Morphologic Features of Carotid Plaque Rupture Assessed by Optical Coherence Tomography

BACKGROUND AND PURPOSE:Rupture of the plaque fibrous cap and subsequent thrombosis are the major causes of stroke. This study evaluated morphologic features of plaque rupture in the carotid artery by using optical coherence tomography in vivo.MATERIALS AND METHODS:Thirty-six carotid plaques with high-grade stenosis were prospectively imaged by optical coherence tomography. “Plaque rupture” was defined as a plaque containing a cavity that had overlying residual fibrous caps. The fibrous cap thickness was measured at its thinnest part for both ruptured and nonruptured plaques. The distance between the minimum fibrous cap thickness site and the bifurcation point was also measured. Optical coherence tomography identified 24 ruptured and 12 nonruptured plaques.RESULTS:Multiple ruptures were observed in 9 (38%) patients: Six patients had 2 ruptures in the same plaque, 2 patients had 3 ruptures in the same plaque, and 1 patient had 5 ruptures in the same plaque. Most (84%) of the fibrous cap disruptions were identified at the plaque shoulder and near the bifurcation point (within a 4.2-mm distance). The median thinnest cap thickness was 80 μm (interquartile range, 70–100 μm), and 95% of ruptured plaques had fibrous caps of <130 μm. Receiver operating characteristic analysis revealed that a fibrous cap thickness of <130 μm was the critical threshold value for plaque rupture in the carotid artery.CONCLUSIONS:Plaque rupture was common in high-grade stenosis and was located at the shoulder of the carotid plaque close to the bifurcation. A cap thickness of <130 μm was the threshold for plaque rupture in the carotid artery.

Rupture of the fibrous cap and subsequent thrombosis are the major causes of cardiovascular events such as heart attack and stroke.^1–3 In a previous study of sudden coronary death, a fibrous cap thickness of 65 μm was chosen as a criterion of instability because for a cap to rupture, the average cap thickness was 23 ± 19 μm; 95% of caps measured <65 μm within a limit of only 2 SDs.¹ Therefore, the fibrous cap thickness of <65 μm is now widely accepted as the definition of in vivo coronary vulnerable plaque that is prone to rupture. Disruption of the fibrous cap is frequently observed in symptomatic carotid plaques^4,5 and is strongly associated with an ulceration appearance on angiography,⁶ which is considered an independent predictor of stroke on long-term follow-up in patients with symptomatic severe carotid stenosis.⁷ Redgrave et al⁸ examined the cross-sections of plaques with high-grade carotid stenosis and found that the optimum fibrous cap thickness for discriminating ruptured and nonruptured plaques was 200 μm; thus, it appears that there is no clear threshold for classifying plaques that are prone to rupture in vivo.Intravascular sonography, which is a widely used imaging method in the field of carotid artery intervention, has an axial resolution of 100–200 μm and a lateral resolution of 250 μm.⁹ Although it can visualize deep structures, intravascular sonography is not a suitable imaging technique for the detection of thin fibrous caps because its resolution is too low. Optical coherence tomography (OCT) has been introduced recently as a high-resolution imaging method.^10,11 The typical OCT image has an axial resolution of 10 μm, approximately 10 times higher than that of any other clinically available diagnostic imaging technique, such as intravascular sonography. OCT provides an accurate representation of the thickness of the fibrous cap that could not be measured by other imaging modalities.¹² In the present study, we evaluated the morphologic features of ruptured plaques in the carotid artery by using OCT.