Characterization of carotid atherosclerosis and detection of soft plaque with use of black-blood MR imaging

BACKGROUND AND PURPOSE: In the treatment of carotid atherosclerosis, the rate of stenosis and characteristics of plaque should be assessed to diagnose vulnerable plaques that increase the risk for cerebral infarction. We performed carotid black-blood (BB) MR imaging to diagnose plaque components and assess plaque hardness based on MR signals.MATERIALS AND METHODS: Three images of BB-MR imaging per plaque were obtained from 70 consecutive patients who underwent carotid endarterectomy (CEA) to generate T1- and T2-weighted images. To evaluate the relative signal intensity (rSI) of plaque components and the relationship between histologic findings and symptoms, we prepared sections at 2-mm intervals from 34 intact plaques. We then calculated the relative overall signal intensity (roSI) of 70 plaques to assess the relationship between MR signal intensity and plaque hardness and symptoms.RESULTS: The characteristics of rSI values on T1- and T2-weighted images of fibrous cap (FC), fibrosis, calcification, myxomatous tissue, lipid core (LC) with intraplaque hemorrhage (IPH), and LC without IPH differed. Symptomatic plaques were associated with FC disruption (P < .001) and LC with IPH (P < .05). The roSI on T1-weighted images was significantly higher for soft than nonsoft plaques. When the roSI cutoff value was set at 1.25 (mean of the roSI), soft plaques were diagnosed with 79.4% sensitivity and 84.4% specificity. The roSI was also significantly higher for symptomatic than for asymptomatic plaques. Soft and nonsoft plaques as well as symptomatic and asymptomatic plaques did not significantly differ on T2-weighted images.CONCLUSION: BB-MR imaging can diagnose plaque components and predict plaque hardness. This procedure provides useful information for planning therapeutic strategies of carotid atherosclerosis.

Carotid atherosclerosis accounts for a large proportion of the causes of cerebral infarction, and accurate diagnostic imaging of carotid stenosis is useful for clarification of the pathogenesis of cerebral infarction and planning of therapy. In the diagnostic imaging of carotid arterial lesions, luminography such as conventional angiography is generally performed to determine the rate of stenosis, and in randomized studies documenting the value of carotid endarterectomy (CEA) in the treatment of carotid atherosclerosis, therapeutic guidelines have been based on stenosis rate.^1–3 Recent studies have also shown the critical importance of diagnosing vulnerable plaques, which are associated with a higher risk for cerebral infarction, by imaging the carotid artery wall itself and determining plaque characteristics.^4,5 Therefore, less invasive and more accurate diagnostic modalities such as carotid ultrasonography (US) for plaque evaluation have considerable importance in the management of patients with carotid atherosclerosis. Carotid US has been widely applied to characterize atherosclerotic plaque, and the content of soft plaque (lipid and hemorrhage) is presently associated with echolucency.^6,7 Furthermore, accumulating evidence indicates that echolucent plaques represent biologically more active disease and are associated with the risk for future stroke.^8,9 In addition, although carotid artery stent placement (CAS) is becoming an increasingly popular alternative to CEA in the treatment of carotid stenosis, several reports have shown that soft plaques are associated with a high incidence of ischemic complication during CAS.^10–12 Therefore, accurate diagnosis of carotid soft plaque seems to be of paramount clinical importance. However, carotid US has some limitations because it is difficult to obtain full images on patients who have a short neck, high carotid bifurcation, or highly calcified plaques.¹³The chemical composition and physical properties of tissues can be determined by MR imaging, which indicates that this diagnostic technique should be useful in plaque characterization. Along with recent advances in imaging devices and techniques, many studies have documented the usefulness of high-resolution MR imaging in the diagnosis of plaque.^14–21 In addition to sorting plaque composition on the basis of MR signal intensity, if soft plaque can be differentiated from nonsoft plaque by overall plaque MR signal intensity, MR imaging will be a simple, objective, and useful method to diagnose carotid atherosclerosis. To our knowledge, however, few studies have closely assessed the MR imaging signals of plaque components by comparing CEA specimens with carotid MR imaging in vivo,²² and the findings on MR imaging of carotid soft plaque have not been described.Our study investigates the benefit of carotid black-blood (BB) MR imaging by evaluating the MR signal intensity of the components of carotid plaque and by detecting soft plaque on the basis of overall plaque MR signal intensity.