Magnetic resonance imaging of carotid plaques: current status and clinical perspectives

Abstract

Rupture of a vulnerable carotid plaque is one of the leading causes of stroke. Carotid magnetic resonance imaging (MRI) is able to visualize all the main hallmarks of plaque vulnerability. Various MRI sequences have been developed in the last two decades to quantify carotid plaque burden and composition. Often, a combination of multiple sequences is used. These MRI techniques have been extensively validated with histological analysis of carotid endarterectomy specimens. High agreement between the MRI and histological measures of plaque burden, intraplaque hemorrhage (IPH), lipid-rich necrotic core (LRNC), fibrous cap (FC) status, inflammation and neovascularization has been demonstrated. Novel MRI sequences allow to generate three-dimensional isotropic images with a large longitudinal coverage. Other new sequences can acquire multiple contrasts using a single sequence leading to a tremendous reduction in scan time. IPH can be easily identified as a hyperintense signal in the bulk of the plaque on strongly T₁-weighted images, such as magnetization-prepared rapid acquisition gradient echo images, acquired within a few minutes with a standard neurovascular coil. Carotid MRI can also be used to evaluate treatment effects. Several meta-analyses have demonstrated a strong predictive value of IPH, LRNC, thinning or rupture of the FC for ischemic cerebrovascular events. Recently, in a large meta-analysis based on individual patient data of asymptomatic and symptomatic individuals with carotid artery stenosis, it was shown that IPH on MRI is an independent risk predictor for stroke, stronger than any known clinical risk parameter. Expert recommendations on carotid plaque MRI protocols have recently been described in a white paper. The present review provides an overview of the current status and applications of carotid plaque MR imaging and its future potential in daily clinical practice.

Keywords: Atherosclerosis; carotid artery; magnetic resonance imaging (MRI); stroke.

Figure 1

Transversal magnetic resonance (MR) images of a carotid plaque in the right carotid artery with intraplaque hemorrhage (IPH). The following MR sequences were acquired (A) pre-contrast T₁w-weighted (T₁w) quadruple inversion recovery (QIR) turbo-spin echo (44), (B) post-contrast T₁w QIR TSE, (C) T₂w TSE, (D) T₁w inversion recovery (IR) turbo-field echo (TFE) and (E)time of flight (TOF). A lipid-rich necrotic core LRNC was identified as a region within the bulk of the plaque that does not show contrast enhancement (* on B) with thin and/or ruptured fibrous cap (small arrow on panel B). On the T1 IR-TFE image, a hyper-intense signal in the bulk of the plaque can be clearly observed, indicating the presence of intraplaque hemorrhage IPH within the area of LRNC (* on panel D). Calcification was identified as low signal intensity on TOF and at least two other weightings (long arrow on A, B and E). Panel (F) shows the plaque contours on the pre-contrast T₁w QIR TSE images (green = outer vessel wall, red = inner vessel wall, yellow = lipid-rich necrotic core, blue = IPH, orange/brown = calcifications).