MRI of rabbit atherosclerosis in response to dietary cholesterol lowering

Effects of withdrawing an atherogenic diet on the atherosclerotic plaque in rabbits

Lifestyle interventions and pharmacotherapy are the most common of non-invasive treatments for atherosclerosis, but the individual effect of diet on plaques remains unclear. The current study aimed to investigate the effect of withdrawing the atherogenic diet on plaque in the aortas of rabbits. Experimental atheroma was induced in 33 rabbits using a 1% high cholesterol diet for 30 days (H-30 d) or 90 days (H-90 d, baseline group). After 90 days of the atherogenic diet, the remaining animals were divided into four groups: A total of 10 rabbits continued to consume the atherogenic diet for 50 days (H-90 d & H-50 d; n=5) or 140 days (H-90 d & H-140 d; n=5). Another 13 rabbits were switched to a chow diet for 50 days (H-90 d & C-50 d; n=7) or 140 days (H-90 d & C-140 d; n=6). A total of 10 age-matched rabbits in the control groups were fed a chow diet for 90 and 230 days, respectively. The en face or cross-sectional plaque areas were determined using oil red O staining and elastic van Gieson staining. Immunohistochemistry analyses were used to assess the macrophages or smooth muscle cell contents. When fed an atherogenic diet for 90 days, the rabbits' abdominal aortas exhibited severe atherosclerotic lesions (the median en face plaque area was 63.6%). After withdrawing the atherogenic diet, the plaque area did not shrink with feeding the chow diet compared with the baseline, but increased to 71.8 or 80.5% after 50 or 140 days, respectively. After removing cholesterol from the diet, the lipids content in the plaques increased during the first 50 days, and then decreased compared with the baseline group. Furthermore, withdrawing the atherogenic diet increased the total collagen content and the percentage of the smooth muscle cells, alleviated macrophage infiltration, decreased the vulnerable index and promoted the cross-linking of collagen. Feeding the rabbits an atherogenic diet followed by removal of cholesterol from the diet did not lead to the regression of established lesions but instead delayed the progression of the lesions and promoted the stabilization of the plaque.

Large-scale gene analysis of rabbit atherosclerosis to discover new biomarkers for coronary artery disease

Atherosclerosis is the pathological basis of coronary artery disease (CAD) and causes high mortality. Thus, early detection is thought to be crucial in reducing the risk of CAD. Uncovering the mechanisms of the progression and regression of atherosclerosis will provide insights into discovering novel biomarkers to identify subjects at risk for CAD and improve prevention. We established atherosclerosis progression and regression in a rabbit model. Then, we extracted mRNA of the abdominal aorta from control, model and recovery groups to perform gene chip analysis. Candidate biomarkers were screened by large-scale gene analysis and validated in patients with CAD or with CAD recovery by ELISA. The differentially expressed genes in the progression and regression of atherosclerosis were mainly enriched in four clusters. Genes associated with inflammation and extracellular matrix were returned to normal or close-to-normal levels much earlier than genes associated with metabolism and sarcoplasmic proliferation, and they were maintained downregulated or upregulated after feeding a normal diet. We then selected four candidate biomarkers and found that lipoprotein lipase (LPL), bone morphogenetic protein 7 and somatostatin concentrations could indicate CAD diagnosis. In addition, LPL and macrophage cationic peptide 2 can be indicators of the prognosis of CAD. Molecular changes during the progression and regression of atherosclerosis in rabbits were revealed, and candidate regulators were identified. The identified factors could be used as novel biomarkers and targets for improving the diagnosis and prognosis of human CAD in the future.

MRI/optical dual-modality imaging of vulnerable atherosclerotic plaque with an osteopontin-targeted probe based on Fe3O4 nanoparticles

Rupture of vulnerable atherosclerotic plaque is the major pathological cause of luminal thrombosis in acute coronary syndromes. Since foamy macrophages have been identified as a prominent component in vulnerable atherosclerotic lesions and osteopontin (OPN) is reported to be highly expressed in foamy macrophages, OPN could be a potential target for vulnerable atherosclerotic plaque imaging. The current study designed an OPN-specific MRI/optical dual-modality probe to detect vulnerable plaques. Fluorescence imaging revealed that 24 h after injection of the Cy5.5-OPN-DMSA-MNPs (COD-MNPs), the atherosclerotic plaques in carotid artery exhibited significant higher signals in high fat diet (HFD) fed mice in comparison to the group injected with Cy5.5-IgG-DMSA-MNPs (CID-MNPs) or normal diet fed group injected with COD-MNPs (1.87 ± 0.19 × 10¹⁰ vs. 0.74 ± 0.04 × 10¹⁰, 0.73 ± 0.03 × 10¹⁰ p/sec/cm²/sr, P < 0.05). Meanwhile, MRI displayed stronger T₂ contrast enhancement 24 h post-injection at the area of atherosclerotic plaques in the carotid of HFD fed group injected with COD-MNPs than group injected with CID-MNPs or normal diet fed group injected with COD-MNPs (post/pre signal ratio: 0.64 ± 0.04 vs. 0.95 ± 0.02, 0.98 ± 0.01, P < 0.05). As a dual-modality molecular probe, the resulting COD-MNPs conjugates exhibit promising potentials for noninvasive detection of vulnerable atherosclerotic plaque in vivo.

Combined non-invasive assessment of endothelial shear stress and molecular imaging of inflammation for the prediction of inflamed plaque in hyperlipidaemic rabbit aortas

AIMS

To evaluate the incremental value of low endothelial shear stress (ESS) combined with high-resolution magnetic resonance imaging (MRI)- and computed tomography angiography (CTA)-based imaging for the prediction of inflamed plaque.

METHODS AND RESULTS

Twelve hereditary hyperlipidaemic rabbits underwent quantitative analysis of plaque in the thoracic aorta with 256-slice CTA and USPIO-enhanced (ultra-small superparamagnetic nanoparticles, P904) 1.5-T MRI at baseline and at 6-month follow-up. Computational fluid dynamics using CTA-based 3D reconstruction of thoracic aortas identified the ESS patterns in the convex and concave curvature subsegments of interest. Subsegments with low baseline ESS exhibited significant increase in wall thickness and plaque inflammation by MRI, in non-calcified plaque burden by CTA, and developed increased plaque size, lipid and inflammatory cell accumulation (high-risk plaque features) at follow-up by histopathology. Multiple regression analysis identified baseline ESS and inflammation by MRI to be independent predictors of plaque progression, while receiver operating curve analysis revealed baseline ESS alone or in combination with inflammation by MRI as the strongest predictor for augmented plaque burden and inflammation (low ESS at baseline: AUC = 0.84, P < 0.001; low ESS and inflammation by molecular MRI at baseline: AUC = 0.89, P < 0.001).

CONCLUSION

Low ESS predicts progression of plaque burden and inflammation as assessed by non-invasive USPIO-enhanced MRI. Combined non-invasive assessment of ESS and imaging of inflammation may serve to predict plaque with high-risk features.

Early in vivo discrimination of vulnerable atherosclerotic plaques that disrupt: A serial MRI study

BACKGROUND AND AIMS

MRI has been validated as a suitable imaging modality for in vivo, non-invasive detection of atherosclerosis and has provided quantitative predictors of high-risk plaque. Here, we apply serial MRI to monitor the natural progression of plaques over a 3-month period in a rabbit model of atherothrombosis to determine differences over time between plaques that ultimately disrupt to form a luminal mural thrombus and plaques that remain stable.

METHODS

Atherosclerotic plaques were induced in 12 male New Zealand White (NZW) rabbits by aortic endothelial injury and a 1% cholesterol diet. The rabbits were imaged 5 times: at baseline, 1, 2, and 3 months, and 48hr after pharmacological triggering for plaque disruption.

RESULTS

Starting at 2 months, plaques that disrupted after triggering exhibited a higher remodeling ratio (RR, 1.05 ± 0.11 vs 0.97 ± 0.10, p = 0.0002) and a larger vessel wall area (VWA, 6.99 ± 1.54 mm(2) vs 6.30 ± 1.37 mm(2), p = 0.0072) than the stable non-disrupted plaques. The same trends were observed at 3 months: plaques that disrupted had a higher RR (1.04 ± 0.02 vs 0.99 ± 0.01, p = 0.0209), VWA (8.19 ± 2.69 mm(2) vs 6.81 ± 1.60 mm(2), p = 0.0001), and increased gadolinium uptake (75.51 ± 13.77% for disrupted vs 31.02 ± 6.45% for non-disrupted, p = 0.0022).

CONCLUSIONS

MR images of plaques that disrupted revealed larger VWAs, RRs, and increased gadolinium uptake at 2 months and continued progression of these vulnerable features between 2 and 3 months. Non-disrupted plaques had an independent history without these hallmarks of vulnerability. Our results show that MRI can provide early detection of plaques at a higher-risk for luminal thrombosis.

Postangioplasty Restenosis Followed with Magnetic Resonance Imaging in an Atherosclerotic Rabbit Model

Rationale and Objectives. Testing a quantitative, noninvasive method to assess postangioplasty vessel wall changes in an animal model. Material and Methods. Six New Zealand white rabbits were subjected to atherosclerotic injury, including cholesterol-enriched diet, deendothelialization, and percutaneous transluminal angioplasty (PTA) in the distal part of abdominal aorta (four weeks after deendothelialization). The animals were examined with a 1.5T MRI scanner at three times as follows: baseline (six weeks after diet start and two days after PTA) and four weeks and 10 weeks after-PTA. Inflow angiosequence (M2DI) and proton-density-weighted sequence (PDW) were performed to examine the aorta with axial slices. To identify the inner and outer vessel wall boundaries, a dynamic contour algorithm (Gradient Vector Flow Snakes) was applied to the images, followed by calculation of the vessel wall dimensions. The results were compared with histopathological analysis. Results. The wall thickness in the lesion was significantly higher than in the control region at 4 and 10 weeks, reflecting induction of experimentally created after-angioplasty lesion. At baseline, no significant difference between the two regions was present. Conclusions. It is possible to follow the development of vessel wall changes after-PTA with MRI in this rabbit model.

Impact of statins on progression of atherosclerosis: rationale and design of SATURN (Study of Coronary Atheroma by InTravascular Ultrasound: effect of Rosuvastatin versus AtorvastatiN)

BACKGROUND

Previous imaging studies have demonstrated that the beneficial impact of high-dose statins on the progression of coronary atherosclerosis associates with their ability to lower levels of low-density lipoprotein cholesterol (LDL-C) and C-reactive protein (CRP) and to raise high-density lipoprotein cholesterol (HDL-C). The Study of Coronary Atheroma by InTravascular Ultrasound: Effect of Rosuvastatin versus AtorvastatiN (SATURN, NCT00620542) aims to compare the effects of high-dose atorvastatin and rosuvastatin on disease progression.

METHODS

A total of 1385 subjects with established coronary artery disease (CAD) on angiography were randomized to receive rosuvastatin 40 mg or atorvastatin 80 mg for 24 months. The primary efficacy parameter will be the nominal change in percent atheroma volume (PAV), determined by analysis of intravascular ultrasound (IVUS) images of matched coronary artery segments acquired at baseline and at 24-month follow-up. The effect of statin therapy on plasma lipids and inflammatory markers, and the incidence of clinical cardiovascular events will also be assessed. The study does not have the statistical power to directly compare the treatment groups with regard to clinical events.

CONCLUSION

Serial IVUS has emerged as a sensitive imaging modality to assess the impact of treatments on arterial structure. In this study, IVUS will be used to determine whether high-dose statins have different effects on plaque progression.

Imaging of atherosclerosis: magnetic resonance imaging

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

In vivo MR imaging of plaque disruption and thrombus formation in an atherosclerotic rabbit model

Our aim is to introduce an atherosclerotic rabbit model for inducing atherosclerosis lesions in rabbits, and to validate the model in vivo with 3T high resolution magnetic resonance imaging of the thrombosis followed a pharmacologically triggered plaque disruption. Twenty male New Zealand White rabbits were randomly allocated into an experimental group (n = 16) and a control group (n = 4). The aortic wall injuries were induced by an intravascular balloon in the experimental group rabbits after feeding them with a high cholesterol diet for 2 weeks. The pharmacological triggering with Russell's viper venom and histamine was performed after totally 16 weeks of intermittent cholesterol feeding. All of the animals underwent both the pre-trigger and post-trigger MR examinations including TOF, T1WI, T2WI and post contrast T1WI. Euthanasia was performed in all rabbits; gross anatomy and histological specimen of aorta were obtained. MR images were analyzed and compared with histological results. Compared with the control group rabbits, the aorta of the experimental group rabbits in the pre-triggered MR images showed an increased vessel wall thickening, luminal narrowing, and vessel wall enhancement. Fourteen rabbits survived the triggering, and 8 of them developed thrombosis (58.1%). No thrombus was found in the control group. The accuracy of the multi-sequences MR including TOF, T1WI, T2WI and post contrast T1WI was 87.1% (27/31) for detecting thrombus. MR data significantly correlated with the histopathology data for both thrombus length (r = 0.94, P < 0.01) and thrombus location (r = 0.85, P < 0.01), respectively. The study demonstrated that MR reliably determined the plaque disruption and thrombus formation in the atherosclerotic rabbit model.

High-resolution magnetic resonance imaging enhanced with superparamagnetic nanoparticles measures macrophage burden in atherosclerosis

BACKGROUND

Macrophages contribute to the progression and acute complications of atherosclerosis. Macrophage imaging may serve as a biomarker to identify subclinical inflamed lesions, to predict future risk, and to aid in the assessment of novel therapies.

METHODS AND RESULTS

To test the hypothesis that nanoparticle-enhanced, high-resolution magnetic resonance imaging (MRI) can measure plaque macrophage accumulation, we used 3-T MRI with a macrophage-targeted superparamagnetic nanoparticle preparation (monocrystalline iron oxide nanoparticles-47 [MION-47]) in cholesterol-fed New Zealand White rabbits 6 months after balloon injury. In vivo MRI visualized thickened abdominal aortas on both T1- and T2-weighted spin-echo images (T1 spin echo, 20 axial slices per animal; T2 spin echo, 28 slices per animal). Seventy-two hours after MION-47 injection, aortas exhibited lower T2 signal intensity compared with before contrast imaging (signal intensity ratio, aortic wall/muscle: before, 1.44 ± 0.26 versus after, 0.95 ± 0.22; 164 slices; P<0.01), whereas T1 spin echo images showed no significant change. MRI on ex vivo specimens provided similar results. Histological studies colocalized iron accumulation with immunoreactive macrophages in atheromata. The magnitude of signal intensity reduction on T2 spin echo in vivo images further correlated with macrophage areas in situ (150 slices; r=0.73). Treatment with rosuvastatin for 3 months yielded diminished macrophage content (P<0.05) and reversed T2 signal intensity changes (P<0.005). Signal changes in rosuvastatin-treated rabbits correlated with reduced macrophage burden (r=0.73). In vitro validation studies showed concentration-dependent MION-47 uptake by human primary macrophages.

CONCLUSION

The magnitude of T2 signal intensity reduction in high-resolution MRI after administration of superparamagnetic phagocytosable nanoparticles can assess macrophage burden in atheromata, providing a clinically translatable tool to identify inflamed plaques and to monitor therapy-mediated changes in plaque inflammation.

High-resolution magnetic resonance imaging enhanced with superparamagnetic nanoparticles measures macrophage burden in atherosclerosis

BACKGROUND

Macrophages contribute to the progression and acute complications of atherosclerosis. Macrophage imaging may serve as a biomarker to identify subclinical inflamed lesions, to predict future risk, and to aid in the assessment of novel therapies.

METHODS AND RESULTS

To test the hypothesis that nanoparticle-enhanced, high-resolution magnetic resonance imaging (MRI) can measure plaque macrophage accumulation, we used 3-T MRI with a macrophage-targeted superparamagnetic nanoparticle preparation (monocrystalline iron oxide nanoparticles-47 [MION-47]) in cholesterol-fed New Zealand White rabbits 6 months after balloon injury. In vivo MRI visualized thickened abdominal aortas on both T1- and T2-weighted spin-echo images (T1 spin echo, 20 axial slices per animal; T2 spin echo, 28 slices per animal). Seventy-two hours after MION-47 injection, aortas exhibited lower T2 signal intensity compared with before contrast imaging (signal intensity ratio, aortic wall/muscle: before, 1.44 ± 0.26 versus after, 0.95 ± 0.22; 164 slices; P<0.01), whereas T1 spin echo images showed no significant change. MRI on ex vivo specimens provided similar results. Histological studies colocalized iron accumulation with immunoreactive macrophages in atheromata. The magnitude of signal intensity reduction on T2 spin echo in vivo images further correlated with macrophage areas in situ (150 slices; r=0.73). Treatment with rosuvastatin for 3 months yielded diminished macrophage content (P<0.05) and reversed T2 signal intensity changes (P<0.005). Signal changes in rosuvastatin-treated rabbits correlated with reduced macrophage burden (r=0.73). In vitro validation studies showed concentration-dependent MION-47 uptake by human primary macrophages.

CONCLUSION

The magnitude of T2 signal intensity reduction in high-resolution MRI after administration of superparamagnetic phagocytosable nanoparticles can assess macrophage burden in atheromata, providing a clinically translatable tool to identify inflamed plaques and to monitor therapy-mediated changes in plaque inflammation.

Cardiovascular diseases as target for imaging

Recent pathophysiological findings have lead to new concepts to identify patients at risk for cardiovascular disease using systemic serum markers or new imaging methodology. New probe technology and progress in imaging techniques have set the base for development of molecular imaging concepts in the cardiovascular systems. The aim of these new imaging techniques is the detection of active biological processes in cardiovascular systems combining specific probes with contrast agents for MRI, SPECT or PET. There are promising strategies mostly in preclinical tests, which will prove clinical applicability in the near future.

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.

Optimal region-of-interest settings for tissue characterization based on ultrasonic elasticity imaging

Pathologic changes in arterial walls significantly influence their mechanical properties. We have developed a correlation-based method, the phased tracking method, for measurement of the regional elasticity of the arterial wall. Using this method, elasticity distributions of lipids, blood clots, fibrous tissue and calcified tissue were measured by in-vitro experiments of excised arteries (mean +/- SD: lipids, 89 +/- 47 kPa; blood clots, 131 +/- 56 kPa; fibrous tissue, 1022 +/- 1040 kPa; calcified tissue, 2267 +/- 1228 kPa). It was found that arterial tissues can be classified into soft tissues (lipids and blood clots) and hard tissues (fibrous tissue and calcified tissue) on the basis of their elasticity. However, there are large overlaps between elasticity distributions of lipids and blood clots and those of fibrous tissue and calcified tissue. Thus, it was difficult to differentiate lipids from blood clots and fibrous tissue from calcified tissue by setting a threshold for a single elasticity value. Therefore, we previously proposed a tissue classification method using the elasticity distribution in each small region. In this method, the elasticity distribution of each small region of interest (ROI) (not a single pixel) in an elasticity image is used to classify lipids, blood clots, fibrous tissue and calcified tissue by calculating the likelihood function for each tissue. In the present study, the optimum size of the ROI and threshold T(o) for the likelihood function were investigated to improve the tissue classification. The ratio of correctly classified pixels to the total number of classified pixels was 29.8% when the size of a small region was 75 microm x 300 microm (a single pixel). The ratio of correctly classified pixels became 35.1% when the size of a small region was 1,500 microm x 1,500 microm (100 pixels). Moreover, a region with an extremely low likelihood with respect to all tissue components was defined as an unclassified region by setting threshold T(o) for the likelihood function to 0.21. The tissue classification of the arterial wall was improved using the elasticity distribution of a small region whose size was larger than the spatial resolution (800 microm x 600 microm) of ultrasound. In this study, the arteries used in construction of the elasticity databases were classified into each tissue using the constructed elasticity databases. Other arteries, which are not used for constructing the elasticity databases, should be classified in future work to thoroughly show the effectiveness of the proposed method.

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.

Aortic plaque imaging and monitoring atherosclerotic plaque interventions

The atherosclerotic process that results in coronary artery disease (CAD) is recognized to be a generalized process that may involve the entire vasculature. The association between CAD and atherosclerotic plaques in the thoracic aorta has often been reported using transesophageal echocardiography. An autopsy study showed plaques in the abdominal aorta, but not in the thoracic aorta, to be severe in patients with cardiac events. However, studies evaluating an association between abdominal aortic plaques and CAD are scarce. Recently, magnetic resonance imaging (MRI) has become a useful tool for the noninvasive evaluation of atherosclerotic plaques in both the thoracic and abdominal aortas. Plaques in the thoracic and abdominal aortas were found to be characteristically associated with hypercholesterolemia and smoking, respectively, suggesting different susceptibilities to risk factors. Because patients have various risk factors, it seems to be preferable to evaluate atherosclerosis in multiple vascular beds than in just 1 bed. Magnetic resonance imaging can evaluate atherosclerosis in multiple vascular beds in the same examination session. Complex aortic plaques, especially in the abdominal aorta, were found to be associated with myocardial infarction and complex coronary lesions, suggesting a link between aortic and coronary plaque instability. Aortic MRI may thus be useful for identifying vulnerable patients. Moreover, MRI is a powerful tool to serially evaluate plaque progression and regression. Intensive lipid-lowering therapy can regress aortic plaques, but the susceptibility to lipid lowering and the process of plaque regression may differ between the thoracic and abdominal aortic plaques.

Targeted contrast agent helps to monitor advanced plaque during progression: a magnetic resonance imaging study in rabbits

OBJECTIVE

Gadofluorine M has been reported to enhance early atherosclerotic plaque signals in magnetic resonance imaging (MRI). The aim of this study was to examine the use of Gadofluorine M to monitor the progression of advanced plaques in a rabbit model.

METHODS

Focal advanced atherosclerosis was induced in the right femoral arteries of 6 New Zealand white rabbits using a combination of cholesterol-enriched diet, and sequential air-desiccation, and balloon-overstretch injury. MRI with conventional 3 contrasts (T1, T2, and proton density [PD]) was performed to monitor the progression of the atherosclerotic plaques with 2 MRI scans separated by 4 to 8 weeks. Gadofluorine M was given intravenously to the rabbits 24 hours before the first MRI scans, and before (n = 3) or during (n = 3) the second MRI scan. The left femoral arteries were used as a control. Histopathologic images localized individual plaque components.

RESULTS

The advanced plaque displayed multilayered neointima that included foam cells, smooth muscle cells, and extracellular matrix. The separate image contrasts offered similar T1-weighted enhancement patterns, but the combination of all 3 contrasts helped to delineate plaque and lumen boundaries. Gadofluorine M strongly enhanced neointima areas with an image contrast (contrast-to-noise ratio [CNR]) of approximately 15, versus 2 in the control femoral arterial wall. With improved images, significant changes in neointima and total plaque volumes over the 4 to 8 weeks between scans could be identified. Gadofluorine M remained within the plaques with significant image enhancements (contrast-to-noise ratio = 5.8) for 2 months after a single injection.

CONCLUSION

This preliminary study in rabbits indicated that Gadofluorine M provides specific enhancements of components associated with advanced atherosclerotic plaques and may help to monitor the progression of the plaque in a rabbit model of atherogenesis.

MRI of early- and late-stage arterial remodeling in a low-level cholesterol-fed rabbit model of atherosclerosis

PURPOSE

To monitor early- and late-stage arterial remodeling following low-level cholesterol (CH) feeding in rabbits using a standardized MRI protocol.

MATERIALS AND METHODS

New Zealand White rabbits were fed a CH diet (0.25% w/w) (n = 15) or normal chow (n = 6) and imaged either at 0, 2, 6, 8, and 11 months ("early-stage") or 12, 14, 16, 18, and 20 months ("late-stage"). T2-weighted fast-spin-echo images ( approximately 200 microm in-plane resolution) of aortic lesions were collected using either a 1.5 or 3.0T MR scanner interfaced with a customized surface RF coil. Luminal (LA), outer vessel wall boundary (OVBA), and vessel wall areas (VWA) were assessed.

RESULTS

Among CH-fed animals in the early-stage group, increased VWA associated with decreased OVBA and a more pronounced decrease in LA was first detectable at 8 months. These changes became more evident between 8 and 11 months. In the late-stage group, lesions continued to grow in response to CH-feeding, as VWA significantly increased at regular 2-month intervals. Beyond 16 months, signal intensity differences (reflecting increased lesion complexity) within the vessel wall were noted.

CONCLUSION

This often-overlooked rabbit model combined with customized MR technology holds tremendous promise for studying the natural progression, regression, and remodeling of atherosclerotic lesions.

Fenofibrate induces plaque regression in hypercholesterolemic atherosclerotic rabbits: In vivo demonstration by high-resolution MRI

INTRODUCTION

Fenofibrate has shown to reduce major cardiovascular events and slow angiographic progression of coronary atherosclerosis. The postulated mechanism of action is via the activation of peroxisomal proliferator-activated receptor-alpha (PPAR-alpha), a nuclear transcription factor that controls a variety of cellular functions. We investigated the anti-atherogenic effects of fenofibrate on previously established experimental atherosclerotic lesions.

METHOD

Atherosclerotic lesions were induced in the abdominal aorta of New Zealand white (NZW) rabbits (n=19) by a combination of a double-balloon injury and a 9-month hypercholesterolemic diet. The rabbits were randomized into placebo or fenofibrate group. The corresponding treatments were added to the hypercholesterolemic diet. All rabbits underwent MRI examination at randomization and after 6 months of treatment, and were then sacrificed for histopathology.

RESULTS

LDL-cholesterol was similarly elevated at randomization and follow-up, and was not significantly modified by fenofibrate therapy. HDL-cholesterol decreased (-27+/-10%, p=0.04) in the placebo and increased (+36.8+/-2%, p=0.04) in the fenofibrate group. MRI showed comparable vessel wall area (VWA) at randomization in both groups. At 15months, a significant increase in VWA was seen in the placebo group (15+/-4%, p=0.007), while fenofibrate treatment was associated with a regression (-11+/-4%, p=0.041) of previously established lesions. Fenofibrate also decreased macrophage and increased smooth muscle cell/collagen content of atherosclerotic lesions.

CONCLUSION

MRI measurements can, in conjunction with in vitro histological measurements, contribute to the understanding of the actions of pharmacologic agents in experimental models of atherosclerosis. Fenofibrate significantly regresses atherosclerotic lesions and induced changes in plaque composition associated with a more "stable" phenotype (reduced macrophages and increased SMC). These observations support the potential anti-atherogenic effects of PPAR-alpha agonists.

Noninvasive imaging of atherosclerotic vessels by MRI for clinical assessment of the effectiveness of therapy

Atherosclerosis and its thrombotic complications are the major cause of morbidity and mortality in the industrialized countries. Despite advances in our understanding of the mechanisms of pathogenesis and new treatment modalities, the absence of an adequate noninvasive method for early detection limits prevention or treatment of patients with various degrees and localizations of atherothrombotic disease. The ideal clinical imaging modality for atherosclerosis should be safe, inexpensive, noninvasive or minimally invasive, accurate, and reproducible, thus allowing longitudinal studies in the same patients. Additionally, the results should correlate with the extent of atherosclerotic disease and have high predictive values for clinical events. In vivo, high-resolution magnetic resonance imaging (MRI) has recently emerged as one of the most promising techniques for the noninvasive study of atherothrombotic disease in several vascular beds such as the aorta, the carotid arteries, and the coronary arteries. Most importantly MRI can be used to characterize plaque composition as it allows the discrimination of lipid core, fibrosis, calcification, and intra-plaque hemorrhage deposits. MRI findings have been extensively validated against pathology in ex vivo studies of carotid, aortic, and coronary artery specimens obtained at autopsy and using experimental models of atherosclerosis. In vivo MRI of carotid arteries of patients referred for endarterectomy has shown a high correlation with pathology and with previous ex vivo results. A recent study in patients with plaques in the thoracic aorta showed that compared with transesophageal echocardiography plaque composition and size are more accurately characterized and measured using in vivo MRI. The composition of the plaque rather than the degree of stenosis determines the patient outcome. Therefore, a reliable noninvasive imaging tool able to detect early atherosclerotic disease in the various regions and identify the plaque composition is clinically desirable. MRI has potential in the detection arterial thrombi and in the definition of thrombus age. MRI has been used to monitor plaque progression and regression in several animal model of atherosclerosis and more recently in human. Advances in diagnosis prosper when they march hand-in-hand with advances in treatment. We stand at the threshold of accurate noninvasive assessment of atherosclerosis. Thus, MRI opens new strategies ranging from screening of high-risk patients for early detection and treatment as well as monitoring the target areas for pharmacological intervention.

Role of Magnetic Resonance and Intravascular Magnetic Resonance in the Detection of Vulnerable Plaques

Noninvasive magnetic resonance imaging (MRI) has been used to determine vascular three-dimensional structure, detect the presence of subclinical atherosclerotic disease in high-risk patient subgroups, and optimize and follow therapy in individual patients. The outstanding soft-tissue-characterizing capabilities of MRI permit depiction of various components of atherothrombotic plaque, including lipid, fibrous tissue, calcium, and thrombus formation. However, noninvasive MRI visualization of coronary arteries is currently limited by the small size of the coronary arteries, the deep arterial location, and arterial motion. The combination of MR imaging and molecular probes offers exciting possibilities of direct visualization of biologic processes within atherosclerotic tissue. The self-contained intravascular MRI probe appears to hold promise in the identification of high-risk coronary atherosclerotic lesions with increased superficial lipid content.

Effects of Aggressive Versus Conventional Lipid-Lowering Therapy by Simvastatin on Human Atherosclerotic Lesions

OBJECTIVES

This study sought to compare the effects of aggressive and conventional lipid lowering by two different dosages of the same statin on early human atherosclerotic lesions using serial noninvasive magnetic resonance imaging (MRI).

BACKGROUND

Regression of atherosclerotic lesions by lipid-lowering therapy has been reported.

METHODS

Using a double-blind design, newly diagnosed hypercholesterolemic patients (n = 51) with asymptomatic aortic and/or carotid atherosclerotic plaques were randomized to 20 mg/day (n = 29) or 80 mg/day (n = 22) simvastatin. Mean follow-up was 18.1 months. A total of 93 aortic and 57 carotid plaques were detected and sequentially followed up by MRI every six months after lipid-lowering initiation. The primary MRI end point was change in vessel wall area (VWA) as a surrogate for atherosclerotic burden.

RESULTS

Both statin doses reduced significantly total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C) versus baseline (p < 0.001). Total cholesterol decreased by 26% versus 33% and LDL-C by 36% versus 46% in the conventional (20 mg) versus aggressive (80 mg) simvastatin groups, respectively. Although the simvastatin 80-mg group had significantly higher baseline TC and LDL-C levels, both groups reached similar absolute values after treatment. A significant reduction in VWA was already observed by 12 months. No difference on vascular effects was detected between the randomized doses. Post-hoc analysis showed that patients reaching mean on-treatment LDL-C < or = 100 mg/dl had larger decreases in plaque size.

CONCLUSIONS

Effective and protracted lipid-lowering therapy with simvastatin is associated with a significant regression of atherosclerotic lesions. No difference in vessel wall changes was seen between high and conventional doses of simvastatin. Changes in vessel wall parameters are more related to LDL-C reduction rather than to the dose of statin.

Atherosclerosis regression and TP receptor inhibition: effect of S18886 on plaque size and composition—a magnetic resonance imaging study

AIMS

Endothelial dysfunction, platelet hyperactivity, and inflammation play a crucial role in atherogenesis. A growing body of evidence suggests that inhibition of the thromboxane A2 (TxA2 or TP) receptor may improve endothelial function and reduce the inflammatory component of atherosclerosis in addition to its demonstrated antiplatelet activity. Consequently, we sought to assess the effect of a novel TP receptor antagonist S18886, on atherosclerotic lesion progression and composition by serial non-invasive magnetic resonance imaging (MRI).

METHODS AND RESULTS

S18886 was compared with control in an experimental model of established aortic atherosclerosis in New Zealand White rabbits (n=10). The animals underwent MRI of the abdominal aorta at the time of randomization and at the end of treatment. Subsequently, animals were euthanized and specimens were stained for histopathology and immunohistochemistry with anti-alpha-actin antibodies for vascular smooth muscle cells (VSMC), anti-RAM-11 for macrophages, anti-caspase-3 for apoptotic cells, anti-MMP-1 for metalloproteinases, and anti-endothelin-1 (ET-1) as a marker of endothelial dysfunction. MRI analysis revealed a significant reduction in total vessel area (TVA) and vessel wall area (VWA) in the S18886 group (P<0.05). Immunostaining analysis showed a significant decrease in RAM-11, caspase-3, MMP-1, ET-1 and an increase in alpha-actin in the treated group (P<0.05 vs. control).

CONCLUSION

Inhibition of the TP receptor by S18886 causes a regression of advanced atherosclerotic plaques. In addition, the reduction in the markers for macrophages, apoptotic cells, metalloproteinases, and endothelin-1 and the increase in VSMC, suggests that S18886 may not only halt the progression of atherosclerosis, but also transform lesions towards a more stable phenotype. The possibility of combining antithrombotic and antiatherosclerotic activity by means of the administration of TP inhibitors deserves further investigation in a clinical setting.

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

One of the current limitations of magnetic resonance imaging (MRI) is the lack of an objective method to classify plaque components. Here we present a cluster analysis technique that can objectively quantify and classify MR images of atherosclerotic plaques. We obtained three-dimensional (3D) images from 12 human coronary artery specimens on a 9.4T imaging system using multicontrast-weighted fast spin-echo (T1-, proton density-, and T2-weighted) imaging with an isotropic voxel size of 39 micro. Spatially enhanced cluster analysis (SECA) was performed on multicontrast MR images, and the resulting segmentation was evaluated against histological tracings. To visualize the overall structure of plaques, the MR images were rendered in 3D. The specimens exhibited lesions of American Heart Association (AHA) plaque classification types I-VI. Both MR images and histological sections were independently reviewed, categorized, and compared. Overall, the classification obtained from the cluster-analyzed MR and histopathology images showed very good agreement for all AHA types (92%, Cohen's kappa = 0.89, P < 0.0001). All plaque types were identified and quantified by SECA with a high degree of correlation between cluster-analyzed MR and manually traced histopathology data. MRI combined with SECA provides an objective method for atherosclerotic plaque component characterization and quantification.

Short term arterial remodelling in the aortae of cholesterol fed New Zealand white rabbits shown in vivo by high-resolution magnetic resonance imaging — implications for human pathology

High-resolution, non-invasive imaging methods are required to monitor progression and regression of atherosclerotic plaques. We investigated the use of MRI to measure changes in plaque volume and vessel remodelling during progression and regression of atherosclerosis in New Zealand White rabbits. Atherosclerotic lesions were induced in the abdominal aorta by balloon injury and cholesterol feeding. MR images (2D) of the abdominal aorta were acquired with cardiac and respiratory gating using a fast spin echo sequence with and without fat-suppression. In an initial study on rabbits treated for 30 weeks we imaged the aortae with a spatial resolution of 250x250 micrometers with a slice thickness of 2 mm and achieved a close correlation between MRI-derived measurements and those made on perfusion pressure-fixed histological sections (r(1) = 0.83, slope p(1) < 0.01). We subsequently imaged 18 rabbits before and periodically during 12 weeks of cholesterol feeding (progression) followed by 12 weeks on normal diet (regression). Aortic wall (atherosclerotic lesion) volume increased significantly during progression and decreased during regression. In contrast, lumen volume increased during progression and did not change during regression. In conclusion, this study confirms that non-invasive, high-resolution MRI can be used to monitor progression and regression of atherosclerosis, each within 3 months and shows, for the first time in a short-term model, that positive remodelling occurs early during progression and persists through regression of atherosclerotic lesions.

Magnetic resonance imaging and computed tomography in assessment of atherosclerotic plaque

The two most promising noninvasive imaging modalities for the study of atherosclerosis are magnetic resonance imaging (MRI) and computed tomography (CT). Both have been shown to be capable of imaging vessel wall structures and differentiating various stages of atherosclerotic wall changes. MRI has been applied in various in vivo human studies to image atherosclerotic plaques in coronary arteries, carotid arteries, and aorta. The latest generation of multidetector row computed tomography (MDCT) systems allows for the noninvasive characterization of different plaque components in various vascular structures. MDCT allows evaluation of the whole arterial vasculature. In addition, MDCT has the ability to visualize the vessel wall and to give a quantitative measurement of calcified and noncalcified plaque. Using either technique, the repeatable, noninvasive study of atherosclerotic disease during its natural history and after therapeutic intervention will enhance our understanding of disease progression and regression. MDCT and MRI, therefore, may help in selecting appropriate treatments.

MRI of coronary artery atherosclerosis in rabbits: Histopathology-MRI correlation and atheroma characterization

BACKGROUND AND OBJECTIVES: We report in vivo magnetic resonance imaging (MRI) characteristics and histopathology correlation of the thrombus formation in atherosclerosis the rabbit animal model. DESIGN AND METHODS: Atherosclerosis was induced in white male rabbits with vegetable ghee followed oxidized diet. Baseline MRI of atherosclerosis-recruited rabbits was done and later animals were used for atheroma histopathology characterization. Contiguous cross-sectional T2-weighted fast spin echo MRI images were compared by coronary histopathology. In all animals, coronary aortic wall thickening and atheroma size was measured using MRI. RESULTS: MRI images and digitized histological sections confirmed intraluminal thrombus in 6 (67%) of the 9 animals. MRI data showed correlation with the histopathology for aortic wall thickness (R2 = 0.82, P < 0.0001), lumen area (R2 = 0.88, P < 0.0001) and plaque size (R2 = 0.77, P < 0.0001). Optimized TE and TR parameters and multicontrast enhancement generated better MRI visibility of vulnerable plaque components. The MRI data evaluated % stenosis, plaque burden. Frequency of plaques, plaque height in aorta and coronary artery atheroma was also assessed by histology. In vivo, MRI determined the presence and size of the thrombus in this animal model of atherosclerosis and histopathology defined the plaque disruption. CONCLUSION: The combination of in vivo MRI and comparison with histopathology images of rabbit coronary thrombus may be a research tool for understanding of the pathogenesis of acute coronary plaques.

Evaluation of a soft atherosclerotic lesion in the rabbit aorta by an invasive IVUS method versus a non-invasive MRI technology

The intravascular ultrasound (IVUS) modality has rapidly gained acceptance for the measurement of arterial plaque thickness and for anatomical characterization. In view, however, of the growing interest in the direct assessment of plaque size after therapeutic modalities directly reducing plaque burden, a non-invasive method such as magnetic resonance imaging (MRI) may be of help for repeated evaluations. The two methods were compared directly on a focal plaque developed at the abdominal aortic level by a combination of local electric lesion followed by a hypercholesterolemic diet. The plaque was fully characterized histopathologically at intervals up to 120 days from lesion induction, and maximal plaque formation was detected at 90 days from electrical injury. Plaques could be well assessed by IVUS at each time point analyzed and data correlated very well to histopathologic findings (r = 0.969, P = 0.0014). The MRI technology provided reliable determinations only at 90 days after lesion induction, i.e. at maximal plaque formation, with excellent correspondence to IVUS determinations (r = 0.989, P = 0.0111). Altogether these findings indicate that the non-invasive MRI technology, when applied to the analysis of arterial plaques of adequate size, can be used successfully for plaque determination, with results comparable to the invasive IVUS technique.

The selective peroxisomal proliferator-activated receptor-gamma agonist has an additive effect on plaque regression in combination with simvastatin in experimental atherosclerosis

OBJECTIVES

We sought to investigate the anti-atherogenic effects of a selective peroxisomal proliferator-activated receptor-gamma (PPAR-gamma) agonist and simvastatin, as well as their combination, over time, in a rabbit model of experimental atherosclerosis.

BACKGROUND

The PPARs are nuclear transcription factors that control a variety of cellular functions, with the potential effects required to induce plaque regression and stabilization.

METHODS

Atherosclerosis was induced in rabbits (n = 37) by the combination of double-balloon injury and a nine-month high-cholesterol (HC) diet. The rabbits were randomized into a continued HC diet, a normal chow (NC) diet, NC plus simvastatin, NC plus PPAR-gamma agonist, and NC plus simvastatin plus PPAR-gamma agonist. All rabbits underwent magnetic resonance imaging (MRI) at randomization and after six months of treatment and were then sacrificed for histopathologic study.

RESULTS

All groups had a similar vessel wall area by MRI (8.45 +/- 0.65 mm(2), p = NS between groups) at randomization. Significant progression was seen in the HC diet group (15 +/- 4%, p < 0.01). In the NC and NC plus PPAR-gamma agonist groups, progression was abolished (-2.5 +/- 3% and -4.5 +/- 5%, respectively; p = NS). The NC plus simvastatin and NC plus simvastatin plus PPAR-gamma agonist groups had significant plaque regression (-12 +/- 4% [p < 0.05] and -22 +/- 4% [p < 0.01], respectively). Regression was independent of plasma lipid levels. All NC groups had similar lipid profiles at the end of treatment. Histopathologic analysis of the NC groups showed a decreased macrophage content and matrix metalloproteinase activity and an increased smooth muscle cell/collagen content of lesions.

CONCLUSIONS

Our data indicate that normalization of plasma lipid levels abolishes progression of atherosclerosis. Simvastatin elicits regression of atherosclerotic lesions, and the combination simvastatin plus PPAR-gamma agonist has additive regression effects on plaque. This is paralleled by structural alterations in plaque composition, which may increase plaque stability. These observations support the beneficial effects of statins on atherosclerosis and show additional anti-atherogenic benefits of combining a PPAR-gamma agonist with simvastatin.

Vascular MRI in the diagnosis and therapy of the high risk atherosclerotic plaque

Disruption of a high risk plaque is known as the primary cause of cardiovascular events. Characterization of arterial wall components has become an essential adjunct in the identification of patients with plaques prone to rupture. Magnetic Resonance Imaging (MRI) has been revealed as one of the noninvasive tools possibly capable of identifying and characterizing high risk atherosclerotic plaque. MRI may facilitate diagnosis, and guide and serially monitor interventional and pharmacological treatment of atherosclerotic disease. In addition, it permits the simultaneous assessment of the anatomy, morphology, and hemodynamics for the study of flow-induced atherogenesis. It possibly will identify asymptomatic patients with subclinical atherosclerosis. This has potential significance for the improvement of strategies in primary and secondary prevention.

In vivo accuracy of multisequence MR imaging for identifying unstable fibrous caps in advanced human carotid plaques

PURPOSE

To evaluate the in vivo accuracy of a multisequence MRI technique for prospectively identifying one feature of the vulnerable plaque-an unstable fibrous cap-in human carotid atherosclerosis.

MATERIALS AND METHODS

The carotid arteries of 18 endarterectomy patients were preoperatively imaged in a 1.5 T scanner using a multisequence protocol that generated four contrast weightings (3D time of flight (ToF), T1, proton density (PD), and T2) at each slice location. With the use of previously published MR criteria, the images of the vessel wall were first examined for evidence of an unstable fibrous cap. The imaging findings were then correlated with the histology from the surgical specimens.

RESULTS

A blinded review of the MR findings with the histologic state of the fibrous cap revealed that 1). assessing the preoperative appearance of the fibrous cap has a high test sensitivity (0.81) and specificity (0.90) for identifying an unstable cap in vivo; and 2). the availability of different contrast weightings facilitated image interpretation when intimal calcifications or flow artifacts obscured the lumen surface.

CONCLUSION

Multisequence MRI can accurately characterize the in vivo state of the fibrous cap. This finding supports the use of these noninvasive techniques to prospectively identify vulnerable plaques.

Characterization of aortic root atherosclerosis in ApoE knockout mice: high-resolution in vivo and ex vivo MRM with histological correlation

In vivo, cardiac-gated, black-blood, and ex vivo magnetic resonance microscopy (MRM) images of the aortic root, and histopathology data were obtained from 12 transgenic and wild-type (WT) mice. MRM was performed using a black-blood imaging spin-echo sequence with upstream and downstream in-flow saturation pulses to obtain aortic root images in three contrast techniques: proton density-weighted (PDW), T(1)- (T(1)W), and T(2)-weighted (T(2)W). Aortic wall thickness and area were measured and correlated with histopathology data (R > 0.90). Ex vivo lesion components (lipid core, fibrous tissue, and cell tissue) were identified and characterized by differing image contrast in PDW, T(1)W, and T(2)W MRM, and by histopathology. The differences between WT and transgenic mice for maximal wall thickness and area were statistically significant (P < 0.05). This study demonstrates the feasibility of in vivo murine aortic root lesion assessment and ex vivo plaque characterization by MRM.

MR imaging for the noninvasive assessment of atherothrombotic plaques

Imaging methods to quantify the progression and regression of atherosclerosis could play a strong role in the management of patients. High-resolution, noninvasive MR imaging may provide exhaustive 3D anatomical information about the lumen and the vessel wall. Furthermore, MR imaging has the ability to characterize plaque composition and microanatomy and therefore to identify lesions vulnerable to rupture or erosion. The high resolution of MR imaging and the development of sophisticated contrast agents offer the promise of molecular in vivo molecular imaging of the plaque. This may aid early intervention (eg, lipid-lowering drug regiments) in both primary and secondary treatment of vascular disease in all arterial beds.

Noncoronary and coronary atherothrombotic plaque imaging and monitoring of therapy by MRI

In the future, the use of imaging methods to quantify the progression and regression of atherosclerosis could play a strong role in the management of patients. High-resolution, noninvasive MRI may provide exhaustive 3-D anatomic information about the lumen and the vessel wall. Furthermore, MRI has the ability to characterize plaque composition and microanatomy and therefore to identify lesions vulnerable to rupture or erosion. The high resolution of MRI and the development of sophisticated contrast agents offer the promise of molecular in vivo molecular imaging of the plaque. This may aid early intervention (e.g., lipid lowering drug regiments) in both primary and secondary treatment of vascular disease in all arterial beds.

MR imaging of atherosclerotic plaque

MRI is a powerful noninvasive imaging tool with high spatial resolution that continues to prove its value in determining atherosclerotic plaque size, volume, and tissue components. Multispectral MRI sequences have been validated to characterize atherosclerotic plaque components in animals; they have recently been applied to human aorta and carotid artery and are being used to identify the vulnerable plaque. The ability to measure wall thickness in human coronary artery wall has been realized. Future developments may allow plaque characterization in the coronary arteries with surface coil imaging, but intravascular MRI may play an important role in this regard. Novel contrast agents for identifying inflammation and thrombus within atherosclerotic plaque will aid in the identification of higher-risk atherosclerotic disease. Lastly, MRI has progressed to the point where it can be used in serial studies of atherosclerotic plaque progression and regression in the face of therapeutic intervention. MRI will continue to evolve an important role in imaging of atherosclerotic plaque.

Intravascular ultrasound imaging in the assessment of atherosclerotic plaques in rabbit abdominal aorta: comparison with histologic findings

RATIONALE AND OBJECTIVES

To examine the correlation between the echogenicity and the components of atherosclerotic plaques in rabbit.

METHODS

The atherosclerotic plaque formation in the abdominal aortas of hyperlipidemic or normolipidemic rabbits was stimulated by inserting polyethylene tubing. Intravascular ultrasound (30-MHz, 4.5 F catheter) investigation was performed at locations in the vessel. The intravascular ultrasound images of the plaques were evaluated and compared with the histologic findings.

RESULTS

Ultrasound images delineated areas showing hyperechoic or hypoechoic ultrasound beams in the plaques. Histologic studies revealed that the hyperechoic areas were closely associated with a dense fibrous extracellular matrix, whereas the hypoechoic areas corresponded to lesions showing a marked accumulation of foamy macrophages or proteoglycan-rich loose myxoid extracellular matrix with smooth muscle cell proliferation.

CONCLUSION

A good correlation between ultrasound images and histologic features was observed. These results suggest that intravascular ultrasound imaging could provide useful information for assessing the tissue characteristics of atherosclerotic lesions.

Non-invasive imaging of plaque vulnerability: an important tool for the assessment of agents to stabilise atherosclerotic plaques

Disruption of a vulnerable atherosclerotic plaque is well-recognised as the primary cause of stroke, non-fatal myocardial infarction and sudden cardiac death. Novel therapeutic agents are being developed to stabilise such plaques. The initial evaluation of these drugs would be facilitated by the use of non-invasive imaging techniques to identify vulnerable plaque and document serial changes in plaque stability. The aim of this review is to explain the characteristics of the leading non-invasive imaging modalities and discuss their role in examining the vulnerable plaque. This knowledge will be extremely important for physicians and scientists involved in the clinical evaluation of novel agents to stabilise the vulnerable plaque.

New understanding of atherosclerosis (clinically and experimentally) with evolving MRI technology in vivo

Atherosclerosis and its thrombotic complications are the major cause of morbidity and mortality in industrialized countries. Despite advances in our understanding of the pathogenetic mechanisms and new treatment modalities, the absence of an adequate noninvasive method for early detection limits the prevention or treatment of patients with various degrees and localizations of atherothrombotic disease. High-resolution magnetic resonance (MR) has recently emerged as one of the most promising techniques for the noninvasive study of atherothrombotic disease. Most importantly, MR can be used to characterize plaque composition and monitor progression. Thus, MR opens new strategies, ranging from the screening of high-risk patients for early detection and treatment as well as the monitoring of target areas for pharmacological intervention.

In vivo magnetic resonance imaging of experimental thrombosis in a rabbit model

The process of atherosclerotic plaque disruption has been difficult to monitor because of the lack of an animal model and the limited ability to directly visualize the plaque and overlying thrombus in vivo. Our aim was to validate in vivo magnetic resonance imaging (MRI) of the thrombus formation after pharmacological triggering of plaque disruption in the modified Constantinides animal model of plaque disruption. Atherosclerosis was induced in 9 New Zealand White male rabbits (3 kg) with aortic balloon endothelial injury followed by a high cholesterol (1%) diet for 8 weeks. After baseline (pretrigger) MRI, the rabbits underwent pharmacological triggering with Russell's viper venom and histamine, followed by another MRI 48 hours later. Contiguous cross-sectional T2-weighted fast spin echo images of the abdominal aorta were compared by histopathology. In all animals, aortic wall thickening was present on the pretrigger MRI. On MRIs performed 48 hours after triggering, a histologically confirmed intraluminal thrombus was visualized in 6 (67%) of the 9 animals. MRI data correlated with the histopathology regarding aortic wall thickness (R=0.77, P<0.0005), thrombus size (R=0.82, P<0.0001), thrombus length (R=0.86, P<0.005), and anatomic location (R=0.98, P<0.0001). In vivo, MRI reliably determines the presence, location, and size of the thrombus in this animal model of atherosclerosis and plaque disruption. The combination of in vivo MRI and the modified Constantinides animal model could be an important research tool for our understanding of the pathogenesis of acute coronary syndromes.

The human high-risk plaque and its detection by magnetic resonance imaging

The study of atherosclerotic disease during its natural history and after therapeutic intervention will enhance our understanding of the progression and regression of this disease and will aid in selecting the appropriate treatments. Several invasive and noninvasive imaging techniques are available to assess vessels in atherosclerotic disease. Most of the standard techniques, however, identify luminal diameter or stenosis, wall thickness, or plaque volume. None of the standard techniques can characterize the composition of an atherosclerotic plaque and therefore are incapable of identifying the high-risk plaques. High-resolution, multicontrast, magnetic resonance imaging (MRI) can noninvasively image vulnerable plaques and characterize plaques in terms of their different components (ie, lipid, fibrous, calcium, or thrombus). Application of MRI opens up whole new areas for diagnosis, prevention, and treatment of atherosclerosis.

Cardiac gated breath-hold black blood MRI of the coronary artery wall: an in vivo and ex vivo comparison

BACKGROUND

High resolution magnetic resonance (MR) imaging of the coronary artery wall in vivo has been limited by the cardiac and respiratory motion, flow artifacts as well as the relatively small size of the coronary arteries. We sought to validate in vivo black blood MR imaging of the coronary artery wall using a double inversion recovery fast spin echo MR imaging sequence with limited breath-holding and cardiac gating for suppression of motion artifacts by comparison with ex vivo MR imaging.

METHODS

Yorkshire albino swine (n = 6) were used in this study and coronary lesions were induced with balloon angioplasty. Four weeks after balloon injury of the coronary arteries MR imaging of the coronary artery lesions was performed. High resolution in vivo and ex vivo images of the coronary artery wall and lesions were obtained using a double inversion recovery fast spin echo sequence in a 1.5 T MR system. There was a statistically significant agreement (p < 0.0001) between measurements of vessel wall area (r = 0.87, slope = 0.87) and maximal wall thickness (r = 0.84, slope = 0.88) from in vivo and ex vivo MR images of the coronary arteries. The mean differences between in vivo and ex vivo measurements were 0.56 +/- 1.98 mm2 for vessel wall area and 0.02 +/- 0.36 mm for maximal wall thickness.

CONCLUSIONS

Using breathholding and cardiac gating, it is possible to perform high resolution MR imaging of the coronary artery wall in vivo with good suppression of motion artifacts with a double inversion recovery fast spin echo black blood imaging sequence.

Carotid plaque characterization by magnetic resonance imaging: review of the literature

Magnetic resonance imaging (MRI) of carotid plaque has undergone significant improvements in the last decade. Early studies utilizing ex vivo specimens and spin-echo or fast spin-echo imaging led to the conclusion that T2 weighting is the best single contrast to characterize carotid plaque morphology. On these images, the fibrous plaque appears bright and the lipid core is dark; thrombus can have variable intensity. There can be an overlap in T2-weighted signal intensities among the various plaque components, which can be partially offset by the use of multispectral analysis of multiple contrast images. With improvements in coil design, sequence design, and main field and gradient capabilities, accurate in vivo differentiation and measurement of these various carotid plaque components should be possible in 3 to 5 years. Ex vivo and in vivo studies have yielded high-resolution measurements of the complex three-dimensional lumen geometry, which are being used to predict hemodynamic forces acting on the lumenal surface. Carotid plaque burden can be accurately measured in vivo today; ongoing longitudinal studies should lead to a better understanding of the relationship between plaque burden and the risk of thromboembolic complications, as well as the effect of diet and drug therapy in hyperlipidemic patients. With these developments in place or soon to be available, MRI of the diseased carotid artery wall may prove to be even more important than magnetic resonance angiography.

The assessment of the vulnerable atherosclerotic plaque using MR imaging: a brief review

The study of atherosclerotic disease during its natural history and after therapeutic intervention may enhance our understanding of the progression and regression of this disease and will aid in selecting the appropriate medical treatments or surgical interventions. Several invasive and non-invasive imaging techniques are available to assess atherosclerotic disease vessels. Most of these techniques are strong in identifying the morphological features of the disease such as lumenal diameter and stenosis or wall thickness, and in some cases provide an assessment of the relative risk associated with the atherosclerotic disease. However, none of these techniques can fully characterize the composition of the atherosclerotic plaque in the vessel wall and therefore are incapable of identifying the vulnerable plaques. High-resolution, multi-contrast, magnetic resonance (MR) can non-invasively image vulnerable plaques and characterize plaques in terms of lipid and fibrous content and identify the presence of thrombus or calcium. Application of MR imaging opens up whole new areas for diagnosis, prevention, and treatment of atherosclerosis.