Quantitative assessment of carotid plaque composition using multicontrast MRI and registered histology

Abstract

MRI is emerging as a promising modality for monitoring carotid atherosclerosis. Multiple MR contrast weightings are required for identification of plaque constituents. In this study, eight MR contrast weightings with proven potential for plaque characterization were used to image carotid endarterectomy specimens. A classification technique was developed to create a tissuespecific map by incorporating information from all MR contrast weightings. The classifier was validated by comparison with micro-CT (calcification only) and with matched histological slices registered to MR images using a nonlinear warping algorithm (other components). A pathologist who was blinded to the classifier results manually segmented digitized histological images. Atherosclerosis is a chronic and progressive disease that is characterized by the accumulation of lipids and fibrous tissue within the artery wall (1). Disruption of atherosclerotic plaques at the carotid bifurcation is believed to be the underlying cause of many embolic strokes; furthermore, the histological composition of carotid atheroma is related to a plaque's vulnerability to rupture (2,3). In general, unstable plaques are characterized by a thin, fibrous cap that separates a pool of extracellular lipid and necrosis from the lumen (4). If the integrity of the fibrous cap is compromised, the necrotic core is exposed to the bloodstream and thrombus formation may occur. Several factors may play a role in triggering plaque rupture, including the size and spatial distribution of various plaque components (5); however, the specific plaque components or mechanisms responsible for plaque rupture and/or erosion and subsequent clinical events have yet to be elucidated (4). Further knowledge regarding the explicit features that confer vulnerability to a lesion may come from studies that track changes in plaque composition over time.MR is a promising noninvasive technique for characterizing atherosclerotic plaque composition. Because of the complexity and variability of atherosclerotic lesions, it is necessary to employ several MR contrast weightings to adequately discriminate between plaque constituents. Ex vivo studies have demonstrated that T 2 -weighted contrast alone is inadequate for measuring the size of the lipid core (6), but a combination of proton density, partial T 2 , T 2 , and diffusion-weighted spin-echo sequences can provide a more accurate assessment of plaque microstructure (7). Multicontrast-weighted MR has also been shown to be necessary for characterizing plaque in vivo. In a series of patients undergoing carotid endarterectomy, no single MR contrast weighting was able to identify lipid-rich necrotic cores and intraplaque hemorrhage; rather, a review of proton density, T 1 , T 2 , and time-of-flight (TOF) contrast weightings was required (8). In a subsequent investigation, these same four contrast weightings provided enough information to successfully classify carotid plaques according to a modified American Heart Association classification scheme (9). These stu...

The influence of composition and location on the toughness of human atherosclerotic femoral plaque tissue

“…Additionally, existing clinical imaging tools are currently capable of characterising the composition of atherosclerotic plaques. Calcified tissue fraction, fibrous tissue and lipid content can be identified by optical coherence topography [70,71] and also by magnetic resonance imaging [72][73][74].…”

Performance of steady‐state free precession for imaging carotid artery disease

“…However, there has been little study in groups with pathology in which complex flow patterns exist. Certainly, in ex vivo carotid artery imaging, without the presence of flowing blood, SSFP has been successful (17). However, its full capability for high-resolution carotid artery imaging under flowing conditions remains in doubt.…”

Characterization of coronary atherosclerotic plaque using multicontrast MRI acquired under simulated in vivo conditions

“…In the present study we were interested in evaluating the feasibility of extending multicontrast characterization procedures from ex vivo to in vivo. This is critical because automatic classification techniques (13,14) reported in the literature rely heavily on techniques developed using ex vivo scans. The automatic plaque characterization algorithm relies on the comparative, rather than absolute, intensity of each plaque constituent.…”

“…The coronary vessels were preserved in formalin after the MRI scan, and then scanned a second time using the same imaging protocol. An automatic plaque characterization technique (12)(13)(14) employing fuzzy C-means (FCM)-based clustering with spatial information incorporated via penalty terms (15) was applied to classify plaque components in multicontrast MR images. Classification results from both the "fresh" and "preserved" scans were compared with each other and with corresponding histological slices.…”