High resolution ex vivo magnetic resonance imaging of in situ coronary and aortic atherosclerotic plaque in a porcine model

Estabilización de la placa de ateroma: un nuevo concepto basado en la biología dinámica de la aterosclerosis

As it is well-known, a thrombus evolving into a disrupted/eroded atherosclerotic plaque causes most acute coronary syndromes. Plaque stabilization via reduction of the lipid core and/or thickening of the fibrous cap is one of the possible mechanisms accounted for the clinical benefits displayed by different anti-atherosclerotic strategies. The concept of plaque stabilization was developed to explain how lipid-lowering agents could decrease adverse coronary events without substantial modifications of the atherosclerotic lesion. A number of imaging modalities (vascular ultrasound, MRI, and coronary computed tomography) are used for non-invasive assessment of atherosclerosis; most of them can identify luminal stenosis, wall thickness and plaque volume and composition, and can even characterize the rupture-prone vulnerable plaques. Several classes of drugs, including statins, ACE inhibitors, -blockers, and antithrombotics, are able to reduce the plaque burden and the incidence of cardiovascular events; this may be attibutable, at least in part, to plaque-stabilizing effects and the improvement of endothelial dysfunction.

High resolution three-dimensional visualization and characterization of coronary atherosclerosis in vitro by synchrotron radiation x-ray microtomography and highly localized x-ray diffraction

Human atherosclerotic plaques in both native and bypass arteries have been visualized using microtomography to provide additional information on the nature of coronary artery disease. Plaques contained within arteries removed from three white males aged 51, 55 and 70 are imaged in three-dimensions with monochromatic synchrotron x-ray radiation. Fields of view are 658 x 658 x 517 voxels. with cubic voxels ranging from 12 to 13 microm on a side. X-ray energies range from 11 to 15 keV (bandpass approximately 10 eV). At lower energies, high local absorption tends to generate reconstruction artefacts, while at higher energies the arterial wall is scarcely visible. At all energies, calcifications are clearly visible and differences are observed between plaques in native arteries (lifetime accumulations) versus bypass arteries (plaques developing in the interval between the heart bypass operation and the autopsy). In order to characterize coronary calcification, a microfocused, 50 microm2, 25 keV x-ray beam was used to acquire powder diffraction data from selected calcifications. Also, large calcifications were removed from the native arteries and imaged with 25 keV x-ray energy. Calcifications are composed of hydroxyapatite crystallites and an amorphous phase. In summary, native calcifications are larger and have a higher fraction of hydroxyapatite than calcifications from the bypass arteries.

Magnetic resonance imaging to identify the high-risk plaque

Atherosclerotic plaque stability depends on 3 factors: (1) lipid core, (2) fibrous cap and its thickness, and (3) inflammation within the cap. Magnetic resonance imaging (MRI) is a noninvasive technique that can provide information on these plaque components using a variety of pulse sequences. Assessment of plaque volume and tissue components and the efficacy of lipid-lowering therapy has been performed in human aorta and carotid arteries. Imaging the coronary wall for plaque burden is a novel application of MRI. Newer approaches also include intravascular and transesophageal MRI techniques. Several emerging MR contrast agents being tested in animal models hold promise for targeted imaging of plaque. MRI is a powerful noninvasive imaging tool with high spatial resolution that continues to prove its value in determining atherosclerotic plaque volume and tissue components.

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.

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.

Progression and regression of atherosclerotic lesions: monitoring with serial noninvasive magnetic resonance imaging

BACKGROUND

Modification or stabilization of atherosclerotic plaques has been proposed as the mechanism responsible for the beneficial clinical effect of lipid-lowering therapies. An imaging modality able to quantify atherosclerotic plaque composition could potentially allow not only the identification of these vulnerable atherosclerotic lesions, but also monitoring of the effects of therapeutic interventions on plaque composition. Our aim was to monitor changes in burden and composition of atherosclerotic plaques in a rabbit model of complex atherosclerosis using serial noninvasive magnetic resonance imaging (MRI).

METHODS AND RESULTS

Aortic atherosclerotic lesions were induced in rabbits and the animals randomized to continue an atherogenic diet (atherosclerosis progression) or resume normal chow (atherosclerosis regression) for 6 months. MRI of the aorta was performed at 3 time points in each rabbit, as follows: baseline, after atherosclerosis induction (9 months old), and after atherosclerosis regression or progression (15 months old). Histopathologic correlation with MRI was performed. There was a significant (P<0.0001) reduction in atherosclerotic burden in the regression group, and an increase in the progression group. There was a significant (P<0.001) reduction in the lipidic component of plaques in the regression group, and an increase in the progression group. A small, nonsignificant increase in the fibrotic plaque components was noted in the regression group, but a significant decrease in the fibrotic composition of lesions in the progression group. A significant correlation (P<0.001) was found between MRI and histopathology for atherosclerotic burden and plaque composition.

CONCLUSIONS

These findings indicate that serial noninvasive MRI can monitor changes in atherosclerotic plaque composition under conditions of atherosclerotic progression and regression.

Clinical imaging of the high-risk or vulnerable atherosclerotic plaque

The study of atherosclerotic disease during its natural history and after therapeutic intervention will enhance our understanding of disease progression and regression and aid in selecting appropriate treatments. Several invasive and noninvasive imaging techniques are available to assess atherosclerotic vessels. Most of the standard techniques identify luminal diameter, stenosis, wall thickness, and plaque volume; however, none can characterize plaque composition and therefore identify the high-risk plaques. We will present the different imaging modalities that have been used for the direct assessment of the carotid, aortic, and coronary atherosclerotic plaques. We will review in detail the use of high-resolution, multicontrast magnetic resonance for the noninvasive imaging of vulnerable plaques and the characterization of plaques in terms of their various components (ie, lipid, fibrous, calcium, or thrombus).

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.

Tomographic (plaque) imaging: state of the art

Tomographic coronary artery plaque imaging is possible noninvasively using x-ray computed tomography (CT) and magnetic resonance imaging (MRI). The pathophysiology of coronary plaque disease is one of repeated inflammation and repair. Imaging of coronary artery calcium, a consequence of this process, is possible using CT, whereas MRI has the potential to examine the lipid and fibrous components of plaque acquisition and 3-dimensional slice registration. Quantitation of coronary artery calcium has been validated using electron-beam CT [EBCT], a unique device that images the entire heart in a single breathold from rapid [100 msec] tomographic scans done in synchrony with the heart cycle. Current mechanical CT devices require 300 to 500 msec per scan and acquire hundreds of tomographic images that then must be retrospectively separated to the required phase of the heart cycle. There are limited correlations with calcium scoring by EBCT versus mechanical CT and the later device has a limitation in situations of low plaque volume and necessitates increased radiation exposure to the patient. MRI has been shown to have the potential to define plaque composition ex vivo or in the aorta, but studies of the heart arteries are so far very limited. Widespread utilization of noninvasive plaque imaging requires that the studies be done consistently and reproducibly. The training of the interpreting physicians is of paramount concern.

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.

Magnetic resonance imaging in coronary heart disease

Modern level of cardiac magnetic resonance imaging (MRI) development already allows its routine use (with proper indications) in coronary heart disease patients for studies of heart morphology and functions, performance of stress tests for evaluation of myocardial perfusion and contractile function. Coronary MRA and some other new MR techniques are close to its wide-scale clinical application. It has been shown that cardiac MRI is a valuable tool for detection of postinfarction scars, aneurysms, pseudoaneurysms, septal defects, mural thrombi and valvular regurgitations. Due to intrinsic advantages of the method it is of special value when these pathological conditions cannot be fully confirmed or excluded with echocardiography. MRI is recognized as the best imaging method for quantification of myocardial thickness, myocardial mass, systolic myocardial thickening, chamber volumes, ejection fraction and other parameters of global and regional systolic and diastolic function. MRI is used in studies of cardiac remodeling in postinfarction patients. The most attractive areas for cardiovascular applications of MRI are assessment of myocardial perfusion and non-invasive coronary angiography. Substantial progress has been achieved in these directions. There are some other new developments in studies of coronary artery disease with MRI. High-resolution MR is used for imaging and quantification of atherosclerotic plaque composition in vivo. Intravascular MR devices suitable for performing imaging-guided balloon angioplasty are created. But before MRI will be widely accepted by the medical community as a important cardiovascular imaging modality several important problems have to be solved. Further technical advances are necessary for clinical implementation of all major diagnostic capabilities of cardiac MRI. The subjective obstacles for growth of clinical applications of cardiac MRI are lack of understanding of its possibilities and benefits both by clinicians and radiologists themselves. So proper training of specialists and promotion of this promising modality among the medical community are necessary.

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.

Predicting plaque rupture: enhancing diagnosis and clinical decision-making in coronary artery disease

Atherosclerosis is the process underlying coronary artery disease, myocardial infarction and cerebrovascular disease and is a leading cause of morbidity and mortality in industrialized countries. The atherosclerotic plaque is often indolent and progressive and may destabilize without warning. Components of the atherosclerotic plaque, including structural, cellular and molecular characteristics, determine its vulnerability to rupture. The imaging techniques currently available utilize invasive and non-invasive methods to characterize coronary artery stenoses. Detection, however, usually occurs late in the course of disease after symptoms have presented. Much effort has recently been directed at early detection and in defining markers of atherosclerotic disease. Our challenge for the future is to find non-invasive imaging modalities that can predict plaque vulnerability before irreversible damage has occurred. Through early detection and a targeted treatment strategy we hope to reduce the burden of ischemic heart disease.

Atherosclerotic aortic component quantification by noninvasive magnetic resonance imaging: an in vivo study in rabbits

OBJECTIVES

We sought to demonstrate the ability that noninvasive in vivo magnetic resonance imaging (MRI) has to quantify the different components within atherosclerotic plaque.

BACKGROUND

Atherosclerotic plaque composition plays a critical role in both lesion stability and subsequent thrombogenicity. Noninvasive MRI is a promising tool for the characterization of plaque composition.

METHOD

Thoracic and abdominal aortic atherosclerotic lesions were induced in rabbits (n = 5). Nine months later, MRI was performed in a 1.5T system. Fast spin-echo sequences (proton density-weighted and T2-weighted [T2W] images) were obtained (in-plane resolution: 350 x 350 microns, slice thickness: 3 mm). Magnetic resonance images were correlated with matched histopathological sections (n = 108).

RESULTS

A significant correlation (p < 0.001) was observed for mean wall thickness and vessel wall area between MRI and histopathology (r = 0.87 and r = 0.85, respectively). The correlation was also present on subanalysis of the thoracic and upper part of the abdominal aorta, susceptible to respiratory motion artifacts. There was a significant correlation for plaque composition (p < 0.05) between MRI and histopathology for the analysis of lipidic (low signal on T2W, r = 0.81) and fibrous (high signal on T2W, r = 0.86) areas with Oil Red O staining. T2-weighted images showed greater contrast than proton density-weighted between these different components of the plaques as assessed by signal intensity ratio analysis with the mean difference in signal ratios of 0.47 (S.E. 0.012, adjusted for clustering of observations within lesions) being significantly different from 0 (t1 = 39.1, p = 0.016).

CONCLUSIONS

In vivo noninvasive high resolution MRI accurately quantifies fibrotic and lipidic components of atherosclerosis in this model. This may permit the serial analysis of therapeutic strategies on atherosclerotic plaque stabilization.

Noninvasive in vivo human coronary artery lumen and wall imaging using black-blood magnetic resonance imaging

BACKGROUND

High-resolution MRI has the potential to noninvasively image the human coronary artery wall and define the degree and nature of coronary artery disease. Coronary artery imaging by MR has been limited by artifacts related to blood flow and motion and by low spatial resolution.

METHODS AND RESULTS

We used a noninvasive black-blood (BB) MRI (BB-MR) method, free of motion and blood-flow artifacts, for high-resolution (down to 0.46 mm in-plane resolution and 3-mm slice thickness) imaging of the coronary artery lumen and wall. In vivo BB-MR of both normal and atherosclerotic human coronary arteries was performed in 13 subjects: 8 normal subjects and 5 patients with coronary artery disease. The average coronary wall thickness for each cross-sectional image was 0.75+/-0.17 mm (range, 0.55 to 1.0 mm) in the normal subjects. MR images of coronary arteries in patients with >/=40% stenosis as assessed by x-ray angiography showed localized wall thickness of 4.38+/-0.71 mm (range, 3.30 to 5.73 mm). The difference in maximum wall thickness between the normal subjects and patients was statistically significant (P<0.0001).

CONCLUSIONS

In vivo high-spatial-resolution BB-MR provides a unique new method to noninvasively image and assess the morphological features of human coronary arteries. This may allow the identification of atherosclerotic disease before it is symptomatic. Further studies are necessary to identify the different plaque components and to assess lesions in asymptomatic patients and their outcomes.

The role of plaque rupture and thrombosis in coronary artery disease

Atherosclerosis and its thrombotic complications are the major cause of morbidity and mortality in the industrialized world. The progression of atherosclerotic plaques in the coronary circulation is dependent on several risk factors. It is now clear that plaque composition is a major determinant of the risk of subsequent plaque rupture and superimposed thrombosis. The vulnerability of plaques to rupture is further determined by extrinsic triggering factors. Following rupture, the fatty core of the plaque and its high content of tissue factor provide a powerful substrate for the activation of the coagulation cascade. Plaque rupture can be clinically silent or cause symptoms of ischaemia depending on thrombus burden and the degree of vessel occlusion. In addition, plaque rupture and subsequent healing is recognized to be a major cause of further rapid plaque progression. This review looks at the mechanisms underlying the development and progression of atherosclerotic plaques, factors leading to plaque rupture and subsequent thrombosis and their clinical consequences. Finally, we speculate on targets for future research.