Carotid Atherosclerotic Disease










  • Multiple contrast weighted imaging sequences (including bright- and black-blood techniques) are needed for accurate assessment of the luminal surface condition and plaque composition.



  • MRI is able to visualize carotid atherosclerosis at different stages and quantify atherosclerotic plaque burden.



  • Assessment of luminal narrowing alone may underestimate atherosclerotic plaque burden and the stage of disease.



  • Gadolinium contrast enhancement is beneficial for identification and quantification of the fibrous cap, lipid-rich necrotic core, and vessel wall inflammatory status.



  • The common carotid artery bifurcation provides an internal fiducial marker for coregistration in serial MRI studies.



  • Key plaque features identified by MRI, such as intraplaque hemorrhage and fibrous cap status, are associated with more rapid plaque progression and subsequent ischemic events.



KEY POINTS


Stroke is a leading cause of long-term disability and is the third most common cause of mortality in many countries. Carotid atherosclerosis is one of the causes of stroke. As a means to prevent cerebrovascular events, carotid endarterectomy has been advocated in patients with high-grade carotid stenosis. However, there is increasing evidence that luminal narrowing may be a poor predictor of carotid plaque vulnerability. The Asymptomatic Carotid Atherosclerosis Study noted that carotid endarterectomy was associated with a reduction in absolute risk for ipsilateral stroke of only 5.9% at 5 years, compared with medical management. If we assume that the projected risk reduction is equally distributed over the course of 5-year follow-up, only one stroke per year would be prevented for every 85 patients undergoing successful endarterectomy.


As such, additional criteria have been sought to better identify patients most at risk of complications from carotid disease. Based on analysis of histologic findings in carotid endarterectomy specimens, fibrous cap rupture, intraplaque hemorrhage, large necrotic cores with thin overlying fibrous caps, plaque neovasculature and inflammatory cell infiltration have been hypothesized to be features of the high-risk, vulnerable plaque.


Although histopathology studies have identified plaque features associated with prior ischemic events, it is not possible to determine risk for future events based on findings in excised tissue. Until recently progress toward prospectively testing the “vulnerable plaque hypothesis” has been hampered by the inability to accurately and reproducibly identify the crucial plaque features theorized to represent the high-risk lesion in vivo.


Cardiovascular magnetic resonance (CMR) imaging has been extensively validated by multiple investigators as an accurate and reproducible method to characterize human carotid atherosclerosis. Furthermore, CMR is ideally suited for serial studies of plaque progression and regression as it is noninvasive and does not involve ionizing radiation.


The case reports and the list of suggested reading that follow were selected to illustrate this chapter’s important points:



Case 1


Figure 25-1 shows application of black-blood technique for identification of luminal and outer wall boundaries of carotid arteries.




Figure 25-1


Appearance of a diseased left internal carotid artery (ICA) with eccentric plaque on T1-weighted CMR (spatial resolution 0.62 × 0.62 mm, pixel size 0.31 × 0.31 mm). The right carotid artery is normal. Note that the lumen area of the diseased left internal carotid artery is similar to that of the normal right ICA. Thus, measurement of luminal stenosis by conventional angiography would underestimate carotid plaque burden in this individual. The expansion seen in the outer boundary of the left ICA is consistent with the phenomenon of compensatory expansive remodelling, originally described by Glagov. ECA, external carotid artery; ICA, internal carotid artery; R, right; L, left.


Assessment of Lumen and Vessel Wall Area




Case 2


A 58-year-old man with a history of hypercholesterolemia presented with right upper extremity weakness of four weeks’ duration. Duplex ultrasonography discovered a 50% to 79% stenosis in the left internal carotid artery and 1% to 15% stenosis in the right carotid artery. The patient underwent a carotid CMR examination before endarterectomy, which revealed the presence of a large lipid-rich necrotic core in the left carotid artery ( Figure 25-2 ).




Figure 25-2


Precontrast T1-weighted MR image (T1W) demonstrating a large eccentric plaque in the common carotid artery. Postcontrast T1-weighted image (CE-T1W) demonstrates enhancement of the fibrous cap and less enhancement of the underlying core. The signal pattern (isointense on TOF, iso- to hyperintense on T1W, and less gadolinium-contrast enhancement on CE T1W) is consistent with presence of a lipid-rich necrotic core. The matched histologic cross section confirms the presence and size of the lipid-rich necrotic core. Note that the addition of gadolinium-contrast enhancement facilitates the delineation of the border of the lipid-rich necrotic core. TOF, time-of-flight-bright blood; with the rest all black-blood techniques; T1W, T1-weighted; CE T1W, contrast-enhanced T1-weighted; PDW, proton density-weighted; T2W, T2-weighted.


Appearance of the Lipid-Rich Necrotic Core




Case 3


A 57-year-old man described multiple episodes consistent with left amaurosis fugax over a period of 5 weeks. Duplex ultrasonography discovered a 80% to 99% left midinternal carotid artery stenosis. A carotid CMR was performed before endarterectomy, which showed a large lipid-rich necrotic core in the left common carotid artery. Use of the contrast-enhanced T1-weighted sequence improved the delineation of lipid-rich necrotic core as well as the overlying fibrous cap ( Figure 25-3 ).




Figure 25-3


Precontrast T1-weighted MR image (T1WI) demonstrating a large eccentric plaque in the common carotid artery (*, lumen of common carotid artery; JV, jugular vein). Postcontrast T1-weighted image (T1WI) demonstrates enhancement of the fibrous cap and less enhancement of the underlying necrotic core. Contours measuring the size of the fibrous cap ( green ) and necrotic core ( yellow ) areas are shown on the Post T1WI on the right. Matched histology from the excised specimen confirmed the corresponding location and size of the fibrous cap ( green ) and necrotic core ( yellow ). The magnified box is a high-power view of the lipid-rich necrotic core containing cholesterol clefts. In a comparison of 108 cross-sectional locations from 21 arteries, quantitative measurements of fibrous cap (FC) length, FC area, and necrotic core (NC) area were collected from matched contrast-enhanced MR images and histology sections. Blinded comparison of corresponding CMR and histology slices showed good correlation for length ( r = 0.73, p < 0.001) and area ( r = 0.80, p < 0.001) of the intact FC. The mean percent necrotic core areas (NC area divided by the wall area) measured by contrast-enhanced CMR and histology were 30.1% and 32.7%, respectively, and were strongly correlated across locations (i = 0.87, p < 0.001).

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Feb 1, 2019 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on Carotid Atherosclerotic Disease
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