Quantification and 3D Reconstruction of Atherosclerotic Plaque Components in Apolipoprotein E Knockout Mice Using Ex Vivo High-Resolution MRI

Abstract

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 (apoEϪ/Ϫ) mice fed Western diet for 10, 20, or 30 weeks were imaged ex vivo (11.7 T; 3D multiecho sequence; resolution 47ϫ47ϫ62.5 m). Using semiautomated histogram-based methods, MRI accurately quantified lipid-rich/necrotic areas in the aortic root (r 2 ϭ0.84; PϽ0.001) and brachiocephalic artery (r 2 ϭ0.90; PϽ0.001) compared with histology. Similarly, cell-rich caps in aortic roots, quantified by MRI and histology, correlated closely (r 2 ϭ0.74; PϽ0.001). 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. Key Words: atherosclerosis Ⅲ brachiocephalic artery Ⅲ apoEϪ/Ϫ mice Ⅲ MRI Ⅲ lipid-rich/necrotic core G enetically engineered mice have become the animal model of choice for study of the pathogenesis of atherosclerosis. 1-3 For example, apolipoprotein E knockout (apoEϪ/Ϫ) mice spontaneously develop atherosclerotic lesions throughout the arterial tree. 4 The standard method for quantification and characterization of mouse atherosclerosis involves serial sectioning and histopathology of the aortic root. 5 However, lesions at this site do not typically show defined lipid cores with thin fibrous caps, which characterize the "vulnerable" plaque in human atherosclerosis. 6,7 More recently, advanced lesions showing features consistent with plaque rupture and intraplaque hemorrhage have been identified in the brachiocephalic artery of apoEϪ/Ϫ mice. 8 -11 Although histopathology can provide valuable information on plaque composition, it is tissue destructive and confined to 2D analyses of a relatively small number of 5 to 10 m thick histological sections, giving limited insight into overall plaque structure and spatial distribution of plaque components. The brachiocephalic artery is technically demanding to embed and to section because of its small size (Ϸ500 m external diameter). Furthermore, preparation and processing for histology introduces size distortion during tissue dehydration and has the potential to disrupt the integrity of the plaque itself. Conclusions-Ex-vivoUsing multicontrast MRI (T1-weighted [T 1 W], T 2 W, proton density-weighted [PDW]) with submillimeter spatial resolution, plaque components have been distinguished without tissue destruction in carotid artery specimens ex vivo 12-14 and in human carotid artery 15 and aorta 16 in vivo. Magnetic resonance (MR) has also been shown to discriminate atherosclerotic plaque components in larger animal models including ...

Flow‐enhanced off‐resonance saturation for remote detection of iron‐based contrast agents

“…Subsequently, this leads to an iron concentration of 45 mM. This volume is approximately three times smaller than the volume of an inflamed ApoE À/À mouse plaque (0.51mm 3 (20)). …”

Generation and 3-Dimensional Quantitation of Arterial Lesions in Mice Using Optical Projection Tomography

“…It gives a more informative indication of the total lesion burden in an artery 4 than is usually obtained with histological methods. Analyzing the entire lesion reduces selection bias and error that will inevitably occur when discrete sections of a vessel are chosen for analysis.…”

“…Whole-mount 3-dimensional imaging technology provides a possible solution to this problem but surprisingly few suitable approaches have been described. This may be due predominantly to the size of mouse arteries which are too large for single-photon confocal microscopy but too small for magnetic resonance imaging (MRI) 4 and X-ray computed tomography (CT) 5 . Application of ex vivo MRI and micro CT to the study of atherosclerotic lesions in mice suggests they offer limited resolution, even in relatively large arteries.…”

“…Application of ex vivo MRI and micro CT to the study of atherosclerotic lesions in mice suggests they offer limited resolution, even in relatively large arteries. Added to this, the relatively long acquisition times required limit throughput (and increase scanning costs) 4,6 . Development of new optical imaging modalities (such as optical coherence tomography 3,7 and photo-acoustic tomography 8 ) offers much potential for improving imaging of lesions in murine arteries.…”

Pitfalls in the assessment of murine atherosclerosis

“…Although experimental atherosclerosis can be assessed using advanced imaging techniques such as angiography, Doppler ultrasound, B-mode ultrasound and magnetic resonance (30)(31)(32), the main methods employed in the quantification of mouse atherosclerosis are those mentioned in this brief review.…”