A noninvasive imaging approach to assess plaque severity: the carotid atherosclerosis score

Abstract

Background and purpose: The presence of IPH and/or FCR in the carotid atherosclerotic plaque indicates a high-risk lesion. The aim of this multicenter cross-sectional study was to establish the characteristics of lesions that may precede IPH and/or FCR. We further sought to construct a CAS that stratifies carotid disease severity.

Materials and methods: Three hundred forty-four individuals from 4 imaging centers with 16%-99% carotid stenosis by duplex sonography underwent carotid MR imaging. In approximately 60% of the study sample (training group), multivariate analysis was used to determine factors associated with IPH and FCR. Statistically significant parameters identified during multivariate analysis were used to construct CAS. CAS was then applied to the remaining arteries (40%, test group), and the accuracy of classification for determining the presence versus absence of IPH or, separately, FCR was determined by ROC analysis and calculation of the AUC.

Results: The maximum proportion of the arterial wall occupied by the LRNC was the strongest predictor of IPH (P < .001) and FCR (P < .001) during multivariate analysis of the training group. The subsequently derived CAS applied to the test group was an accurate classifier of IPH (AUC = 0.91) and FCR (AUC = 0.93). Compared with MRA stenosis, CAS was a stronger classifier of both IPH and FCR.

Conclusions: LRNC quantification may be an effective complementary strategy to stenosis for classifying carotid atherosclerotic disease severity. CAS forms the foundation for a simple imaging-based risk-stratification system in the carotid artery to classify severity of atherosclerotic disease.

Fig 1.

In vivo identification of IPH, FCR, and ulceration. Each row contains multicontrast axial images from a single location in the carotid artery. The outer wall boundary (white arrowheads) of either the internal or common carotid artery, the lumen of the internal or common (asterisk) carotid artery, the external (cross, where applicable) carotid artery, and the JV are identified. The top row is of the right internal carotid artery from a 69-year-old man imaged at MSU. IPH (arrows) is characterized by a hyperintense signal intensity on TOF and T1WI. The second row is of the right internal carotid artery from an 80-year-old man imaged at PLA. An FCR (white arrow) is evident on TOF imaging by the hyperintense signal intensity extending from the lumen into the plaque and the absence of a fibrous cap on T2WI and CE-T1WI. The third row is of the left common carotid artery from a 74-year-old man imaged at PLA. An ulceration (black arrows) is present. Notably, the varying appearance of the ulcer between the different contrast weightings is due to reduced flow suppression caused by turbulent flow in the ulcerated region, particularly after contrast administration.

Fig 2.

ROC curve analyses for the classification of IPH (A) and FCR (C) in the training set by maximum percentage LRNC. Adjacent to each ROC curve is a cumulative prevalence plot for IPH (B) and FCR (D) versus maximum percentage LRNC. The cumulative prevalence plots clearly depict changes in slope (arrows) for both IPH (B) and FCR (D), which were subsequently used to construct the CAS. E, ROC curve analysis for prediction of the presence of LRNC in the training set by maximum wall thickness, maximum total vessel area, and maximum NWI. F, In the cumulative prevalence plot for LRNC by maximum wall thickness, LRNC was absent in lesions with a maximum wall thickness <2 mm.

Fig 3.

Flow diagram of CAS. Subjects with a maximum wall thickness >2 mm require additional evaluation. Further categorization of lesions can be determined by the size of the maximum percentage LRNC. Examples of matched cross-sectional images from 3 contrast weightings (TOF, T1WI, and CE-T1WI) for each category are provided. Images corresponding to CAS 1 (maximum wall thickness <2 mm) are from the left common carotid artery of a 69-year-old man imaged at MSU. The plaque in the CAS 2 (maximum percentage LRNC, ≤20%) example is from the right internal carotid artery of a 63-year-old man imaged at UW. There is a small LRNC (arrow) present on postcontrast imaging. Of note, there are also flow artifacts visible in the lumen—common artifacts in images distal to the bifurcation. The lesion in CAS 3 (maximum percentage LRNC, >20% and ≤40%) is from the left common carotid artery of a 65-year-old man imaged at AH. A noticeable LRNC (arrows) is present in both the anterior and posterior arterial wall. An example of a large LRNC (arrow) without IPH in the left common carotid artery of a 64-year-old man imaged at PLA is shown for CAS 4 (maximum percentage LRNC, >40%). Arrowheads indicate the outer wall boundary; asterisk, the lumen.

Fig 4.

Prevalence of key features in the carotid artery is shown for each MR imaging location relative to the bifurcation. Distance from the bifurcation (0) is labeled on the x-axis, where positive and negative integers represent locations within the internal and common carotid arteries, respectively.

Fig 5.

ROC curve analyses for IPH (A) and FCR (B) by using either CAS or MRA stenosis.