Predicting Carotid Plaque Characteristics Using Quantitative Color-Coded T1-Weighted MR Plaque Imaging: Correlation with Carotid Endarterectomy Specimens

BACKGROUND AND PURPOSE:MR plaque imaging is used to evaluate the risk of embolic complications during carotid endarterectomy and carotid artery stent placement. However, its performance for characterizing intraplaque components has varied across studies and is generally suboptimal. Hence, we correlated MR imaging results with histologic findings to determine whether a combination of high-contrast T1-weighted imaging and quantitative image analysis could readily determine plaque characteristics.MATERIALS AND METHODS:We prospectively examined 40 consecutive patients before carotid endarterectomy by using a 1.5T scanner and axial T1-weighted spin-echo images under optimized scanning conditions. The percentage areas of intraplaque fibrous tissue, lipid/necrosis, and hemorrhage were calculated automatically by using the software with previously reported cutoff values and were compared with those of the specimens. The thickness of the fibrous cap was also measured manually.RESULTS:The percentage areas of fibrous, lipid/necrotic, and hemorrhagic components were 5.7%–98.7%, 1.3%–65.7%, and 0%–82.0%, respectively, as determined by the MR images, whereas the corresponding values were 4.8%–92.3%, 7.0%–93.8%, and 0%–70.4%, respectively, as determined by histologic examination. Significant positive correlation and agreement were observed between MR images and histologic specimens (r = 0.92, 0.79, and 0.92; intraclass correlation coefficients = 0.91, 0.67, and 0.89; respectively). Thickness of the fibrous caps on MR images (0.21–0.87 mm) and in the specimens (0.14–0.83 mm) also showed positive correlation and agreement (r = 0.61, intraclass correlation coefficient = 0.59).CONCLUSIONS:Quantitative analysis of high-contrast T1-weighted images can accurately evaluate the composition of carotid plaques in carotid endarterectomy candidates.

Estimation of the composition of carotid plaques is important to help identify the risk of embolic events during carotid endarterectomy or carotid artery stent placement. Ultrasonography has commonly been used for this purpose, but it is not useful in cases of extensive calcification or if the plaque is in a higher location; furthermore, ultrasonography lacks reliable and versatile quantitative metrics.^1–4 Thus, as a complement to ultrasonography, MR plaque imaging is now widely used to estimate plaque composition. However, plaque characterization remains unsatisfactory, presumably owing to the deterioration of image contrast, secondary to inappropriate scanning techniques and/or protocols.⁵ Recently, a nongated T1-weighted spin-echo (SE) technique with appropriately fixed scanning parameters was reported to improve and stabilize intraplaque contrast, compared with a cardiac-gated black-blood fast spin-echo technique in which T1-weighting tended to be attenuated and fluctuated according to a heart-rate-dependent setting of the TR.⁶ The nongated T1-weighted spin-echo method has been found to allow accurate estimation of the main plaque components with minimal overlap,⁷ suggesting potential advantages over other imaging modalities in terms of plaque characterization. However, quantitative evaluation of intraplaque composition has not been achieved by this method. Hence, in the present study, we used a quantitative color-coded image-analysis software to characterize intraplaque distribution by percentage area of fibrous, lipid/necrotic, and hemorrhagic tissues and to determine the accuracy of this method for predicting plaque characteristics by correlating the results with histologic findings from the carotid endarterectomy specimens.