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The Vulnerable, Or High-Risk, Atherosclerotic Plaque

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The Vulnerable, Or High-Risk, Atherosclerotic Plaque Note: This copy is for your personal non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, contact us at www.rsna.org/rsnarights. REVIEW The Vulnerable, or High-Risk, Ⅲ Atherosclerotic Plaque: Noninvasive MR Imaging for Characterization and Assessment1 REVIEWS AND COMMENTARY Tobias Saam, MD “Vulnerable” plaques are atherosclerotic plaques that have Thomas S. Hatsukami, MD a high likelihood to cause thrombotic complications, such Norihide Takaya, MD, PhD as myocardial infarction or stroke. Plaques that tend to Baocheng Chu, MD, PhD progress rapidly are also considered to be vulnerable. Hunter Underhill, MD Besides luminal stenosis, plaque composition and mor- William S. Kerwin, PhD phology are key determinants of the likelihood that a Jianming Cai, MD, PhD plaque will cause cardiovascular events. Noninvasive mag- Marina S. Ferguson, MT netic resonance (MR) imaging has great potential to en- Chun Yuan, PhD able characterization of atherosclerotic plaque composi- tion and morphology and thus to help assess plaque vulner- ability. A classification for clinical, as well as pathologic, evaluation of vulnerable plaques was recently put forward in which five major and five minor criteria to define vulner- able plaques were proposed. The purpose of this review is to summarize the status of MR imaging with regard to depiction of the criteria that define vulnerable plaques by using existing MR techniques. The use of MR imaging in animal models and in human disease in various vascular beds, particularly the carotid arteries, is presented. ௠ RSNA, 2007 1 From the Departments of Radiology (T.S., N.T., B.C., H.U., W.S.K., J.C., M.S.F., C.Y.) and Surgery (T.S.H.), Uni- versity of Washington, Seattle, Wash; and Surgical Ser- vice, VA Puget Sound Health Care System, Seattle, Wash (T.S.H.). Received October 31, 2005; revision requested December 14; revision received January 17, 2005; ac- cepted March 2; final version accepted July 17; final re- view and update by T.S. January 9, 2007. Address cor- respondence to T.S., Department of Clinical Radiology, Grosshadern Campus, University of Munich, Marchioni- nistr 15, 81377 Munich, Germany (e-mail: Tobias_ [email protected]). ஽ RSNA, 2007 64 Radiology: Volume 244: Number 1—July 2007 REVIEW: MR Imaging of the Vulnerable Atherosclerotic Plaque Saam et al omplications of cardiovascular must be able to help determine the pa- will focus on the current status of MR as disease, including stroke, myocar- tient-specific risk of experiencing a car- a diagnostic imaging method to help Cdial infarction, and sudden car- diovascular event. identify the features of vulnerable diac death, are the most common Imaging methods have the potential plaques. The structure of this article is causes of death in the world (1). Ath- to not only be used as a screening tool based on two recently published con- erosclerotic disease accounts for ap- for the presence of atherosclerosis but sensus documents (3,4) by a group of proximately 25% of ischemic strokes also to help distinguish stable from vul- experienced researchers in atheroscle- (2) and for the majority of myocardial nerable plaques and ultimately to distin- rosis, including pathologists, clinicians, infarctions and sudden cardiac deaths guish patients with low versus those molecular biologists, and imaging scien- (3,4). Despite major advances in the with high risk of cardiovascular compli- tists, in which the key features of the treatment of atherosclerosis, a larger cations. Methods commonly used in vulnerable plaque were defined. The au- percentage of individuals with the dis- atherosclerosis imaging include B-mode thors of those documents argue that ease who are apparently healthy die ultrasonography (US), intravascular knowledge of luminal diameter is not without prior symptoms (3). The chal- US, conventional angiography, com- sufficient to determine the vulnerability lenge for screening and diagnostic puted tomography (CT), and magnetic of an atherosclerotic lesion, and they methods is to identify patients at high resonance (MR) imaging. Each of these propose five major and five minor crite- risk who have lesions that are vulnera- imaging modalities has advantages and ria for the detection of vulnerable ble to thrombosis, so-called vulnerable disadvantages, and all have been re- plaques. These plaque features were plaques, before the event occurs. To tai- viewed elsewhere (5,6). This article will based on studies of coronary arteries lor and improve treatment strategies, focus on the particular potential of MR and included a thin cap with a large these screening and diagnostic methods imaging to depict the key features of the lipid-necrotic core, active inflammation, vulnerable plaque. MR imaging is well a fissured plaque, stenosis greater than suited for this role because it is noninva- 90%, endothelial denudation with or Essentials sive, does not involve ionizing radiation, without superficial platelet aggregation Ⅲ The challenge for imaging meth- enables visualization of the vessel lumen and fibrin deposition, endothelial dys- ods is to enable identification of and wall (7,8), and can be repeated se- function, calcified nodules, intraplaque patients with high-risk lesions rially to track progression or regres- hemorrhage, glistening yellow plaques that are vulnerable to thrombosis, sion. Furthermore, the excellent soft- (seen at angioscopy), and outward re- so-called vulnerable plaques, be- tissue contrast provided by MR imaging modeling (see Table 1). fore the occurrence of cardiovas- allows evaluation of compositional and In this review, we will discuss the cular complications. morphologic features of atherosclerotic role of MR imaging in the identification Ⅲ Noninvasive MR imaging has plaques (9–15). This information is cru- of each of the features that define the great potential to help character- cial because, besides luminal stenosis, vulnerable plaque, as proposed in the ize atherosclerotic plaque compo- plaque composition and morphology are consensus documents (3,4). sition and morphology and thus to key determinants of a plaque’s vulnera- enable assessment of plaque vul- bility with regard to causing cardiovas- Major Criteria nerability. cular events (3,4). Ⅲ Serial MR imaging of atheroscle- Authors of several review articles rotic plaques can provide useful (7,8,16) have described in detail the Thin Cap with Large Lipid-Necrotic Core insights on the natural history of technical aspects of the MR imaging Virmani et al (17) defined the fibrous vulnerable plaques and might be characterization of human atheroscle- cap as a distinct layer of connective tis- useful in identifying plaques that rotic plaque, including hardware con- sue completely covering the lipid-ne- are progressing toward a vulnera- siderations, imaging sequences, and im- crotic core. The lipid-necrotic core, ble state. aging protocols. Results of the accuracy which is frequently surrounded by mac- Ⅲ A recent prospective study dem- of MR imaging, compared with histo- rophages, consists of large amounts of onstrated that certain vulnerable logic findings, for measurements of extracellular lipid, cholesterol crystals, plaque features identified on MR plaque burden, tissue characterization, and necrotic debris (17,18). Lesions images are associated with the and fibrous cap status have also been occurrence of subsequent cere- reported (7,8,16). Choudhury et al (7) brovascular events. focused on present and future MR appli- Published online Ⅲ Most of the plaque imaging data cations for the characterization of ath- 10.1148/radiol.2441051769 are based on larger vessels, such erosclerotic plaques, including real-time Radiology 2007; 244:64–77 as the carotid arteries, and fur- vascular intervention, new contrast ther advances in temporal and agents, and molecular imaging. Abbreviation: TOF ϭ time of flight spatial resolution are needed for The present review is a continuation coronary plaque imaging. of previous review articles (8,16) and Authors stated no financial relationship to disclose. Radiology: Volume 244: Number 1—July 2007 65 REVIEW: MR Imaging of the Vulnerable Atherosclerotic Plaque Saam et al with a large lipid-necrotic core and a identified the major components of ca- wall, was similar on MR images and thin fibrous cap are considered to be rotid atherosclerotic plaque on MR im- histologic specimens, with values of most likely to rupture (19). ages and serially sectioned carotid end- 30.1% Ϯ 12.5 (standard deviation) and MR imaging is able to help charac- arterectomy specimens, respectively 32.7% Ϯ 12.3, respectively. Further- terize all major plaque components, in- (Fig 1). MR measurements of the lipid- more, area measurements were strongly cluding the lipid-necrotic core, by de- necrotic core did not differ significantly correlated (r ϭ 0.85; P Ͻ .001). picting particular combinations of signal from findings on histologic specimens The ability of MR imaging to help intensities of each component on im- (23.7% vs 20.3%; P ϭ .1), and a strong determine the status and thickness of ages obtained with different contrast correlation between MR and histologic the fibrous cap will be discussed in the weightings (see Table 2 for details). Ini- area measurements was found (r ϭ following section. tial experiments involving ex vivo imag- 0.75; P Ͻ .001) (9). ing of endarterectomy specimens by us- Comparison of T1-weighted images Fissured Plaque ing T1-, T2-, and intermediate-weighted obtained before and after contrast ma- Virmani et al (17) described plaque
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