Serial magnetic resonance imaging of experimental atherosclerosis allows visualization of lesion characteristics and lesion progression in vivo

The aorta wall of patients presenting to the emergency department with acute myocardial infarction by cardiac magnetic resonance

BACKGROUND

Inflammation has been shown to be a major component in the pathophysiology of acute coronary syndrome (ACS). In patients presenting with acute myocardial infarction (AMI), a critical component of the ACS spectrum, multiple coronary arteries are involved during this inflammatory process. In addition to the coronary vasculature, the inflammatory cascade has also been shown to affect the carotid arteries and possibly the aorta.

PURPOSE

To assess the involvement of the aorta during AMI by cardiac magnetic resonance (CMR).

METHODS

We prospectively evaluated the aortic wall by CMR in 123 patients. 78 patients were enrolled from the emergency department (ED), who presented with chest pain and were classified as either: (1) AMI: elevated troponin levels and typical chest pain or (2) non-cardiac chest pain (CP): negative troponins and a normal stress test or normal cardiac catheterization. We compared these 2 groups to a group of 45 asymptomatic diabetic patients. The descending thoracic aortic wall area (AWA) and maximal aortic wall thickness (AWT) were measured using a double inversion recovery T-2 weighted, ECG-gated, spin echo sequence by CMR.

RESULTS

Patients with AMI were older, more likely to smoke, had a higher incidence of claudication, and had higher CRP levels. The AWA and maximal AWT were greater in patients who presented to the ED with ACS (2.11+/-0.17 mm(2), and 3.17+/-0.19 mm, respectively) than both patients presenting with non-cardiac CP (1.52+/-0.58 mm(2), p<0.001; and 2.57+/-0.10 mm, p<0.001) and the diabetic patients (1.38+/-0.58 mm(2), p<0.001; and 2.30+/-0.131 mm, p<0.001). The difference in the aortic wall characteristics remained significant after correcting for body mass index, hyperlipidemia, statins and C-reactive protein. There was no difference in maximal AWT or AWA between patients with non-cardiac CP and patients with diabetes.

CONCLUSION

Patients with AMI have a significantly greater maximal aortic wall thickness and area compared to patients with non-cardiac CP. Longitudinal studies are needed to assess whether this increase is due to inflammation or a higher atherosclerotic burden.

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.

Quantification and 3D reconstruction of atherosclerotic plaque components in apolipoprotein E knockout mice using ex vivo high-resolution MRI

OBJECTIVE

To investigate the ability of high-resolution MRI to determine composition and microanatomy of atherosclerosis in mouse aortic root and brachiocephalic artery.

METHODS AND RESULTS

Aortic root and brachiocephalic arteries of apolipoprotein E knockout (apoE-/-) mice fed Western diet for 10, 20, or 30 weeks were imaged ex vivo (11.7 T; 3D multiecho sequence; resolution 47x47x62.5 microm). Using semiautomated histogram-based methods, MRI accurately quantified lipid-rich/necrotic areas in the aortic root (r2=0.84; P<0.001) and brachiocephalic artery (r2=0.90; P<0.001) compared with histology. Similarly, cell-rich caps in aortic roots, quantified by MRI and histology, correlated closely (r2=0.74; P<0.001). Reconstruction of segmented brachiocephalic arteries in 3D provided unique insights into plaque microanatomy and enabled volumetric quantification of plaque and lipid-rich/necrotic core. Between 10 and 30 weeks, 3D measurement identified an 11.6-fold increase in plaque volume (versus 4.1-fold for 2D) and a 21.3-fold increase in plaque lipid-rich/necrotic core volume (versus 6.4-fold for 2D), indicating superior power of 3D quantification.

CONCLUSIONS

Ex-vivo high-resolution 3D MRI accurately quantified lipid-rich/necrotic core and cell-rich cap areas in atherosclerotic lesions in apoE-/- mice. Reconstruction and volumetric quantification of segmented brachiocephalic arteries demonstrated greater sensitivity in detecting changes in plaque size and lipid composition over time than 2D analysis.

MRI and characterization of atherosclerotic plaque: emerging applications and molecular imaging

Noninvasive high-resolution magnetic resonance has the potential to image atherosclerotic plaque and to determine its composition and microanatomy. This review summarizes the rationale for plaque imaging and describes the characteristics of plaque by use of existing MRI techniques. The use of MRI in human disease and in animal models, particularly in rabbits and mice, is presented. Present and future applications of MRI, including real-time vascular intervention, new contrast agents, and molecular imaging, are also discussed.