High-Resolution Three-Dimensional Aortic Magnetic Resonance Angiography and Quantitative Vessel Wall Characterization of Different Atherosclerotic Stages in a Rabbit Model

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.

Ex-vivo Assessment of Coronary Artery Atherosclerosis by Magnetic Resonance Imaging: Correlation with Histopathology

Introduction: In recent years, high-resolution magnetic resonance imaging (MRI) has emerged as a very promising technique for studying atherosclerotic disease in humans. Aim: In the present study we sought to determine whether MRI allowed for the morphological characterization of the coronary vessel wall and atherosclerotic plaques using histopathological assessment as the reference standard. Methods: The study population consisted of 13 patients who died of acute myocardial infarction and underwent autopsy. The proximal portions of the coronary arteries were excised and were evaluated both by MRI and by histopathology. For each arterial segment, the following parameters were calculated through manual planimetry: 1. total vessel area (TVA); 2. luminal area (LA) and 3. plaque area (PA). Results: A total of 207 coronary artery cross-sections were found to be suitable for analysis by both MRI and histopathology and were included in the final analyses. Both methods demonstrated moderate to good agreement for the quantification of TVA (mean difference = 2.4±2.4 mm², 95‰ limits of agreement from -2.4 to +7.2 mm²; CCC = 0.69, 95‰ CI from 0.63 to 0.75), LA (mean difference = 0.0±1.7 mm², 95‰ limits of agreement from -3.3 to + 3.3 mm²; CCC = 0.84, 95‰ CI from 0.80 to 0.88) and PA (mean difference = 2.4±2.4 mm², 95‰ limits of agreement from -2.3 to + 7.1 mm²; CCC = 0.64, 95‰ CI from 0.58 to 0.71). Conclusion: In this ex vivo experimental model we demonstrated good agreement between coronary artery morphometrical measurements obtained by high-resolution MRI and by histopathology.

Regions of low endothelial shear stress colocalize with positive vascular remodeling and atherosclerotic plaque disruption: an in vivo magnetic resonance imaging study

BACKGROUND

Local hemodynamic factors, particularly low endothelial shear stress (ESS), play a role in the focal formation of atherosclerosis. We used in vivo MRI to investigate the role of the magnitude of ESS on vascular remodeling, plaque burden, and disruption using a rabbit model of controlled atherothrombosis.

METHODS AND RESULTS

Atherosclerosis was induced in New Zealand white rabbits by cholesterol diet and endothelial denudation. MRI was performed before (pretrigger) and after (posttrigger) inducing plaque disruption with Russell viper venom and histamine. Of the 134 vascular segments studied, 28 contained thrombus (disrupted) and 106 did not (nondisrupted). Disrupted plaques were histologically characterized by a thin, inflamed fibrous cap, a dense lipid core, and mural thrombus. Pretriggered MRI revealed that disrupted plaques clustered at regions with low mean ESS (11.55±5.3 versus 20.9±9.74 dynes/cm(2); P<0.001) and low peak ESS (21.5±11.2 versus 49.2±21.5 dynes/cm(2); P<0.001) compared with nondisrupted plaques. The peak ESS negatively correlated with the plaque area (r=-0.56, P<0.001) and remodeling ratio (r=-0.4, P=0.008). There was also a negative correlation between the mean ESS and the remodeling ratio (r=-0.55, P<0.001). Both the peak ESS and the mean ESS did not correlate with the % stenosis; there was a weak but statistically significant correlation with the % cross-sectional narrowing (r=0.3, P=0.002 and r=0.2, P=0.04, respectively). Receiver operating characteristic analysis showed that both mean (AUC=0.78; 95% CI, 0.69-0.87) and peak ESS (AUC=0.85; 95% CI, 0.78-0.93) identified disrupted plaques.

CONCLUSIONS

We demonstrated that low ESS is associated with plaque burden, positive vascular remodeling, and plaque disruption in a rabbit model. Assessment of ESS by noninvasive MRI might be useful for assessing atherosclerotic risk.

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

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

Advances in molecular imaging of atherosclerosis and myocardial infarction: shedding new light on in vivo cardiovascular biology

Molecular imaging of the cardiovascular system heavily relies on the development of new imaging probes and technologies to facilitate visualization of biological processes underlying or preceding disease. Molecular imaging is a highly active research discipline that has seen tremendous growth over the past decade. It has broadened our understanding of oncologic, neurologic, and cardiovascular diseases by providing new insights into the in vivo biology of disease progression and therapeutic interventions. As it allows for the longitudinal evaluation of biological processes, it is ideally suited for monitoring treatment response. In this review, we will concentrate on the major accomplishments and advances in the field of molecular imaging of atherosclerosis and myocardial infarction with a special focus on magnetic resonance imaging.

Automated tracing of the adventitial contour of aortoiliac and peripheral arterial walls in CT angiography (CTA) to allow calculation of non-calcified plaque burden

Aortoiliac and lower extremity arterial atherosclerotic plaque burden is a risk factor for the development of visceral and peripheral ischemic and aneurismal vascular disease. While prior research allows automated quantification of calcified plaque in these body regions using CT angiograms, no automated method exists to quantify soft plaque. We developed an automatic algorithm that defines the outer wall contour and wall thickness of vessels to quantify non-calcified plaque in CT angiograms of the chest, abdomen, pelvis, and lower extremities. The algorithm encodes the search space as a constrained graph and calculates the outer wall contour by deriving a minimum cost path through the graph, following the visible outer wall contour while minimizing path tortuosity. Our algorithm was statistically equivalent to a reference standard made by two reviewers. Absolute error was 1.9 ± 2.3% compared to the inter-observer variability of 3.9 ± 3.6%. Wall thickness in vessels with atherosclerosis was 3.4 ± 1.6 mm compared to 1.2 ± 0.4 mm in normal vessels. The algorithm shows promise as a tool for quantification of non-calcified plaque in CT angiography. When combined with previous research, our method has the potential to quantify both non-calcified and calcified plaque in all clinically significant systemic arteries, from the thoracic aorta to the arteries of the calf, over a wide range of diameters. This algorithm has the potential to enable risk stratification of patients and facilitate investigations into the relationships between asymptomatic atherosclerosis and a variety of behavioral, physiologic, pathologic, and genotypic conditions.

Roles of the WHHL rabbit in translational research on hypercholesterolemia and cardiovascular diseases

Conquering cardiovascular diseases is one of the most important problems in human health. To overcome cardiovascular diseases, animal models have played important roles. Although the prevalence of genetically modified animals, particularly mice and rats, has contributed greatly to biomedical research, not all human diseases can be investigated in this way. In the study of cardiovascular diseases, mice and rats are inappropriate because of marked differences in lipoprotein metabolism, pathophysiological findings of atherosclerosis, and cardiac function. On the other hand, since lipoprotein metabolism and atherosclerotic lesions in rabbits closely resemble those in humans, several useful animal models for these diseases have been developed in rabbits. One of the most famous of these is the Watanabe heritable hyperlipidemic (WHHL) rabbit, which develops hypercholesterolemia and atherosclerosis spontaneously due to genetic and functional deficiencies of the low-density lipoprotein (LDL) receptor. The WHHL rabbit has been improved to develop myocardial infarction, and the new strain was designated the myocardial infarction-prone WHHL (WHHLMI) rabbit. This review summarizes the importance of selecting animal species for translational research in biomedical science, the development of WHHL and WHHLMI rabbits, their application to the development of hypocholesterolemic and/or antiatherosclerotic drugs, and future prospects regarding WHHL and WHHLMI rabbits.

Magnetic resonance angiography: current status in the planning and follow-up of endovascular treatment in lower-limb arterial disease

Magnetic resonance angiography (MRA) has become an established imaging modality in the management of lower-limb arterial disease, with emerging roles in treatment planning and follow-up. Contrast-enhanced MRA is now the most widely used technique with clinically acceptable results in the majority of patients. Difficulties in imaging and image interpretation are recognised in certain subgroups, including patients with critical limb ischaemia as well as patients with stents. Although newer contrast agents and refined imaging protocols may offer some solutions to these problems, this optimism is balanced by concerns about the toxicity of certain gadolinium chelates. Further development of interventional MRA remains one of the most significant challenges in the development of magnetic resonance imaging-guided peripheral vascular intervention. The status of MRA in managing patients with lower-limb arterial disease in current clinical practice is reviewed.

Advances in magnetic resonance (2008)

Current advances in magnetic resonance, as a diagnostic modality, are discussed in the context of publications from Investigative Radiology during 2007 and 2008. The articles relating to this topic, published during the past 2 years, are reviewed by anatomic region. The discussion concludes with a consideration of magnetic resonance contrast media, focusing on studies published in the journal, and examining in particular the potential impact of nephrogenic systemic fibrosis.

Advances in magnetic resonance (2007)

Advances in clinical magnetic resonance (MR) are discussed in this review in the context of publications from Investigative Radiology during 2006 and 2007. The articles relevant to this topic, published during this 2 year time period, are considered as organized by anatomic region. An additional final focus of discussion is in regards to those studies involving MR contrast media.