron emission tomography and dynamic contrast-enhanced MRI have been proposed to quantitatively assess plaque inflammation by probing macrophages and neovessels, respectively. We examined their correlation to study the in vivo relationship between macrophage and neovessel activities in atherogenesis.

Methods—

Forty-one patients (34 men; aged 65±12 years) with a total of 68 carotid plaques (thickness ≥2 mm on ultrasound; 20 symptomatic) were assessed by both 18 F-fluorodeoxyglucose positron emission tomography/computed tomography and dynamic contrast-enhanced MRI within 2 weeks, measured as target-to-background ratio and transfer constant ( K trans ), respectively.

Results—

Overall, the correlation between target-to-background ratio and K trans was weak and marginal ( r =0.22; P =0.068). They were correlated in the symptomatic plaques ( r =0.59; P =0.006) but not in the asymptomatic plaques ( r =0.07; P =0.625; P =0.033 for difference in r ). Neither target-to-background ratio nor K trans was significantly higher in the symptomatic plaques, but both showed an inverse relationship with time since last neurological event ( r =−0.94 and −0.69 for target-to-background ratio and K trans , respectively).

Conclusions—

The correlation between 18 F-fluorodeoxyglucose positron emission tomography and dynamic contrast-enhanced MRI measurements varied with clinical conditions, pointing to a complex interplay between macrophages and neovessels under different pathophysiological conditions. The moderate correlation shown only in symptomatic plaques indicates the presence of acute plaque inflammation with increased metabolic activity and cytokine production by inflammatory cells.

Homologous HOmologous Black-Bright-blood and flexible Interleaved imaging sequence (HOBBI) for dynamic contrast-enhanced MRI of the vessel wall

PURPOSE

To present a HOmologous Black-Bright-blood and flexible Interleaved imaging (HOBBI) sequence for dynamic contrast-enhanced magnetic resonance imaging (MRI) of the vessel wall.

THEORY AND METHODS

A HOBBI sequence is proposed to acquire high-spatial-resolution black-blood and high-temporal-resolution bright-blood dynamic contrast-enhanced images in an interleaved fashion. Black-blood imaging allows for thin vessel wall evaluation, whereas bright-blood imaging obtains the arterial input function accurately. A simulation was performed to assess the accuracy of the pharmacokinetic parameters [transfer constant (K(trans) ) and fractional plasma volume (vp )] generated from HOBBI. In vivo evaluation was also used to validate HOBBI in an animal model of aortic atherosclerosis.

RESULTS

In the simulation test, the estimated K(trans) and vp measured by HOBBI were more accurate than those from black-blood dynamic contrast-enhanced-MRI. In the animal model testing, K(trans) and vp also demonstrated good interscan reproducibility (K(trans) : ICC = 0.77, vp : ICC = 0.72, respectively). Additionally, K(trans) showed a significant increase from 1 month (0.026 ± 0.013 min(-1) ) to 2 months (0.069 ± 0.018 min(-1) ) in animal model plaque progression after balloon injury.

CONCLUSION

The proposed HOBBI sequence was demonstrated to be feasible and accurate in estimating the pharmacokinetic parameters of the atherosclerotic vessel wall, and has potential to become an early screening tool for atherosclerosis disease.

Atherosclerosis and atheroma plaque rupture: imaging modalities in the visualization of vasa vasorum and atherosclerotic plaques

Invasive angiography has been widely accepted as the gold standard to diagnose cardiovascular pathologies. Despite its superior resolution of demonstrating atherosclerotic plaque in terms of degree of lumen stenosis, the morphological assessment for the plaque is insufficient for the analysis of plaque components, and therefore, unable to predict the risk status or vulnerability of atherosclerotic plaque. There is an increased body of evidence to show that the vasa vasorum play an important role in the initiation, progression, and complications of atherosclerotic plaque leading to major adverse cardiac events. This paper provides an overview of the evidence-based reviews of various imaging modalities with regard to their potential value for comprehensive characterization of the composition, burden, and neovascularization of atherosclerotic plaque.

Contrast-enhanced cardiovascular magnetic resonance imaging of coronary vessel wall: state of art

Coronary wall imaging by cardiovascular magnetic resonance (CMR) emerges as a promising method to detect vascular injury and remodeling directly within the coronary vascular wall. In this review, the current evidence on coronary wall enhancement using CMR is presented and summarized, with particular focus on its ability to detect inflammation in atherosclerosis, Takayasu's arteritis, acute coronary syndromes and immune-mediated inflammatory vasculitides. The authors review the possible mechanisms of coronary wall contrast enhancement on CMR and discuss the technical considerations and limitations. Lastly, the potential clinical applications and possibilities for future research are proposed.

Atherosclerotic plaque inflammation quantification using dynamic contrast-enhanced (DCE) MRI

Inflammation plays an important role in atherosclerosis. Given the increasing interest in using in-vivo imaging methods to study the physiology and treatment effects in atherosclerosis, noninvasive intraplaque inflammation quantitative method is needed. Dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) has been proposed and validated to quantitatively characterize atherosclerotic plaque inflammation. Recent studies have optimized the imaging protocol, pharmacokinetic modeling techniques. All of these technical advances further promoted DCE-MRI to clinical investigations in plaque risk assessment and therapeutic response monitor. Although larger clinical studies are still needed, DCE-MRI has been proven to be a promising tool to reveal more about intraplaque inflammation by in vivo quantitative inflammation imaging.

Three-dimensional dynamic contrast enhanced imaging of the carotid artery with direct arterial input function measurement

PURPOSE

Kinetic analysis using dynamic contrast enhanced MRI to assess neovascularization of carotid plaque requires images with high spatial and temporal resolution. This work demonstrates a new three-dimensional (3D) dynamic contrast enhanced imaging sequence, which directly measures the arterial input function with high temporal resolution yet maintains the high spatial resolution required to identify areas of increased adventitial neovascularity.

THEORY AND METHODS

The sequence consists of multiple rapid acquisitions of a saturation prepared dynamic 3D gradient recalled echo (GRE) sequence temporally interleaved with multiple acquisitions of a 2D slice. The saturation recovery time was adjusted to maintain signal linearity with the very different contrast agent concentrations in the 2D slice and 3D volume. The K(trans) maps were obtained from the 3D dynamic contrast measurements while the 2D slice was used to obtain the arterial input function. Calibration and dynamic studies are presented.

RESULTS

For contrast agent concentrations up to 5 mM, a saturation recovery time for the 2D slice of 20 ms resulted in less than a 10% deviation from the desired linear response of signal intensity with contrast agent concentration. The corresponding saturation recovery time of 83 ms for the 3D volume maintained less than a 10% deviation from the linear response up to contrast agent concentrations of 2 mM while a contrast agent concentration of 5 mM had almost a 30% deviation. There was a significant improvement in signal attenuation (9 ± 3% versus 23 ± 5% at 40 cm/s) when flow compensation was added to the slice select gradients. For patient studies, volume transfer and plasma fraction maps were calculated with data from the proposed sequence.

CONCLUSION

This work demonstrated a novel sequence for 3D dynamic contrast enhanced imaging with a simultaneously acquired 2D slice that directly measures the arterial input function with high temporal resolution. Acquisition parameters can be adjusted to accommodate the full range of contrast agent concentration values to be encountered and the kinetic parameters obtained were consistent with expected values.

T1 gadolinium enhancement of intracranial atherosclerotic plaques associated with symptomatic ischemic presentations

BACKGROUND AND PURPOSE

Contrast enhancement of intracranial atherosclerotic plaques has recently been investigated using high field and high resolution MR imaging as a risk factor in the development of ischemic stroke. We studied the reliability of conventional MR imaging at 1.5T in evaluating intraplaque enhancement and its relationship with acute cerebrovascular ischemic presentations in patients with severe intracranial atherosclerotic disease.

MATERIALS AND METHODS

We retrospectively identified and analyzed 19 patients with 22 high-grade intracranial atherosclerotic disease plaques (>70% stenosis) in vessels cross-sectionally visualized by neuroanatomic MR imaging. Atherosclerotic plaques were classified as asymptomatic or symptomatic. Two blinded neuroradiologists independently ranked each lesion for the presence of intraplaque enhancement by use of a 5-point scale (1-5). Furthermore, plaque enhancement was quantified as the relative change in T1WI spin-echo signal intensity (postcontrast/precontrast) in the vessel wall at the site of each intracranial atherosclerotic disease lesion.

RESULTS

Intraplaque enhancement was observed in 7 of 10 (70%) symptomatic plaques, in contrast to 1 of 12 (8%) asymptomatic plaques. Interobserver reliability correlated well for intraplaque enhancement (κ = 0.82). The degree of relative plaque enhancement in symptomatic versus asymptomatic lesions (63% versus 23%) was statistically significant (P = .001, t test).

CONCLUSIONS

In this pilot study, we determined that intraplaque enhancement could be reliably evaluated with the use of cross-sectional imaging and analysis of vessels/plaques by use of conventional neuroanatomic MR imaging protocols. In addition, we observed a strong association between intraplaque enhancement in severe intracranial atherosclerotic disease lesions and ischemic events with the use of conventional MR imaging. Our preliminary study suggests that T1 gadolinium-enhancing plaques may be an indicator of progressing or symptomatic intracranial atherosclerotic disease.

MR imaging of the arterial vessel wall: molecular imaging from bench to bedside

Cardiovascular diseases remain the leading cause of morbidity and mortality in the Western world and developing countries. In clinical practice, in vivo characterization of atherosclerotic lesions causing myocardial infarction, ischemic stroke, and other complications remains challenging. Imaging methods, limited to the assessment luminal stenosis, are the current reference standard for the assessment of clinically significant coronary and carotid artery disease and the guidance of treatment. These techniques do not allow distinction between stable and potentially vulnerable atherosclerotic plaque. Magnetic resonance (MR) imaging is a modality well suited for visualization and characterization of the relatively thin arterial vessel wall, because it allows imaging with high spatial resolution and excellent soft-tissue contrast. In clinical practice, atherosclerotic plaque components of the carotid artery and aorta may be differentiated and characterized by using unenhanced vessel wall MR imaging. Additional information can be gained by using clinically approved nonspecific contrast agents. With the advent of targeted MR contrast agents, which enhance specific molecules or cells, pathologic processes can be visualized at a molecular level with high spatial resolution. In this article, the pathophysiologic changes of the arterial vessel wall underlying the development of atherosclerosis will be first reviewed. Then basic principles and properties of molecular MR imaging contrast agents will be introduced. Additionally, recent advances in preclinical molecular vessel wall imaging will be reviewed. Finally, the clinical feasibility of arterial vessel wall imaging at unenhanced and contrast material-enhanced MR imaging of the aortic, carotid, and coronary vessel wall will be discussed.

Molecular MRI of atherosclerosis

Despite advances in prevention, risk assessment and treatment, coronary artery disease (CAD) remains the leading cause of morbidity and mortality in Western countries. The lion's share is due to acute coronary syndromes (ACS), which are predominantly triggered by plaque rupture or erosion and subsequent coronary thrombosis. As the majority of vulnerable plaques does not cause a significant stenosis, due to expansive remodeling, and are rather defined by their composition and biological activity, detection of vulnerable plaques with x-ray angiography has shown little success. Non-invasive vulnerable plaque detection by identifying biological features that have been associated with plaque progression, destabilization and rupture may therefore be more appropriate and may allow earlier detection, more aggressive treatment and monitoring of treatment response. MR molecular imaging with target specific molecular probes has shown great promise for the noninvasive in vivo visualization of biological processes at the molecular and cellular level in animals and humans. Compared to other imaging modalities; MRI can provide excellent spatial resolution; high soft tissue contrast and has the ability to simultaneously image anatomy; function as well as biological tissue composition and activity.

Combined 18F-FDG PET-CT and DCE-MRI to assess inflammation and microvascularization in atherosclerotic plaques

BACKGROUND AND PURPOSE

Hallmarks of vulnerable atherosclerotic plaques are inflammation that can be assessed with 18fluorine-fluorodeoxyglucose positron emission tomography/computed tomography, and increased neovascularization that can be evaluated by dynamic contrast-enhanced-MRI. It remains unclear whether these parameters are correlated or represent independent imaging parameters. This study determines whether there is a correlation between inflammation and neovascularization in atherosclerotic carotid plaques.

METHODS

A total of 58 patients with transient ischemic attack or minor stroke in the carotid territory and ipsilateral carotid artery stenosis of 30% to 69% were included. All patients underwent positron emission tomography/computed tomography and dynamic contrast-enhanced-MRI of the carotid plaque. 18Fluorine-fluorodeoxyglucose standard uptake values with target/background ratio were determined. Neovascularization was quantified by the mean (leakage) volume transfer constant Ktrans. Spearman rank correlation coefficients between target/background ratio and Ktrans were calculated.

RESULTS

Images suitable for further analysis were obtained in 49 patients. A weak but significant positive correlation between target/background ratio and mean Ktrans (Spearman ρ=0.30 [P=0.035]) and 75th percentile Ktrans (Spearman ρ=0.29 [P=0.041]) was found.

CONCLUSIONS

There is a weak but significant positive correlation between inflammation on positron emission tomography/computed tomography and neovascularization as assessed with dynamic contrast-enhanced-MRI. Future studies should investigate which imaging modality has the highest predictive value for recurrent stroke, as these are not interchangeable.

CLINICAL TRIAL REGISTRATION URL

http://www.clinicaltrials.gov. Unique identifier: NCT00451529.

Registration of dynamic contrast-enhanced MRI of the common carotid artery using a fixed-frame template-based squared-difference method

PURPOSE

This study examines template-based squared-difference registration for motion correction in dynamic contrast-enhanced (DCE) MRI studies of the carotid artery wall and compares the results of fixed-frame template-based registration with a previously proposed consecutive-frame registration method.

MATERIALS AND METHODS

Ten T1-weighted black-blood, turbo spin-echo DCE-MRI studies of the carotid artery wall were used to test template-based squared-difference registration. An intermediate image from each series was selected as the fixed-frame template for registration. Squared-difference minimization was used to align each image and template. Time-intensity curves generated from data aligned with template-based squared-difference registration were compared with gold standard curves created by drawing regions of interest on each image in the series. The results were also compared with unregistered data and data after consecutive-frame squared-difference registration.

RESULTS

An analysis of variance test of root mean-square error values between gold standard curve and curves from unregistered data and data registered with consecutive-frame and fixed-frame template-based methods was significant (P < 0.005) with template-based squared-difference registration producing curves that most closely matched the gold standard.

CONCLUSION

A fixed-frame template-based squared-difference registration method was proposed and validated for alignment of DCE-MRI of carotid arteries.

Suitability of pharmacokinetic models for dynamic contrast-enhanced MRI of abdominal aortic aneurysm vessel wall: a comparison

Purpose

Increased microvascularization of the abdominal aortic aneurysm (AAA) vessel wall has been related to AAA progression and rupture. The aim of this study was to compare the suitability of three pharmacokinetic models to describe AAA vessel wall enhancement using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI).

Materials and Methods

Patients with AAA underwent DCE-MRI at 1.5 Tesla. The volume transfer constant (K^trans), which reflects microvascular flow, permeability and surface area, was calculated by fitting the blood and aneurysm vessel wall gadolinium concentration curves. The relative fit errors, parameter uncertainties and parameter reproducibilities for the Patlak, Tofts and Extended Tofts model were compared to find the most suitable model. Scan-rescan reproducibility was assessed using the interclass correlation coefficient and coefficient of variation (CV). Further, the relationship between K^trans and AAA size was investigated.

Results

DCE-MRI examinations from thirty-nine patients (mean age±SD: 72±6 years; M/F: 35/4) with an mean AAA maximal diameter of 49±6 mm could be included for pharmacokinetic analysis. Relative fit uncertainties for K^trans based on the Patlak model (17%) were significantly lower compared to the Tofts (37%) and Extended Tofts model (42%) (p<0.001). K^trans scan-rescan reproducibility for the Patlak model (ICC = 0.61 and CV = 22%) was comparable with the Tofts (ICC = 0.61, CV = 23%) and Extended Tofts model (ICC = 0.76, CV = 22%). K^trans was positively correlated with maximal AAA diameter (Spearman’s ρ = 0.38, p = 0.02) using the Patlak model.

Conclusion

Using the presented imaging protocol, the Patlak model is most suited to describe DCE-MRI data of the AAA vessel wall with good K^trans scan-rescan reproducibility.

The complementary roles of dynamic contrast-enhanced MRI and 18F-fluorodeoxyglucose PET/CT for imaging of carotid atherosclerosis

PURPOSE

Inflammation and neovascularization in vulnerable atherosclerotic plaques are key features for severe clinical events. Dynamic contrast-enhanced (DCE) MRI and FDG PET are two noninvasive imaging techniques capable of quantifying plaque neovascularization and inflammatory infiltrate, respectively. However, their mutual role in defining plaque vulnerability and their possible overlap has not been thoroughly investigated. We studied the relationship between DCE-MRI and (18)F-FDG PET data from the carotid arteries of 40 subjects with coronary heart disease (CHD) or CHD risk equivalent, as a substudy of the dal-PLAQUE trial (NCT00655473).

METHODS

The dal-PLAQUE trial was a multicenter study that evaluated dalcetrapib, a cholesteryl ester transfer protein modulator. Subjects underwent anatomical MRI, DCE-MRI and (18)F-FDG PET. Only baseline imaging and biomarker data (before randomization) from dal-PLAQUE were used as part of this substudy. Our primary goal was to evaluate the relationship between DCE-MRI and (18)F-FDG PET data. As secondary endpoints, we evaluated the relationship between (a) PET data and whole-vessel anatomical MRI data, and (b) DCE-MRI and matching anatomical MRI data. All correlations were estimated using a mixed linear model.

RESULTS

We found a significant inverse relationship between several perfusion indices by DCE-MRI and (18)F-FDG uptake by PET. Regarding our secondary endpoints, there was a significant relationship between plaque burden measured by anatomical MRI with several perfusion indices by DCE-MRI and (18)F-FDG uptake by PET. No relationship was found between plaque composition by anatomical MRI and DCE-MRI or (18)F-FDG PET metrics.

CONCLUSION

In this study we observed a significant, weak inverse relationship between inflammation measured as (18)F-FDG uptake by PET and plaque perfusion by DCE-MRI. Our findings suggest that there may be a complex relationship between plaque inflammation and microvascularization during the different stages of plaque development. (18)F-FDG PET and DCE-MRI may have complementary roles in future clinical practice in identifying subjects at high risk of cardiovascular events.

Imaging of the carotid artery vulnerable plaque

Atherosclerosis involving the carotid arteries has a high prevalence in the population worldwide. This condition is significant because accidents of the carotid artery plaque are associated with the development of cerebrovascular events. For this reason, carotid atherosclerotic disease needs to be diagnosed and those determinants that are associated to an increased risk of stroke need to be identified. The degree of stenosis typically has been considered the parameter of choice to determine the therapeutical approach, but several recently published investigations have demonstrated that the degree of luminal stenosis is only an indirect indicator of the atherosclerotic process and that direct assessment of the plaque structure and composition may be key to predict the development of future cerebrovascular ischemic events. The concept of "vulnerable plaque" was born, referring to those plaque's parameters that concur to the instability of the plaque making it more prone to the rupture and distal embolization. The purpose of this review is to describe the imaging characteristics of "vulnerable carotid plaques."

Use of perfusion imaging and other imaging techniques to assess risks/benefits of acute stroke interventions

The advent of multimodal neuroimaging has provided acute stroke care providers with an armamentarium of sophisticated imaging options to utilize for guidance in clinical decision-making and management of acute ischemic stroke patients. Here, we propose a framework and potential algorithm-based methodology for imaging modality selection and utilization for the purpose of achieving optimal stroke clinical care. We first review imaging options that may best inform decision-making regarding revascularization eligibility, with a focus on the imaging modalities that best identify critical inclusion and exclusion criteria. Next, we review imaging methods that may guide the successful achievement of revascularization once it has been deemed desirable and feasible. Further, we review imaging modalities that may best assist in both the noninterventional care of acute stroke as well as the identification of stroke-mimics. Finally, we review imaging techniques under current investigation that show promise to improve future acute stroke management.

The vulnerable coronary plaque: update on imaging technologies

Several studies have been carried out on vulnerable plaque as the main culprit for ischaemic cardiac events. Historically, the most important diagnostic technique for studying coronary atherosclerotic disease was to determine the residual luminal diameter by angiographic measurement of the stenosis. However, it has become clear that vulnerable plaque rupture as well as thrombosis, rather than stenosis, triggers most acute ischaemic events and that the quantification of risk based merely on severity of the arterial stenosis is not sufficient. In the last decades, substantial progresses have been made on optimisation of techniques detecting the arterial wall morphology, plaque composition and inflammation. To date, the use of a single technique is not recommended to precisely identify the progression of the atherosclerotic process in human beings. In contrast, the integration of data that can be derived from multiple methods might improve our knowledge about plaque destabilisation. The aim of this narrative review is to update evidence on the accuracy of the currently available non-invasive and invasive imaging techniques in identifying components and morphologic characteristics associated with coronary plaque vulnerability.

SHILO, a novel dual imaging approach for simultaneous HI-/LOw temporal (Low-/Hi-spatial) resolution imaging for vascular dynamic contrast enhanced cardiovascular magnetic resonance: numerical simulations and feasibility in the carotid arteries

BACKGROUND

Dynamic contrast enhanced (DCE) cardiovascular magnetic resonance (CMR) is increasingly used to quantify microvessels and permeability in atherosclerosis. Accurate quantification depends on reliable sampling of both vessel wall (VW) uptake and contrast agent dynamic in the blood plasma (the so called arterial input function, AIF). This poses specific challenges in terms of spatial/temporal resolution and matched dynamic MR signal range, which are suboptimal in current vascular DCE-CMR protocols. In this study we describe a novel dual-imaging approach, which allows acquiring simultaneously AIF and VW images using different spatial/temporal resolution and optimizes imaging parameters for the two compartments. We refer to this new acquisition as SHILO, Simultaneous HI-/LOw-temporal (low-/hi-spatial) resolution DCE-imaging.

METHODS

In SHILO, the acquisition of low spatial resolution single-shot AIF images is interleaved with segments of higher spatial resolution images of the VW. This allows sampling the AIF and VW with different spatial/temporal resolution and acquisition parameters, at independent spatial locations. We show the adequacy of this temporal sampling scheme by using numerical simulations. Following, we validate the MR signal of SHILO against a standard 2D spoiled gradient recalled echo (SPGR) acquisition with in vitro and in vivo experiments. Finally, we show feasibility of using SHILO imaging in subjects with carotid atherosclerosis.

RESULTS

Our simulations confirmed the superiority of the SHILO temporal sampling scheme over conventional strategies that sample AIF and tissue curves at the same time resolution. Both the median relative errors and standard deviation of absolute parameter values were lower for the SHILO than for conventional sampling schemes. We showed equivalency of the SHILO signal and conventional 2D SPGR imaging, using both in vitro phantom experiments (R2 =0.99) and in vivo acquisitions (R2 =0.95). Finally, we showed feasibility of using the newly developed SHILO sequence to acquire DCE-CMR data in subjects with carotid atherosclerosis to calculate plaque perfusion indices.

CONCLUSIONS

We successfully demonstrate the feasibility of using the newly developed SHILO dual-imaging technique for simultaneous AIF and VW imaging in DCE-CMR of atherosclerosis. Our initial results are promising and warrant further investigation of this technique in wider studies measuring kinetic parameters of plaque neovascularization with validation against gold standard techniques.

MRI of carotid atherosclerosis

OBJECTIVE

Although MRI is widely used to observe atherosclerosis impacts on the vessel lumen, MRI also depicts the size of the plaque itself, its composition, and plaque inflammation, providing information beyond simple stenosis. This article summarizes the state of evidence for a clinical role for MRI of carotid atherosclerosis.

CONCLUSION

MRI of carotid atherosclerosis has a proven role in pharmaceutical trials and may improve patient management once large-scale clinical trials have been completed.

Contrast enhancement by lipid-based MRI contrast agents in mouse atherosclerotic plaques; a longitudinal study

The use of contrast-enhanced MRI to enable in vivo specific characterization of atherosclerotic plaques is increasing. In this study the intrinsic ability of two differently sized gadolinium-based contrast agents to enhance atherosclerotic plaques in ApoE(-/-) mice was evaluated with MRI. We obtained a kinetic profile for contrast enhancement, as the literature data on optimal imaging time points is scarce, and assessed the longer-term kinetics. Signal enhancement in the wall of the aortic arch, following intravenous injection of paramagnetic micelles and liposomes, was followed for 1 week. In vivo T(1)-weighted MRI plaque enhancement characteristics were complemented by fluorescence microscopy of NIR(664) incorporated in the contrast agents and quantification of tissue and blood Gd-DTPA. Both micelles and liposomes enhanced contrast in T(1)-weighted MR images of plaques in the aortic arch. The average contrast-to-noise ratio increased after liposome or micelle injection to 260 or 280% respectively, at 24 h after injection, compared with a pre-scan. A second wave of maximum contrast enhancement was observed around 60-72 h after injection, which only slowly decreased towards the 1 week end-point. Confocal fluorescence microscopy and whole body fluorescence imaging confirmed MRI-findings of accumulation of micelles and liposomes. Plaque permeation of contrast agents was not strongly dependent on the contrast agent size in this mouse model. Our results show that intraplaque accumulation over time of both contrast agents leads to good plaque visualization for a long period. This inherent intraplaque accumulation might make it difficult to discriminate passive from targeted accumulation. This implies that, in the development of targeted contrast agents on a lipid-based backbone, extensive timing studies are required.

3-D carotid multi-region MRI segmentation by globally optimal evolution of coupled surfaces

In this paper, we propose a novel global optimization based 3-D multi-region segmentation algorithm for T1-weighted black-blood carotid magnetic resonance (MR) images. The proposed algorithm partitions a 3-D carotid MR image into three regions: wall, lumen, and background. The algorithm performs such partitioning by simultaneously evolving two coupled 3-D surfaces of carotid artery adventitia boundary (AB) and lumen-intima boundary (LIB) while preserving their anatomical inter-surface consistency such that the LIB is always located within the AB. In particular, we show that the proposed algorithm results in a fully time implicit scheme that propagates the two linearly ordered surfaces of the AB and LIB to their globally optimal positions during each discrete time frame by convex relaxation. In this regard, we introduce the continuous max-flow model and prove its duality/equivalence to the convex relaxed optimization problem with respect to each evolution step. We then propose a fully parallelized continuous max-flow-based algorithm, which can be readily implemented on a GPU to achieve high computational efficiency. Extensive experiments, with four users using 12 3T MR and 26 1.5T MR images, demonstrate that the proposed algorithm yields high accuracy and low operator variability in computing vessel wall volume. In addition, we show the algorithm outperforms previous methods in terms of high computational efficiency and robustness with fewer user interactions.

Characterisation of carotid plaques with ultrasound elastography: feasibility and correlation with high-resolution magnetic resonance imaging

OBJECTIVES

To evaluate the ability of ultrasound non-invasive vascular elastography (NIVE) strain analysis to characterise carotid plaque composition and vulnerability as determined by high-resolution magnetic resonance imaging (MRI).

METHODS

Thirty-one subjects with 50 % or greater carotid stenosis underwent NIVE and high-resolution MRI of internal carotid arteries. Time-varying strain images (elastograms) of segmented plaques were generated from ultrasonic raw radiofrequency sequences. On MRI, corresponding plaques and components were segmented and quantified. Associations between strain parameters, plaque composition and symptomatology were estimated with curve-fitting regressions and Mann-Whitney tests.

RESULTS

Mean stenosis and age were 72.7 % and 69.3 years, respectively. Of 31 plaques, 9 were symptomatic, 17 contained lipid and 7 were vulnerable on MRI. Strains were significantly lower in plaques containing a lipid core compared with those without lipid, with 77-100 % sensitivity and 57-79 % specificity (P < 0.032). A statistically significant quadratic fit was found between strain and lipid content (P < 0.03). Strains did not discriminate symptomatic patients or vulnerable plaques.

CONCLUSIONS

Ultrasound NIVE is feasible in patients with significant carotid stenosis and can detect the presence of a lipid core with high sensitivity and moderate specificity. Studies of plaque progression with NIVE are required to identify vulnerable plaques.

KEY POINTS

• Non-invasive vascular elastography (NIVE) provides additional information in vascular ultrasound • Ultrasound NIVE is feasible in patients with significant carotid stenosis • Ultrasound NIVE detects a lipid core with high sensitivity and moderate specificity • Studies on plaque progression with NIVE are required to identify vulnerable plaques.

Molecular imaging to identify the vulnerable plaque--from basic research to clinical practice

Cardiovascular disease (CVD) is still the leading cause of death in the Western World. Adverse outcomes of CVD include stroke, myocardial infarction, and heart failure. Atherosclerosis is considered to be the major cause of CVD and is estimated to cause half of all deaths in developed countries. Atherosclerotic lesions of the vessel wall may obstruct blood flow mechanically through stenosis, but rupture of atherosclerotic plaques causing formation of occlusive thrombi is far more prevalent. Unfortunately, conventional diagnostic tools fail to assess whether a plaque is vulnerable to rupture. Research over the past decade identified the biological processes that are implicated in the course towards plaque rupture, like cell death and inflammation. Knowledge about plaque biology propelled the development of imaging techniques that target biologic processes in order to predict the vulnerable plaque. This paper discusses novel and existing molecular imaging targets and addresses advantages and disadvantages of these targets and respective imaging techniques in respect of clinical application and socio-economic impact.

High-resolution magnetic resonance imaging of carotid atherosclerosis identifies vulnerable carotid plaques

OBJECTIVE

Carotid magnetic resonance imaging (MRI) may be a useful tool in characterizing carotid plaque vulnerability, but large studies are still lacking. The purpose of this study was to assess carotid MRI features of vulnerable plaque in a large study and the changes in carotid plaque morphology with respect to time since the neurological event.

METHODS

We included 161 patients with carotid plaque more than 3 mm thick. All patients underwent carotid MRI to obtain 3-T high-resolution magnetic resonance sequences. Large lipid core, intraplaque hemorrhage (IPH), fibrous cap rupture (FCR), and gadolinium enhancement (GE) were assessed and classified as present or absent. Prevalences of these features were then compared between symptomatic and asymptomatic patients and time since stroke.

RESULTS

Seven patients were excluded because of poor image quality. Of the remaining 154 patients, 52 were symptomatic and 102 were asymptomatic. The prevalences of IPH (39 vs 16%; P = .002), FCR (30 vs 9%; P = .001), and GE (75 vs 55%; P = .015) were significantly higher in symptomatic than asymptomatic patients. After multivariate analysis, the prevalences of IPH (odds ratio, 2.6; P = .023) and FCR (odds ratio, 2.8; P = .038) were still significantly higher. The prevalence of IPH was significantly higher in symptomatic patients with plaque regardless of the time since the neurological event. For FCR, the difference between symptomatic and asymptomatic patients was significant only during the first 15 days after the neurological event.

CONCLUSIONS

Carotid MRI can identify plaque features that are associated with symptomatic presentation and may be indicative of plaque vulnerability. These features may ultimately be used in the management of extracranial carotid stenosis.

Utilizing imaging tools in lipidology: examining the potential of MRI for monitoring cholesterol therapy

Lipid abnormalities play important roles in the development of atherosclerosis. Lipid therapies result in alterations in atherosclerotic plaques including halting of progression of the plaque, lipid transport out of the plaque and reducing inflammatory activity, which lead to plaque morphologies that are less prone to disruption, the main cause of clinical events. In order to investigate and monitor plaque morphological changes during lipid therapy in vivo we need an imaging method that can provide accurate assessment of plaque tissue components and activity. MRI of atherosclerosis has been validated as a reliable assessment of the size of the vessel lumen, but also the size of the plaque, its tissue composition and plaque activity, including inflammation. The purpose of this review is to summarize the state of evidence for the direct assessment of atherosclerotic plaque and its change by MRI, and to establish the proven role of MRI of atherosclerosis in pharmaceutical trials with lipid therapy.

Carotid artery disease and stroke: assessing risk with vessel wall MRI

Although MRI is widely used to diagnose stenotic carotid arteries, it also detects characteristics of the atherosclerotic plaque itself, including its size, composition, and activity. These features are emerging as additional risk factors for stroke that can be feasibly acquired clinically. This paper summarizes the state of evidence for a clinical role for MRI of carotid atherosclerosis.

Dynamic contrast-enhanced MR imaging of carotid atherosclerotic plaque: model selection, reproducibility, and validation

PURPOSE

To compare four known pharmacokinetic models for their ability to describe dynamic contrast material-enhanced magnetic resonance (MR) imaging of carotid atherosclerotic plaques, to determine reproducibility, and to validate the results with histologic findings.

MATERIALS AND METHODS

The study was approved by the institutional medical ethics committee. Written informed consent was obtained from all patients. Forty-five patients with 30%-99% carotid stenosis underwent dynamic contrast-enhanced MR imaging. Plaque enhancement was measured at 16 time points at approximately 25-second image intervals by using a gadolinium-based contrast material. Pharmacokinetic parameters (volume transfer constant, K(trans); extracellular extravascular volume fraction, v(e); and blood plasma fraction, v(p)) were determined by fitting a two-compartment model to plaque and blood gadolinium concentration curves. The relative fit errors and parameter uncertainties were determined to find the most suitable model. Sixteen patients underwent imaging twice to determine reproducibility. Carotid endarterectomy specimens from 16 patients who were scheduled for surgery were collected for histologic validation. Parameter uncertainties were compared with the Wilcoxon signed rank test. Reproducibility was assessed by using the coefficient of variation. Correlation with histologic findings was evaluated with the Pearson correlation coefficient.

RESULTS

The mean relative fit uncertainty (±standard error) for K(trans) was 10% ± 1 with the Patlak model, which was significantly lower than that with the Tofts (20% ± 1), extended Tofts (33% ± 3), and extended graphical (29% ± 3) models (P < .001). The relative uncertainty for v(p) was 20% ± 2 with the Patlak model and was significantly higher with the extended Tofts (46% ± 9) and extended graphical (35% ± 5) models (P < .001). The reproducibility (coefficient of variation) for the Patlak model was 16% for K(trans) and 26% for v(p). Significant positive correlations were found between K(trans) and the endothelial microvessel content determined on histologic slices (Pearson ρ = 0.72, P = .005).

CONCLUSION

The Patlak model is most suited for describing carotid plaque enhancement. Correlation with histologic findings validated K(trans) as an indicator of plaque microvasculature, and the reproducibility of K(trans) was good.

Gadolinium-Based Contrast Agents for Vessel Wall Magnetic Resonance Imaging (MRI) of Atherosclerosis

Cardiovascular disease due to atherosclerosis is the number one killer in the Western world, and threatens to become the major cause of morbidity and mortality worldwide. It is therefore paramount to develop non-invasive methods for the detection of high-risk, asymptomatic individuals before the onset of clinical symptoms or events. In the recent past, great strides have been made in the understanding of the pathological mechanisms involved in the atherosclerotic cascade down to the molecular details. This has allowed the development of contrast agents that can aid in the in vivo characterization of these processes. Gadolinium chelates are among the contrast media most commonly used in MR imaging. Originally used for MR angiography for the detection and quantification of vascular stenosis, more recently they have been applied to improve characterization of atherosclerotic plaques. In this manuscript, we will briefly review gadolinium-chelates (Gd) based contrast agents for non-invasive MR imaging of atherosclerosis. We will first describe Gd-based non-targeted FDA approved agents, used routinely in clinical practice for the evaluation of neovascularization in other diseases. Secondly, we will describe non-specific and specific targeted contrast agents, which have great potential for dissecting specific biological processes in the atherosclerotic cascade. Lastly, we will briefly compare Gd-based agents to others commonly used in MRI and to other imaging modalities.

Detection of high-risk atherosclerotic plaque: report of the NHLBI Working Group on current status and future directions

The leading cause of major morbidity and mortality in most countries around the world is atherosclerotic cardiovascular disease, most commonly caused by thrombotic occlusion of a high-risk coronary plaque resulting in myocardial infarction or cardiac death, or embolization from a high-risk carotid plaque resulting in stroke. The lesions prone to result in such clinical events are termed vulnerable or high-risk plaques, and their identification may lead to the development of pharmacological and mechanical intervention strategies to prevent such events. Autopsy studies from patients dying of acute myocardial infarction or sudden death have shown that such events typically arise from specific types of atherosclerotic plaques, most commonly the thin-cap fibroatheroma. However, the search in human beings for vulnerable plaques before their becoming symptomatic has been elusive. Recently, the PROSPECT (Providing Regional Observations to Study Predictors of Events in the Coronary Tree) study demonstrated that coronary plaques that are likely to cause future cardiac events, regardless of angiographic severity, are characterized by large plaque burden and small lumen area and/or are thin-cap fibroatheromas verified by radiofrequency intravascular ultrasound imaging. This study opened the door to identifying additional invasive and noninvasive imaging modalities that may improve detection of high-risk atherosclerotic lesions and patients. Beyond classic risk factors, novel biomarkers and genetic profiling may identify those patients in whom noninvasive imaging for vulnerable plaque screening, followed by invasive imaging for risk confirmation is warranted, and in whom future pharmacological and/or device-based focal or regional therapies may be applied to improve long-term prognosis.

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.

Progression of experimental lesions of atherosclerosis: assessment by kinetic modeling of black-blood dynamic contrast-enhanced MRI

Pharmacokinetic modeling of dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) is used to noninvasively characterize neovasculature and inflammation in atherosclerotic vessels by estimating perfusion characteristics, such as fractional plasma volume vp and transfer constant Ktrans. DCE-MRI has potential to study the evolution of nascent lesions involving early pathological changes. However, currently used bright-blood DCE-MRI approaches are difficult to apply to small lesions because of the difficulty in separating the signal in the thin vessel wall from the adjacent lumen. By suppressing the lumen signal, black-blood DCE-MRI techniques potentially provide a better tool for early atherosclerotic lesion assessment. However, whether black-blood DCE-MRI can detect temporal changes in physiological kinetic parameters has not been investigated for atherosclerosis. This study of balloon-injured New Zealand White rabbits used a reference-region-based pharmacokinetic model of black-blood DCE-MRI to evaluate temporal changes in early experimental atherosclerotic lesions of the abdominal aorta. Six rabbits were imaged at 3 and 6 months after injury. Ktrans was found to increase from 0.10±0.03 min(-1) to 0.14±0.05 min(-1) (P=0.01). In histological analysis of all twelve rabbits, Ktrans showed a significant correlation with macrophage content (R=0.70, P=0.01). These results suggest black-blood DCE-MRI and a reference-region kinetic model could be used to study plaque development and therapeutic response in vivo.

Emerging engineered magnetic nanoparticulate probes for molecular MRI of atherosclerosis: how far have we come?

Atherosclerosis is a chronic, progressive, immunoinflammatory disease of the large and medium-sized arteries, and a major cause of cardiovascular diseases. Atherosclerosis often progresses silently for decades until the occurrence of a major catastrophic clinical event such as myocardial infarction, cardiac arrest and stroke. The main challenge in the diagnosis and management of atherosclerosis is to develop a safe, noninvasive technique that is accurate and reproducible, which can detect the biologically active high-risk vulnerable plaques (with ongoing active inflammation, angiogenesis and apoptosis) before the occurrence of an acute clinical event. This article reviews the events involved in the pathogenesis of atherosclerosis in light of recently advanced understanding of the molecular pathogenesis of the disease. Next, we elaborate on the interesting developments in molecular MRI, by describing the recently engineered magnetic nanoparticulate probes targeting clinically promising molecular and cellular players/processes, involved in early atherosclerotic lesion formation to plaque rupture and erosion.

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.

Non-invasive anatomic and functional imaging of vascular inflammation and unstable plaque

Over the last several decades, basic cardiovascular research has significantly enhanced our understanding of pathobiological processes leading to formation, progression, and complications of atherosclerotic plaques. By harnessing these advances in cardiovascular biology, imaging has advanced beyond its traditional anatomical domains to a tool that permits probing of particular molecular structures to image cellular behaviour and metabolic pathways involved in atherosclerosis. From the nascent atherosclerotic plaque to the death of inflammatory cells, several potential molecular and micro-anatomical targets for imaging with particular selective imaging probes and with a variety of imaging modalities have emerged from preclinical and animal investigations. Yet, substantive barriers stand between experimental use and wide clinical application of these novel imaging strategies. Each of the imaging modalities described herein faces hurdles-for example, sensitivity, resolution, radiation exposure, reproducibility, availability, standardization, or costs. This review summarizes the published literature reporting on functional imaging of vascular inflammation in atherosclerotic plaques emphasizing those techniques that have the greatest and/or most immediate potential for broad application in clinical practice. The prospective evaluation of these techniques and standardization of protocols by multinational networks could serve to determine their added value in clinical practice and guide their development and deployment.

Understanding the genetics of coronary artery disease through the lens of noninvasive imaging

Coronary artery disease is a common condition with a known heritable component that has spurred interest in genetic research for decades, resulting in a handful of candidate genes and an appreciation for the complexity of its genetic contributions. Recent advances in sequencing technologies have resulted in large-scale association studies, possibly adding to our current understanding of the genetics of coronary artery disease. Sifting through the statistical noise, however, requires the selection of effective phenotypic markers. New imaging technologies have improved our ability to detect subclinical atherosclerosis in a safe and reproducible manner in large numbers of patients. In this article, we propose that advances in imaging technology have generated improved phenotypic markers for genetic association studies of coronary artery disease.

Imaging the efficacy of anti-inflammatory liposomes in a rabbit model of atherosclerosis by non-invasive imaging

Nanomedicine can provide a potent alternative to current therapeutic strategies for atherosclerosis. For example, the encapsulation of anti-inflammatory drugs into liposomes improves their pharmacokinetics and biodistribution, thereby enhancing bioavailability to atherosclerotic plaques and improving therapeutic efficacy. The evaluation of this type of experimental therapeutics can greatly benefit from in vivo evaluation to assess biological changes, which can be performed by non-invasive imaging techniques, such as ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) and dynamic contrast enhanced magnetic resonance imaging (DCE-MRI). Here, we will illustrate the methods for inducing atherosclerosis in a rabbit model, the production of anti-inflammatory liposomes and monitoring of therapeutic efficacy of experimental therapeutics with the above-mentioned imaging techniques.

Antitumor efficacy of 34.5ENVE: a transcriptionally retargeted and "Vstat120"-expressing oncolytic virus

Here, we describe the construction and testing of a novel herpes simplex virus type 1 (HSV-1) derived oncolytic virus (OV): 34.5ENVE (viral ICP34.5 Expressed by Nestin promotor and Vstat120 Expressing), for the treatment of cancer. This virus showed significant glioma-specific killing and antiangiogenic effects in vitro and in vivo. Treatment of subcutaneous and intracranial glioma-bearing mice with 34.5ENVE showed a significant increase in median survival of mice in four different glioma models. Histology and dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) revealed reduced microvessel density (MVD) and increased tumoral necrosis in 34.5ENVE-treated tumor tissue compared to control OV-treated tumor tissue. Collectively, these results describe the construction, efficacy, and impact on tumor microenvironment of a transcriptionally driven OV armed with Vstat120 gene expression. These preclinical results will facilitate future clinical testing of 34.5ENVE.

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.