Quantification of human atherosclerotic plaques using spatially enhanced cluster analysis of multicontrast-weighted magnetic resonance images

Targeting Nanostrategies for Imaging of Atherosclerosis

Despite the progress in cardiovascular research, atherosclerosis still represents the main cause of death worldwide. Clinically, the diagnosis of Atherosclerotic Cardiovascular Disease (ASCVD) relies on imaging methodologies including X-ray angiography and computed tomography (CT), which however still fails in the identification of patients at high risk of plaque rupture, the main cause of severe clinical events as stroke and heart attack. Magnetic resonance imaging, which is characterized by very high spatial resolution, could provide a better characterization of atherosclerotic plaque (AP) anatomy and composition, aiding in the identification of “vulnerable” plaques. In this context, hydrogel matrices, which have been demonstrated able to boost relaxometric properties of Gd-based contrast agents (CAs) by the effect of Hydrodenticity, represent a valuable tool towards the precision imaging of ASCVD improving the performance of this class of CAs while reducing systemic toxicity. In particular, hydrogel nanoparticles encapsulating Gd-DTPA can further contribute to providing CA-specific accumulation in the AP by nanoparticle surface decoration triggering an active targeting of the AP with the overall effect of allowing an earlier and more accurate diagnosis. In this work, we tested crosslinked Hyaluronic Acid Nanoparticles (cHANPs) in the complex environment of human atherosclerotic plaque. In addition, the surface of cHANPs was decorated with the antibody anti-CD36 (Ab36-cHANPs) for the active targeting of AP-associated macrophages. Results demonstrate that the Hydrodenticity of cHANPs and Ab36-cHANPs is preserved in this complex system and, preliminarily, that interaction of these probes with the AP is present.

Atherosclerosis inflammation and burden in young adult smokers and vapers measured by PET/MR

BACKGROUND AND AIMS

Electronic cigarette (EC) use is popular among youth, touted as a safer alternative to smoking and promoted as a tool to aid in smoking cessation. EC cardiovascular safety however is not well established. The aim of this study was to examine cardiovascular consequences of EC use by evaluating their effect on the entire atherosclerotic cascade in young adults using noninvasive combined positron emission tomography (PET)/magnetic resonance imaging (MR) and comparing EC use with age matched smokers of traditional cigarettes and nonsmoking controls.

METHODS

Carotid PET/MR was applied to look at vascular inflammation (18-fluorodeoxyglucose (FDG)-PET) and plaque burden (multi-contrast MR of vessel wall) from 60 18-30 year-old subjects (20 electronic cigarette users, 20 traditional smokers and 20 nonsmokers).

RESULTS

Groups were reasonably well balanced in terms of age, gender, demographics, cardiovascular risk and most biomarkers. There were no differences in vascular inflammation as measured by 18-FDG-PET target to background ratios (TBR) between EC users, traditional cigarette smokers and nonsmokers. However, measures of carotid plaque burden - wall area, normalized wall index, and wall thickness - measured from MR were significantly higher in both traditional smokers and EC users than in nonsmokers.

CONCLUSIONS

Young adult EC users, smokers and nonsmokers in our study did not exhibit vascular inflammation as defined by 18-F-FDG-PET TBR max, but smokers and EC users had significantly more carotid plaque burden compared to matched nonsmokers. Results could indicate that vaping does not cause an increase in vascular inflammation as measured by FDG-PET.

Classifications of atherosclerotic plaque components with T1 and T2* mapping in 11.7 T MRI

BACKGROUND AND AIMS

Histopathology is the gold standard for analysis of atherosclerotic plaques but has drawbacks due to the destructive nature of the method. Ex vivo MRI is a non-destructive method to image whole plaques. Our aim was to use quantitative high field ex vivo MRI to classify plaque components, with histology as gold standard.

METHODS

Surgically resected carotid plaques from 12 patients with recent TIA or stroke were imaged at 11.7 T MRI. Quantitative T1/T2* mapping sequences and qualitative T1/T2* gradient echo sequences with voxel size of 30 × 30 × 60 μm3 were obtained prior to histological preparation, sectioning and staining for lipids, inflammation, hemorrhage, and fibrous tissue. Regions of interest (ROI) were selected based on the histological staining at multiple levels matched between histology and MRI. The MRI parameters of each ROI were then analyzed with quadratic discriminant analysis (QDA) for classification.

RESULTS

A total of 965 ROIs, at 70 levels matched between histology and MRI, were registered based on histological staining. In the nine plaques where three or more plaque components were possible to co-localize with MRI, the mean degree of misclassification by QDA was 16.5 %. One of the plaques contained mostly fibrous tissue and lipids and had no misclassifications, and two plaques mostly contained fibrous tissue. QDA generally showed good classification for fibrous tissue and lipids, whereas plaques with hemorrhage and inflammation had more misclassifications.

CONCLUSION

11.7 T ex vivo high field MRI shows good visual agreement with histology in carotid plaques. T1/T2* maps analyzed with QDA is a promising non-destructive method to classify plaque components, but with a higher degree of misclassifications in plaques with hemorrhage or inflammation.

Segmentation of carotid arterial walls using neural networks

BACKGROUND

Automated, accurate, objective, and quantitative medical image segmentation has remained a challenging goal in computer science since its inception. This study applies the technique of convolutional neural networks (CNNs) to the task of segmenting carotid arteries to aid in the assessment of pathology.

AIM

To investigate CNN’s utility as an ancillary tool for researchers who require accurate segmentation of carotid vessels.

METHODS

An expert reader delineated vessel wall boundaries on 4422 axial T2-weighted magnetic resonance images of bilateral carotid arteries from 189 subjects with clinically evident atherosclerotic disease. A portion of this dataset was used to train two CNNs (one to segment the vessel lumen and the other to segment the vessel wall) with the remaining portion used to test the algorithm’s efficacy by comparing CNN segmented images with those of an expert reader.

RESULTS

Overall quantitative assessment between automated and manual segmentations was determined by computing the DICE coefficient for each pair of segmented images in the test dataset for each CNN applied. The average DICE coefficient for the test dataset (CNN segmentations compared to expert’s segmentations) was 0.96 for the lumen and 0.87 for the vessel wall. Pearson correlation values and the intra-class correlation coefficient (ICC) were computed for the lumen (Pearson = 0.98, ICC = 0.98) and vessel wall (Pearson = 0.88, ICC = 0.86) segmentations. Bland-Altman plots of area measurements for the CNN and expert readers indicate good agreement with a mean bias of 1%-8%.

CONCLUSION

Although the technique produces reasonable results that are on par with expert human assessments, our application requires human supervision and monitoring to ensure consistent results. We intend to deploy this algorithm as part of a software platform to lessen researchers’ workload to more quickly obtain reliable results.

Imaging plaque inflammation in asymptomatic cocaine addicted individuals with simultaneous positron emission tomography/magnetic resonance imaging

BACKGROUND

Chronic cocaine use is associated with stroke, coronary artery disease and myocardial infarction, resulting in severe impairments or sudden mortality. In the absence of clear cardiovascular symptoms, individuals with cocaine use disorder (iCUD) seeking addiction treatment receive mostly psychotherapy and psychiatric pharmacotherapy, with no attention to vascular disease (i.e., atherosclerosis). Little is known about the pre-clinical signs of cardiovascular risk in iCUD and early signs of vascular disease are undetected in this underserved population.

AIM

To assess inflammation, plaque burden and plaque composition in iCUD aiming to detect markers of atherosclerosis and vascular disease.

METHODS

The bilateral carotid arteries were imaged with positron emission tomography/magnetic resonance imaging (PET/MRI) in iCUD asymptomatic for cardiovascular disease, healthy controls, and individuals with cardiovascular risk. PET with 18F-fluorodeoxyglucose (¹⁸F-FDG) evaluated vascular inflammation and 3-D dark-blood MRI assessed plaque burden including wall area and thickness. Drug use and severity of addiction were assessed with standardized instruments.

RESULTS

The majority of iCUD and controls had carotid FDG-PET signal greater than 1.6 but lower than 3, indicating the presence of mild to moderate inflammation. However, the MRI measure of wall structure was thicker in iCUD as compared to the controls and cardiovascular risk group, indicating greater carotid plaque burden. iCUD had larger wall area as compared to the healthy controls but not as compared to the cardiovascular risk group, indicating structural wall similarities between the non-control study groups. In iCUD, wall area correlated with greater cocaine withdrawal and craving.

CONCLUSION

These preliminary results show markers of carotid artery disease burden in cardiovascular disease-asymptomatic iCUD. Broader trials are warranted to develop protocols for early detection of cardiovascular risk and preventive intervention in iCUD.

A Convolutional Neural Network for Automatic Characterization of Plaque Composition in Carotid Ultrasound

Characterization of carotid plaque composition, more specifically the amount of lipid core, fibrous tissue, and calcified tissue, is an important task for the identification of plaques that are prone to rupture, and thus for early risk estimation of cardiovascular and cerebrovascular events. Due to its low costs and wide availability, carotid ultrasound has the potential to become the modality of choice for plaque characterization in clinical practice. However, its significant image noise, coupled with the small size of the plaques and their complex appearance, makes it difficult for automated techniques to discriminate between the different plaque constituents. In this paper, we propose to address this challenging problem by exploiting the unique capabilities of the emerging deep learning framework. More specifically, and unlike existing works which require a priori definition of specific imaging features or thresholding values, we propose to build a convolutional neural network (CNN) that will automatically extract from the images the information that is optimal for the identification of the different plaque constituents. We used approximately 90 000 patches extracted from a database of images and corresponding expert plaque characterizations to train and to validate the proposed CNN. The results of cross-validation experiments show a correlation of about 0.90 with the clinical assessment for the estimation of lipid core, fibrous cap, and calcified tissue areas, indicating the potential of deep learning for the challenging task of automatic characterization of plaque composition in carotid ultrasound.

New SPECT and PET radiopharmaceuticals for imaging cardiovascular disease

Nuclear cardiology has experienced exponential growth within the past four decades with converging capacity to diagnose and influence management of a variety of cardiovascular diseases. Single photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI) with technetium-99m radiotracers or thallium-201 has dominated the field; however new hardware and software designs that optimize image quality with reduced radiation exposure are fuelling a resurgence of interest at the preclinical and clinical levels to expand beyond MPI. Other imaging modalities including positron emission tomography (PET) and magnetic resonance imaging (MRI) continue to emerge as powerful players with an expanded capacity to diagnose a variety of cardiac conditions. At the forefront of this resurgence is the development of novel target vectors based on an enhanced understanding of the underlying pathophysiological process in the subcellular domain. Molecular imaging with novel radiopharmaceuticals engineered to target a specific subcellular process has the capacity to improve diagnostic accuracy and deliver enhanced prognostic information to alter management. This paper, while not comprehensive, will review the recent advancements in radiotracer development for SPECT and PET MPI, autonomic dysfunction, apoptosis, atherosclerotic plaques, metabolism, and viability. The relevant radiochemistry and preclinical and clinical development in addition to molecular imaging with emerging modalities such as cardiac MRI and PET-MR will be discussed.

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.

Classification of coronary atherosclerotic plaques ex vivo with T1, T2, and ultrashort echo time CMR

OBJECTIVES

This study sought to determine whether the classification of human coronary atherosclerotic plaques with T1, T2, and ultrashort echo time (UTE) cardiac magnetic resonance (CMR) would correlate well with atherosclerotic plaque classification by histology.

BACKGROUND

CMR has been extensively used to classify carotid plaque, but its ability to characterize coronary plaque remains unknown. In addition, the detection of plaque calcification by CMR remains challenging. Here, we used T1, T2, and UTE CMR to evaluate atherosclerotic plaques in fixed post-mortem human coronary arteries. We hypothesized that the combination of T1, T2, and UTE CMR would allow both calcified and lipid-rich coronary plaques to be accurately detected.

METHODS

Twenty-eight plaques from human donor hearts with proven coronary artery disease were imaged at 9.4-T with a T1-weighted 3-dimensional fast low-angle shot (FLASH) sequence (250-μm resolution), a T2-weighted rapid acquisition with refocused echoes (RARE) sequence (in-plane resolution 0.156 mm), and an UTE sequence (300-μm resolution). Plaques showing selective hypointensity on T2-weighted CMR were classified as lipid-rich. Areas of hypointensity on the T1-weighted images, but not the UTE images, were classified as calcified. Hyperintensity on the T1-weighted and UTE images was classified as hemorrhage. Following CMR, histological characterization of the plaques was performed with a pentachrome stain and established American Heart Association criteria.

RESULTS

CMR showed high sensitivity and specificity for the detection of calcification (100% and 90%, respectively) and lipid-rich necrotic cores (90% and 75%, respectively). Only 2 lipid-rich foci were missed by CMR, both of which were extremely small. Overall, CMR-based classification of plaque was in complete agreement with the histological classification in 22 of 28 cases (weighted κ = 0.6945, p < 0.0001).

CONCLUSIONS

The utilization of UTE CMR allows plaque calcification in the coronary arteries to be robustly detected. High-resolution CMR with T1, T2, and UTE contrast enables accurate classification of human coronary atherosclerotic plaque.

Relationship between particulate matter exposure and atherogenic profile in "Ground Zero" workers as shown by dynamic contrast enhanced MR imaging

In this pilot study, we hypothesize that dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) has the potential to evaluate differences in atherosclerosis profiles in patients subjected to high (initial dust cloud) and low (after 13 September 2001) particulate matter (PM) exposure. Exposure to PM may be associated with adverse health effects leading to increased morbidity. Law enforcement workers were exposed to high levels of particulate pollution after working at "Ground Zero" and may exhibit accelerated atherosclerosis. 31 subjects (28 male) with high (n = 19) or low (n = 12) exposure to PM underwent DCE-MRI. Demographics (age, gender, family history, hypertension, diabetes, BMI, and smoking status), biomarkers (lipid profiles, hs-CRP, BP) and ankle-brachial index (ABI) measures (left and right) were obtained from all subjects. Differences between the high and low exposures were compared using independent samples t test. Using linear forward stepwise regression with information criteria model, independent predictors of increased area under curve (AUC) from DCE-MRI were determined using all variables as input. Confidence interval of 95 % was used and variables with p > 0.1 were eliminated. p < 0.05 was considered significant. Subjects with high exposure (HE) had significantly higher DCE-MRI AUC uptake (increased neovascularization) compared to subjects with lower exposure (LE). (AUC: 2.65 ± 0.63 HE vs. 1.88 ± 0.69 LE, p = 0.016). Except for right leg ABI, none of the other parameters were significantly different between the two groups. Regression model indicated that only HE to PM, CRP > 3.0 and total cholesterol were independently associated with increased neovascularization (in decreasing order of importance, all p < 0.026). HE to PM may increase plaque neovascularization, and thereby potentially indicate worsening atherogenic profile of "Ground Zero" workers.

Intracranial atherosclerotic plaque enhancement in patients with ischemic stroke

BACKGROUND AND PURPOSE

Inflammation of an atherosclerotic plaque is a well-known risk factor in the development of ischemic stroke and myocardial infarction. MR imaging is capable of characterizing inflammation by assessing plaque enhancement in both extracranial carotid arteries and coronary arteries. Our goal was to determine whether enhancing intracranial atherosclerotic plaque was present in the vessel supplying the territory of infarction by using high-resolution vessel wall MR imaging.

MATERIALS AND METHODS

High-resolution vessel wall 3T MR imaging studies performed in 29 patients with ischemic stroke and intracranial vascular stenoses were reviewed for presence and strength of plaque enhancement.

RESULTS

Sixteen patients were studied during the acute phase (<4 weeks from acute stroke), 5 patients in the subacute phase (4-12 weeks), and 8 patients in the chronic phase (>12 weeks) of the ischemic injury. In all of the acute phase patients, atherosclerotic plaque in the vessel supplying the stroke territory demonstrated strong enhancement. There was a trend of decreasing enhancement as the time of imaging relative to the ischemic event increased.

CONCLUSIONS

Strong pathologic enhancement of intracranial atherosclerotic plaque was seen in all patients imaged within 4 weeks of ischemic stroke in the vessel supplying the stroke territory. The strength and presence of enhancement of the atherosclerotic plaque decreased with increasing time after the ischemic event. These findings suggest a relationship between enhancing intracranial atherosclerotic plaque and acute ischemic stroke.

Multi-feature-based plaque characterization in ex vivo MRI trained by registration to 3D histology

We present a new method for automated characterization of atherosclerotic plaque composition in ex vivo MRI. It uses MRI intensities as well as four other types of features: smoothed, gradient magnitude and Laplacian images at several scales, and the distances to the lumen and outer vessel wall. The ground truth for fibrous, necrotic and calcified tissue was provided by histology and μCT in 12 carotid plaque specimens. Semi-automatic registration of a 3D stack of histological slices and μCT images to MRI allowed for 3D rotations and in-plane deformations of histology. By basing voxelwise classification on different combinations of features, we evaluated their relative importance. To establish whether training by 3D registration yields different results than training by 2D registration, we determined plaque composition using (1) a 2D slice-based registration approach for three manually selected MRI and histology slices per specimen, and (2) an approach that uses only the three corresponding MRI slices from the 3D-registered volumes. Voxelwise classification accuracy was best when all features were used (73.3 ± 6.3%) and was significantly better than when only original intensities and distance features were used (Friedman, p < 0.05). Although 2D registration or selection of three slices from the 3D set slightly decreased accuracy, these differences were non-significant.

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

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

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

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

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.

T1-weighted MRI for the detection of coronary artery plaque haemorrhage

OBJECTIVE

Hyperintense areas in atherosclerotic plaques on pre-contrast T1-weighted MRI have been shown to correlate with intraplaque haemorrhage. We evaluated the presence of T1 hyperintensity in coronary artery plaques in coronary artery disease (CAD) patients and correlated results with multi-detector computed tomography (MDCT) findings.

METHODS

Fifteen patients with CAD were included. Plaques detected by MDCT were categorised based on their Hounsfield number. T1-weighted inversion recovery (IR) MRI prepared coronary MRI for the detection of plaque and steady-state free-precession coronary MR-angiography for anatomical correlation was performed. After registration of MDCT and MRI, regions of interest were defined on MDCT-visible plaques and in corresponding vessel segments acquired with MRI. MDCT density and MR signal measurement were performed in each plaque.

RESULTS

Forty-three plaques were identified with MDCT. With IR-MRI 5/43 (12%) plaques were hyperintense, 2 of which were non-calcified and 3 mixed. Average signal-to-noise and contrast-to-noise ratios of hyperintense plaques were 15.7 and 9.1, compared with 5.6 and 1.2 for hypointense plaques. Hyperintense plaques exhibited a significantly lower CT density than hypointense plaques (63.6 vs. 140.8). There was no correlation of plaque signal intensity with degree of stenosis.

CONCLUSION

T1-weighted IR-MRI may be useful for non-invasive detection and characterisation of intraplaque haemorrhage in coronary artery plaques.

Future requirement for arterial wall imaging modalities in the evaluation of novel anti-atherosclerotic therapies

BACKGROUND

While arterial wall imaging has been used to characterize progression of atherosclerosis, there remain limitations to this approach.

SCOPE

A selective overview of emerging modalities to image the artery wall and highlight how they may be used to evaluate emerging anti-atherosclerotic agents.

FINDINGS

Ongoing developments appear to enable assessment of composition and molecular properties of plaque in addition to quantitation of burden. Non-invasiveness and correlation with clinical outcome remains a challenge.

CONCLUSION

New developments in imaging should enhance the ability to provide early characterization of the potential therapeutic efficacy of experimental agents.

Macrophage-specific lipid-based nanoparticles improve cardiac magnetic resonance detection and characterization of human atherosclerosis

OBJECTIVES

We sought to determine whether gadolinium (Gd)-containing lipid-based nanoparticles (NPs) targeting the macrophage scavenger receptor-B (CD36) improve cardiac magnetic resonance (CMR) detection and characterization of human atherosclerosis.

BACKGROUND

Gd-containing lipid-based NPs targeting macrophages have improved MR detection of murine atherosclerosis.

METHODS

Gadolinium-containing untargeted NPs, anti-CD36 NPs, and nonspecific Fc-NPs were created. Macrophages were incubated with fluorescent targeted and nontargeted NPs to determine uptake via confocal microscopy and inductively coupled plasma mass spectroscopy (ICP-MS) quantified Gd uptake. Human aortic specimens were harvested at autopsy. With a 1.5-T scanner, T1, T2, and PDW 3-dimensional scans were performed along with post-contrast scans after 24 h incubation. The T1 and cluster analyses were performed and compared with immunohistopathology.

RESULTS

The NPs had a mean diameter of 125 nm and 14,900 Gd-ions, and relaxivity was 37 mmol/l(-1)s(-1) at 1.5-T and 37 degrees C. Confocal microscopy and ICP-MS demonstrated significant in vitro macrophage uptake of targeted NPs, whereas non-targeted NPs had minimal uptake. On T1 imaging, targeted NPs increased contrast-to-noise ratio (CNR) by 52.5%, which was significantly greater than Fc-NPs (CNR increased 17.2%) and nontargeted NPs (CNR increased 18.7%) (p = 0.001). Confocal fluorescent microscopy showed that NPs target resident macrophages, whereas the untargeted NPs and Fc-NPs are found diffusely throughout the plaque. Targeted NPs had a greater signal intensity increase in the fibrous cap compared with non-targeted NPs.

CONCLUSIONS

Macrophage-specific (CD36) NPs bind human macrophages and improve CMR detection and characterization of human aortic atherosclerosis. Thus, macrophage-specific NPs could help identify high-risk human plaque before the development of an atherothrombotic event.

Structural and functional imaging by MRI

Magnetic resonance imaging is one of the most exciting techniques for noninvasive molecular imaging of the cardiovascular system. The article will describe challenges, solutions and results of magnetic resonance plaque imaging ex-vivo, in the experimental animal and in patients.

Quantitative segmentation of principal carotid atherosclerotic lesion components by feature space analysis based on multicontrast MRI at 1.5 T

The purpose of this paper is to evaluate the capability of feature space analysis (FSA) for quantifying the relative volumes of principal components (thrombus, calcification, fibrous, normal intima, and lipid) of atherosclerotic plaque tissue in multicontrast magnetic resonance images (mc-MRI) acquired in a setup resembling clinical conditions ex vivo. Utilizing endogenous contrast, proton density, T1-weighted, and T2-weighted images were acquired for 13 carotid endarterectomy (CEA) tissues under near-clinical conditions (human 1.5 T GE Excite scanner with sequence parameters comparable to an in vivo acquisition). An FSA algorithm was utilized to segment and quantify the principal components of atherosclerotic plaques. Pilot in vivo mc-MRI images were analyzed in the same way as the ex vivo images for exploring the possible adaptation of this technique to in vivo imaging. Relative abundance of principal plaque components in CEA tissues as determined by mc-MRI/FSA were compared to those measured by histology. Mean differences +/- standard deviations were 5.8 +/- 4.1% for thrombus, 1.5 +/-1.4 % for calcification, 4.0 +/-2.8% for fibrous, 8.2 +/- 10% for normal intima, and 2.4 +/- 2.2% for lipid. Reasonable quantitative agreement between the classification results obtained with FSA and histological data was obtained for near-clinical imaging conditions. Combination of mc-MRI and FSA may have an application for determining atherosclerotic lesion composition and monitoring treatment in vivo.

Imaging atherosclerotic plaque inflammation

Inflammation within atherosclerotic plaques is one of the main drivers of atherosclerotic plaque rupture, which frequently leads to clinical events such as myocardial infarction and stroke. Current gold standard techniques such as X-ray angiography and ultrasound can rapidly report on luminal encroachment but give no readout on inflammatory state of the plaque. We summarize several alternative imaging techniques--CT, MRI, and nuclear imaging--that are close to the clinical arena, and we provide the relative advantages of each.

Identification of different heart tissues from MRI C-SENC images using an unsupervised multi-stage fuzzy clustering technique

PURPOSE

To objectively characterize different heart tissues from functional and viability images provided by composite-strain-encoding (C-SENC) MRI.

MATERIALS AND METHODS

C-SENC is a new MRI technique for simultaneously acquiring cardiac functional and viability images. In this work, an unsupervised multi-stage fuzzy clustering method is proposed to identify different heart tissues in the C-SENC images. The method is based on sequential application of the fuzzy c-means (FCM) and iterative self-organizing data (ISODATA) clustering algorithms. The proposed method is tested on simulated heart images and on images from nine patients with and without myocardial infarction (MI). The resulting clustered images are compared with MRI delayed-enhancement (DE) viability images for determining MI. Also, Bland-Altman analysis is conducted between the two methods.

RESULTS

Normal myocardium, infarcted myocardium, and blood are correctly identified using the proposed method. The clustered images correctly identified 90 +/- 4% of the pixels defined as infarct in the DE images. In addition, 89 +/- 5% of the pixels defined as infarct in the clustered images were also defined as infarct in DE images. The Bland-Altman results show no bias between the two methods in identifying MI.

CONCLUSION

The proposed technique allows for objectively identifying divergent heart tissues, which would be potentially important for clinical decision-making in patients with MI.

Fractionated Feridex and positive contrast: in vivo MR imaging of atherosclerosis

Macrophages have been identified as a critical factor in the pathogenesis of atherosclerosis. Ultrasmall iron oxide particles (USPIOs) have been used to passively target intraplaque macrophages. For dextran-based USPIOs, uptake into macrophages may be modulated by particle size. The aim of the current study was to test the efficacy of fractionated Feridex with respect to macrophage uptake in atherosclerotic rabbits. Fractionation of Feridex resulted in a 15-nm USPIO that exhibited a blood half-life of 15.9 h and liver retention of 6.4%. Blood clearance and liver retention of Feridex was 0.46 h and 60%, following administration of 4.8 mg Fe/kg Feridex. Atherosclerotic rabbits were administered 0.5 or 4.8 mg Fe/kg dosages of either fractionated Feridex or Feridex. MRI was performed at 1.5T over a 24-h time period postinjection. Perls and RAM-11 staining was performed to identify iron deposition. MRI showed a dose-dependent signal loss using conventional gradient echo (GRE) sequences following administration of fractionated Feridex. Even at low dose, significant signal loss was observed that correlated with histology. No signal attenuation or iron deposition was observed in the vessel wall of rabbits administered Feridex. Results of this study suggest that it may be possible to optimize USPIOs for intraplaque macrophage detection.

Glioma grading by using histogram analysis of blood volume heterogeneity from MR-derived cerebral blood volume maps

PURPOSE

To retrospectively compare the diagnostic accuracy of an alternative method used to grade gliomas that is based on histogram analysis of normalized cerebral blood volume (CBV) values from the entire tumor volume (obtained with the histogram method) with that of the hot-spot method, with histologic analysis as the reference standard.

MATERIALS AND METHODS

The medical ethics committee approved this study, and all patients provided informed consent. Fifty-three patients (24 female, 29 male; mean age, 48 years; age range, 14-76 years) with histologically confirmed gliomas were examined with dynamic contrast material-enhanced 1.5-T magnetic resonance (MR) imaging. CBV maps were created and normalized to unaffected white matter (normalized CBV maps). Four neuroradiologists independently measured the distribution of whole-tumor normalized CBVs and analyzed this distribution by classifying the values into area-normalized bins. Glioma grading was performed by assessing the normalized peak height of the histogram distributions. Logistic regression analysis and interobserver agreement were used to compare the proposed method with a hot-spot method in which only the maximum normalized CBV was used.

RESULTS

For the histogram method, diagnostic accuracy was independent of the observer. Interobserver agreement was almost perfect for the histogram method (kappa = 0.923) and moderate for the hot-spot method (kappa = 0.559). For all observers, sensitivity was higher with the histogram method (90%) than with the hot-spot method (55%-76%).

CONCLUSION

Glioma grading based on histogram analysis of normalized CBV heterogeneity is an alternative to the established hot-spot method, as it offers increased diagnostic accuracy and interobserver agreement.

Multimodality imaging of atherosclerosis (magnetic resonance imaging/computed tomography/positron emission tomography-computed tomography)

This review discusses the field of atherosclerosis imaging with magnetic resonance imaging, computed tomography and positron emission tomography techniques, and highlights important publications in this area. Future directions and challenges ahead for plaque imaging are also highlighted.

Magnetic resonance imaging of carotid atherosclerosis: plaque analysis

OBJECTIVES

The Computer-Aided System for CArdiovascular Disease Evaluation (CASCADE) has been developed for streamlined, automated analysis of carotid artery magnetic resonance imaging to measure atherosclerotic plaque burden and composition in vivo. The purpose of this investigation was to assess the performance of CASCADE compared with manual outlining.

METHODS

Magnetic resonance images were obtained from 26 subjects with 16% to 79% carotid artery stenosis by duplex ultrasound who were imaged twice in a 2-week period with a multiple-slice, multiple-contrast magnetic resonance imaging protocol as part of the Outcome of Rosuvastatin treatment on carotid artery atheroma: a magnetic resonance Imaging ObservatioN trial. Manual outlining was used to identify the boundaries of the lumen, wall, necrotic core (NC), and calcifications. After 6 months, the analysis was repeated using CASCADE. For each data set, the contours were used to compute the maximal normalized wall index (NWI; wall area divided by total vessel area), maximal wall thickness (WT), and the average NC and calcified (CA) areas per slice. Agreement between manual and automated reviews and the scan-scan measurement reproducibilities were evaluated.

RESULTS

Pearson correlation between manual and automated analyses was 0.94 for maximal NWI, 0.86 for maximal WT, 0.84 for NC, and 0.96 for CA. Intraclass correlation coefficients for manual and automated analyses were 0.90 and 0.97 for maximal NWI, 0.89 and 0.95 for maximal WT, 0.95 and 0.87 for NC, and 0.96 and 0.94 for CA, respectively.

CONCLUSIONS

Automated analysis tools are capable of providing accurate and reproducible measurements of carotid atherosclerotic burden and composition when compared with manually outlined results.

High-resolution CT imaging of carotid artery atherosclerotic plaques

BACKGROUND AND PURPOSE

Plaque morphologic features have been suggested as a complement to luminal narrowing measurements for assessing the risk of stroke associated with carotid atherosclerotic disease, giving rise to the concept of "vulnerable plaque." The purpose of this study was to evaluate the ability of multidetector-row CT angiography (CTA) to assess the composition and characteristics of carotid artery atherosclerotic plaques with use of histologic examination as the gold standard.

MATERIALS AND METHODS

Eight patients with transient ischemic attacks who underwent carotid CTA and "en bloc" endarterectomy were enrolled in a prospective study. An ex vivo micro-CT study of each endarterectomy specimen was obtained, followed by histologic examination. A systematic comparison of CTA images with histologic sections and micro-CT images was performed to determine the CT attenuation associated with each component of the atherosclerotic plaques. A computer algorithm was subsequently developed that automatically identifies the components of the carotid atherosclerotic plaques, based on the density of each pixel. A neuroradiologist's reading of this computer analysis was compared with the interpretation of the histologic slides by a pathologist with respect to the types and characteristics of the carotid plaques.

RESULTS

There was a 72.6% agreement between CTA and histologic examination in carotid plaque characterization. CTA showed perfect concordance for calcifications. A significant overlap between densities associated with lipid-rich necrotic core, connective tissue, and hemorrhage limited the reliability of individual pixel readings to identify these components. However, CTA showed good correlation with histologic examination for large lipid cores (kappa = 0.796; P < .001) and large hemorrhages (kappa = 0.712; P = .102). CTA performed well in detecting ulcerations (kappa = 0.855) and in measuring the fibrous cap thickness (R(2) = 0.77; P < .001).

CONCLUSION

The composition of carotid atherosclerotic plaques determined by CTA reflects plaque composition defined by histologic examination.

Molecular imaging of alpha v beta3 integrin expression in atherosclerotic plaques with a mimetic of RGD peptide grafted to Gd-DTPA

AIMS

The integrin alpha v beta3 is highly expressed in atherosclerotic plaques by medial and intimal smooth muscle cells and by endothelial cells of angiogenic microvessels. In this study, we have assessed non-invasive molecular magnetic resonance imaging (MRI) of plaque-associated alpha v beta3 integrin expression on transgenic ApoE-/- mice with a low molecular weight peptidomimetic of Arg-Gly-Asp (mimRGD) grafted to gadolinium diethylenetriaminepentaacetate (Gd-DTPA-g-mimRGD). The analogous compound Eu-DTPA-g-mimRGD was employed for an in vivo competition experiment and to confirm the molecular targeting. The specific interaction of mimRGD conjugated to Gd-DTPA or to 99mTc-DTPA with alpha v beta3 integrin was furthermore confirmed on Jurkat T lymphocytes.

METHODS AND RESULTS

The mimRGD was synthesized and conjugated to DTPA. DTPA-g-mimRGD was complexed with GdCl3.6H2O, EuCl3.6H2O, or with [99mTc(CO)3(H2O)3]+. MRI evaluation was performed on a 4.7 T Bruker imaging system. Blood pharmacokinetics of Gd-DTPA-g-mimRGD were assessed in Wistar rats and in c57bl/6j mice. The presence of angiogenic blood vessels and the expression of alpha v beta3 integrin were confirmed in aorta specimens by immunohistochemistry. Gd-DTPA-g-mimRGD produced a strong enhancement of the external structures of the aortic wall and of the more profound layers (possibly tunica media and intima). The aortic lumen seemed to be restrained and distorted. Pre-injection of Eu-DTPA-g-mimRGD diminished the Gd-DTPA-g-mimRGD binding to atherosclerotic plaque and confirmed the specific molecular targeting. A slower blood clearance was observed for Gd-DTPA-g-mimRGD, as indicated by a prolonged elimination half-life and a diminished total clearance.

CONCLUSION

The new compound is potentially useful for the diagnosis of vulnerable atherosclerotic plaques and of other pathologies characterized by alpha v beta3 integrin expression, such as cancer and inflammation. The delayed blood clearance, the significant enhancement of the signal-to-noise ratio, and the low immunogenicity of the mimetic molecule highlight its potential for an industrial and clinical implementation.

Quantification of carotid atherosclerotic plaque components using feature space analysis and magnetic resonance imaging

Atherosclerosis is one of the main causes of cardiovascular disease, accounting for more than one third of all deaths in the United States, there is a growing need to develop non-invasive techniques to assess the severity of atherosclerotic plaque burden. Recent research has suggested that not the size of the atherosclerotic plaque but rather its composition is indicative for plaque rupture as the underlying event of stroke and acute coronary syndrome. With its excellent soft-tissue contrast, magnetic resonance imaging (MRI) is a favored modality for examining plaque composition. In an ex-vivo study, aimed to show the feasibility of quantifying the components of carotid atherosclerotic plaques in-vivo, we acquired multi-contrast MRI images of 13 freshly excised endarterectomy tissues with commercially available MRI sequences and a human surface coil. Feature space analysis (FSA) was utilized in four representative tissues to determine the total relative abundance of calcific, lipidic, fibrotic, thrombotic and normal components as well as in consecutive 2 mm sections across the carotid bifurcation in each tissue. Excellent qualitative agreement between the FSA results and the results obtained from histological methods was observed. This study demonstrates the feasibility of combining MRI with FSA to quantify carotid atherosclerotic plaques in-vivo.

Imaging techniques for the vulnerable coronary plaque

The goal of this article is to illustrate the main invasive and noninvasive diagnostic modalities to image the vulnerable coronary plaque, which is responsible for acute coronary syndrome. The main epidemiologic and histological issues are briefly discussed in order to provide an adequate background. Comprehensive coronary atherosclerosis imaging should involve visualization of the entire coronary artery tree and plaque characterization, including three-dimensional morphology, relationship with the lumen, composition, vascular remodelling and presence of inflammation. No single technique provides such a comprehensive description, and no available modality extensively identifies the vulnerable plaque. In particular, we describe multislice computed tomography, which at present seems to be the most promising noninvasive tool for an exhaustive image-based quantification of coronary atherosclerosis.

Biomarkers of atherosclerosis and the potential of MRI for the diagnosis of vulnerable plaque

Atherosclerosis is a chronic inflammatory vascular disease. As it is an inflammation process, many cellular and molecular events are involved at each step of the progression of atherosclerosis from an early fatty streak lesion to a highly dangerous rupture-prone plaque. Magnetic resonance imaging (MRI) is a well-established diagnostic tool for many kinds of chronic inflammation in various systems and organs, and recent improvements in spatial resolution and contrast strategies make it a promising technique for the characterization of inflammatory vessel walls. The first part of this review will briefly introduce the main cellular and molecular processes involved in atherosclerotic lesions; the second part will focus on the use of high-resolution MRI and present-generation contrast agents for plaque characterization; and the third part will present some recent and ongoing cellular and molecular MRI studies of atherosclerosis.

Carotid plaque classification: defining the certainty with which plaque components can be differentiated

Multicontrast-weighted MRI has the potential to become a powerful tool for assessment of atherosclerotic plaque. However, similarities in MR properties across plaque components limit the certainty with which these components can be differentiated. An understanding of MRI's underlying limitations in distinguishing atherosclerotic plaque components, and optimization of key parameters (including the set of components investigated and contrast weightings used) are required. In this study we analyzed endarterectomy specimens using multicontrast MRI and compared the results with matching histological findings to determine the probability of error, an unbiased measure of the underlying error caused by similarity in the spectral characteristics of components. The total error was >40% when five distinct components were investigated, but this was halved when components with similar functions and intensities were grouped together. When three contrast weightings were used to view plaque, diffusion-weighted imaging (DWI) proved valuable for separating hemorrhage from necrotic core, and "hemorrhage + necrotic" from "loose connective tissue + fibrous tissue." A two-way interaction between contrast weightings and components demonstrated that the value of a contrast can be exploited or marginalized depending on the choice of contrast weightings used.

Reproducibility of high-resolution MRI for the identification and the quantification of carotid atherosclerotic plaque components: consequences for prognosis studies and therapeutic trials

BACKGROUND AND PURPOSE

Although MRI is increasingly proposed to investigate composition of carotid atherosclerosis, its reproducibility has rarely been addressed. We assessed the reproducibility of MRI for the identification and quantification of carotid atherosclerotic plaque components.

METHODS

Using published criteria, 2 readers independently analyzed the carotid MRI (1.5-T MR units with a 4-channel phased-array surface coil, Machnet) of 85 consecutive patients with symptomatic (40% to 69% according to NASCET method) or asymptomatic (60% or greater) carotid artery stenosis enrolled in an ongoing prognostic study. One reader reevaluated all images. Fibrous cap was also secondarily identified independently on T2-weighted and time-of-flight (TOF) images.

RESULTS

Intraobserver agreement was substantial for the identification of calcifications (kappa [kappa]=0.70; 95% CI: 0.54 to 0.86) and lipid-rich/necrotic core (LR/NC) (kappa=0.69; 0.31 to 0.86), almost perfect for hemorrhages (kappa=0.82; 0.68 to 0.96), and moderate (kappa=0.58; 0.27 to 0.88) and fair (kappa=0.33; 0.09 to 0.56) for fibrous cap identification on T2-weighted and TOF images, respectively. Interobserver agreement was substantial for the identification of calcifications (kappa=0.74; 0.59 to 0.89) and hemorrhages (kappa=0.62; 0.43 to 0.81), and moderate for LR/NC (kappa=0.58; 0.20 to 0.95). Agreement was fair for fibrous cap identification on both T2-weighted (kappa=0.28; -0.03 to 0.59) and on TOF images (kappa=0.26; 0.04 to 0.48). Agreement between T2 and TOF images for fibrous cap identification was slight (kappa=0.16; 0.01 to 0.31). Intra- and interobserver reproducibility for quantitative area measurements of vessel, lumen, plaque, LR/NC, and fibrous components was high with intraclass correlation coefficients ranging from 0.73 to 0.99. However, for the LR/NC, the interval delimited by the Bland-Altman graphs was wide in comparison to the mean.

CONCLUSIONS

Vessel and plaque quantification is reproducible. Reproducibility of MRI for identifying and quantifying carotid plaque components is overall acceptable, but there is still significant variability that should be taken into account in the design of prognosis studies and clinical trials. Reproducibility for fibrous cap identification needs to be improved.

High-resolution 3D arteriography of chronic total peripheral occlusions using a T1-W turbo spin-echo sequence with inner-volume imaging

Percutaneous revascularization of peripheral artery chronic total occlusion (CTO) is challenging under X-ray guidance without direct image feedback, due to poor visualization of the obstructed segment and underappreciation of vessel tortuosity. Operators are required to steer interventional devices relatively "blindly," and therefore procedural failure or perforation may occur. Alternatively, MRI may allow complete visualization of both patent and occluded arterial segments. We designed and implemented a 3D high-resolution, T(1)-weighted (T(1)-W) turbo spin-echo (TSE) MRI sequence with inner-volume (IV) imaging to enable detailed peripheral artery CTO imaging. Using this sequence, high-resolution volumes of interest (VOIs) around the vessel were achieved within 5-10 min. This imaging approach may be used for rapid pre- and postprocedural evaluations, and as a 3D roadmap that can be overlaid during real-time X-, MR-, or XMR-guided catheterization. Experiments were successfully performed on a carotid CTO model in swine ex vivo, and in peripheral arteries in normal volunteers and patients in vivo. Delineation of the vascular architecture, including contrast differences between the patent and occluded artery segments, and lesion morphology heterogeneity were visualized.

Diabetes mellitus and noninvasive imaging of atherosclerosis

With the aging of the baby-boom generation, non-insulin-requiring diabetes mellitus (type 2) has become a worldwide pandemic. The causes of the disease are complex and multifactorial and include genetic disposition, lifestyle choices, abnormal aging processes, and alterations in metabolic processes. The impetus for diagnosing the preclinical effects of diabetes generally has been the individual patient's risk for cardiovascular diseases, heart attack, stroke, and peripheral vascular diseases. This review highlights the newer noninvasive imaging methods designed to provide the early prevention of preclinical atherosclerosis.

Gadolinium mixed-micelles: effect of the amphiphile on in vitro and in vivo efficacy in apolipoprotein E knockout mouse models of atherosclerosis

Gadolinium (Gd) micelles are nanoparticles that incorporate phospholipids, surfactants, and lipophilic Gd complexes. Preliminary studies have shown that lipid-based nanoparticles may penetrate atherosclerotic plaque. The aim of the current study was to prepare, characterize, and evaluate in vivo the efficacy of two Gd micelle formulations using apolipoprotein E knockout (ApoE(-/-)) mouse models of atherosclerosis. Gd micelles were prepared using two different amphiphiles but similar GdDTPA lipids, surfactants, and fluorescent labels. The results indicate that the choice of amphiphile may affect the particle size, relaxivity, and blood clearance in wild-type mice (WT). However, the in vivo MR efficacy, with respect to uptake in the vessel wall of ApoE(-/-) mice, was not affected by the amphiphile used. Significant wall enhancement of ApoE(-/-) mice was observed following administration of 0.015 and 0.038 mmol Gd/kg of both micelle formulations. No significant enhancement of the vessel wall of WT mice was observed for any of the dosages or formulations tested. Additionally, liver uptake 24 hr post-injection (p.i.) was not influenced by the choice of amphiphile. The results of this study strongly suggest that liver uptake and wall enhancement may be regulated by the surface properties of the micelle and not by other factors, such as micelle size.

Characterization of coronary atherosclerotic plaque using multicontrast MRI acquired under simulated in vivo conditions

PURPOSE

To compare coronary atherosclerotic plaque characterization using multicontrast MRI on: 1) freshly excised vessels under simulated in vivo conditions, and 2) preserved vessels.

MATERIALS AND METHODS

T1-weighted (T1W), T2-weighted (T2W), proton density-weighted (PDW), and diffusion-weighted (DW) MR images were acquired on 13 freshly excised human coronary arteries from explanted hearts. Vessels were imaged in an MR-compatible tissue culture chamber using a 4.7 Tesla small-bore MR scanner. Eight vessels were then preserved in buffered formalin and rescanned following the same imaging protocol. A three-dimensional spatially penalized fuzzy C-means (3D-SPFCM) technique was applied to classify different plaque constituents. The classification results from vessels under "fresh" and "preserved" conditions were compared with corresponding histological sections.

RESULTS

For most plaque constituents, the plaque characterization results show no significant difference between fresh and preserved scans, and little difference between scans and the histological reference standard. In the case of thrombus, apparent signal changes between fresh and preserved images were identified. Overall, MR scans conducted under preserved conditions provided a 1.8% to 17.5% greater signal-to-noise ratio (SNR) than those conducted in the fresh stage.

CONCLUSION

Preservation of coronary vessels did not alter the contrast between plaque tissues on multicontrast MRI, and did not significantly change the results of plaque constituent characterization.

Magnetic resonance imaging of vulnerable atherosclerotic plaques: Current imaging strategies and molecular imaging probes

The vulnerability or destabilization of atherosclerotic plaques has been directly linked to plaque composition. Imaging modalities, such as magnetic resonance (MR) imaging, that allow for evaluation of plaque composition at a cellular and molecular level, could further improve the detection of vulnerable plaque and may allow for monitoring the efficacy of antiatherosclerotic therapies. In this review we focus on MR imaging strategies for the detection and evaluation of atherosclerotic plaques and their composition. We highlight recent advancements in the development of MR pulse sequences, computer image analysis, and the use of commercially available MR contrast agents, such as gadopentic acid (Gd-DTPA), for plaque characterization. We also discuss molecular imaging strategies that are currently being used to design specific imaging probes targeted to biochemical and cellular markers of atherosclerotic plaque vulnerability.

MRI to detect atherosclerosis with gadolinium-containing immunomicelles targeting the macrophage scavenger receptor

The ability to specifically image macrophages may enable improved detection and characterization of atherosclerosis. In this study we evaluated the in vitro uptake of gadolinium (Gd)-containing immunomicelles (micelles linked to macrophage-specific antibody), micelles, and standard contrast agents by murine macrophages, and sought to determine whether immunomicelles and micelles improve ex vivo imaging of apolipoprotein E knockout (ApoE KO) murine atherosclerosis. Murine RAW 264.7 macrophages were incubated with Gd-DTPA, micelles, and immunomicelles. Cell pellets were prepared and imaged using a 1.5 T MR system with an inversion recovery spin-echo sequence to determine the in vitro T1 values. Ex vivo analysis of mouse aortas was performed using a 9.4T MR system with a high-spatial-resolution sequence (78x39x78 microm3). The T1 value was significantly decreased in cells treated with micelles compared to Gd-DTPA (P<0.0001), and in cells incubated at 4 degrees C with immunomicelles compared to micelles (P<0.05). Ex vivo MRI signal intensity (SI) was significantly increased by 81% and 20% in aortas incubated with immunomicelles and micelles, respectively. Confocal microscopy demonstrated in vitro and ex vivo uptake of fluorescent immunomicelles by macrophages. Immunomicelles and micelles improve in vitro and ex vivo MR detection of macrophages, and may prove useful in the detection of macrophage-rich plaques.

Automated classification of atherosclerotic plaque from magnetic resonance images using predictive models

The information contained within multicontrast magnetic resonance images (MRI) promises to improve tissue classification accuracy, once appropriately analyzed. Predictive models capture relationships empirically, from known outcomes thereby combining pattern classification with experience. In this study, we examine the applicability of predictive modeling for atherosclerotic plaque component classification of multicontrast ex vivo MR images using stained, histopathological sections as ground truth. Ten multicontrast images from seven human coronary artery specimens were obtained on a 9.4 T imaging system using multicontrast-weighted fast spin-echo (T1-, proton density-, and T2-weighted) imaging with 39-mum isotropic voxel size. Following initial data transformations, predictive modeling focused on automating the identification of specimen's plaque, lipid, and media. The outputs of these three models were used to calculate statistics such as total plaque burden and the ratio of hard plaque (fibrous tissue) to lipid. Both logistic regression and an artificial neural network model (Relevant Input Processor Network-RIPNet) were used for predictive modeling. When compared against segmentation resulting from cluster analysis, the RIPNet models performed between 25 and 30% better in absolute terms. This translates to a 50% higher true positive rate over given levels of false positives. This work indicates that it is feasible to build an automated system of plaque detection using MRI and data mining.

Feasibility of in vivo identification of endogenous ferritin with positive contrast MRI in rabbit carotid crush injury using GRASP

In vivo markers that allow for detection of ferritin within atheromatous plaque may be useful for identifying iron-catalyzed hydroxyl-radical formation and subsequent lipid peroxidation. Recently, a positive contrast MR technique--GRadient echo Acquisition for Superparamagnetic particles/suscePtibility (GRASP)--was used to identify the presence of magnetic entities in phantom models. The aim of the current study was to determine the feasibility of using GRASP in conjunction with conventional T(2) (*)-weighted (T(2) (*)W) gradient-echo (GRE) sequences for identifying ferritin/hemosiderin deposition using in vitro and in vivo models of thrombus. In vitro thrombi were prepared by incubating blood with ferritin. MRI was performed using conventional GRE sequences and GRASP. The results indicate that GRASP was able to verify ferritin deposition in in vitro thrombi. In vivo thrombi were created using a crush injury model in rabbits. The signal enhancement obtained using conventional GRE sequences and GRASP was compared with the location of iron deposition by histology. In all of the animals the GRASP signal correlated with signal loss by conventional GRE, and ferritin/hemosiderin deposition by histology. GRASP sequences in combination with conventional GRE sequences may be used to detect the presence of ferritin deposition in in vitro thrombi and in vivo crush-injured rabbit carotid arteries.