Extracranial carotid vulnerable plaque imaging is a maturing field in both the clinical and research realms, providing information that goes beyond standard luminal imaging techniques. The North American Symptomatic Carotid Endarterectomy Trial and the European Carotid Surgery Trial have formed the basis of reliance on carotid luminal stenosis measurements to determine the risk for future stroke events and the need for surgical intervention. The degree of luminal narrowing, however, is not the only determining factor for stroke risk. Factors such as plaque composition and morphology, independent of their hemodynamic effect on the lumen, have been shown to contribute to the risk of plaque rupture and future cerebrovascular events. Some of the factors that contribute to vulnerable atherosclerotic plaque include prominent lipid necrotic cores, fibrous cap rupture, intraplaque hemorrhage, plaque inflammation, ulceration, and neovascularity.

Cross-sectional imaging, including MRI and CT, has been extensively studied to determine its ability to identify vulnerable plaque characteristics. MRI carotid vulnerable plaque imaging has contributed significantly to plaque imaging and characterization over the past 10–15 years. MR imaging is emerging as the best technique to assess for high-risk plaque features that can affect patient management. CT angiography has also proven adept at detecting atherosclerotic plaque irregularities and ulcerations, as well as prominent lipid necrotic cores and plaque calcification.

The University of Washington Vascular Imaging Lab has been at the forefront of carotid MRI vulnerable plaque imaging research, and these techniques have now been translated to the clinical realm. Plaque imaging can uncover the presence of fibrous cap rupture and vulnerable plaque characteristics that could lead to rupture, in patients with cryptogenic stroke and TIA. This can modify treatment and potentially result in surgical intervention in a patient who otherwise would have been managed by conservative therapy under current guidelines. Larger multicenter prospective trials—investigating the use of carotid MRI vessel wall imaging

in the evaluation algorithm of carotid atherosclerotic disease—that include newer therapies such as statins in the conservative therapy arm are needed to cement the role of this new imaging technique into the evaluation tree, but these imaging techniques have shown significant promise to date. Our clinical imaging protocol includes 2D double inversion recovery (DIR) black-blood T2, T1 pre- and postcontrast, 3D T1 MPRAGE noncontrast, and 2D TOF MRA of the carotid bulbs and postcontrast MRA of the neck using 4-channel carotid coils. This allows for adequate imaging of vulnerable carotid plaque in addition to a neck MRA in a protocol that lasts less than 30 minutes.

In the current issue of AJNR News Digest, we highlight work performed in carotid vulnerable plaque imaging that has furthered our understanding of the pathophysiologic mechanisms of plaque evolution and outlined imaging techniques for plaque characterization.

One article, by Standish et al, highlights what is currently known about the pathophysiology and classification of atherosclerotic disease and the current state of carotid vulnerable plaque imaging. CT, MR, and sonographic imaging are discussed, with summaries of the literature findings.  Optical coherence tomography (OCT), a novel intravascular imaging technique that may have future applications for carotid plaque characterization, is also discussed. This technique provides exquisite spatial resolution and pathologic characterization of coronary artery plaques not offered by current imaging techniques, and could provide the same for extracranial carotid imaging. The article discusses potential applications of OCT for carotid imaging.

A second article, by Wintermark et al, evaluates the ability of ex vivo high-resolution CT imaging to distinguish plaque composition, as compared with histopathologic evaluation post–carotid endarterectomy in symptomatic patients with greater than 50% extracranial carotid stenosis.

The next group of articles focuses on MR vessel wall imaging techniques, as applied to the carotid arteries. A multicenter trial sought to determine which MR plaque imaging features were most highly associated with intraplaque hemorrhage and fibrous cap rupture. The group also developed and validated a carotid atherosclerosis score based on atherosclerotic plaque imaging characteristics, that could serve to stratify plaques based on disease severity and risk. Another study aimed to determine the degree of plaque burden and composition in patients with 0% stenosis. This was accomplished by performing carotid vulnerable plaque imaging on arteries with 0% stenosis, in patients who had >50% atherosclerotic stenosis of the contralateral carotid bulb. Qiao et al evaluated symptomatic and asymptomatic patients with contrast-enhanced vessel wall imaging to determine the correlation between intraplaque hemorrhage and neovascularity with ischemic symptoms.

 

Image modified from: Standish BA, Spears J, Marotta TR, et al. Vascular Wall Imaging of Vulnerable Atherosclerotic Carotid Plaques: Current State of the Art and Potential Future of Endovascular Optical Coherence Tomography.