Inflammation in carotid atherosclerotic plaque: a dynamic contrast-enhanced MR imaging study

Abstract

Purpose: To prospectively evaluate if there is an association between plaque enhancement at magnetic resonance (MR) imaging and proinflammatory cardiovascular risk factors and plaque content.

Materials and methods: This study was performed with informed consent, HIPAA compliance, and institutional review board approval. Contrast agent dynamics within carotid plaques were measured in 30 patients (29 men, one woman; mean age, 67.7 years +/- 10.7 [standard deviation]) who were scheduled to undergo carotid endarterectomy. Measurements were based on kinetic modeling of images obtained at 15-second intervals during which a gadolinium-based contrast agent was injected. The time-varying signal intensities within the plaques were used to estimate the fractional plasma volume (vp) and transfer constant (Ktrans) of contrast material into the extracellular space. Pearson correlation coefficients were computed between blinded MR measurements and histologic measurements of plaque composition, including macrophages, neovasculature, necrotic core, calcification, loose matrix, and dense fibrous tissue. Correlation coefficients or mean differences were computed regarding clinical markers of cardiovascular risk.

Results: Analyzable MR images and histologic results were obtained in 27 patients. Measurements of Ktrans correlated with macrophage (r = 0.75, P < .001), neovasculature (r = 0.71, P < .001), and loose matrix (r = 0.50, P = .01) content. Measurements of v(p) correlated with macrophage (r = 0.54, P = .004), neovasculature (r = 0.68, P < .001), and loose matrix (r = 0.42, P = .03) content. For clinical parameters, significant associations were correlated with Ktrans only, with decreased high-density lipoprotein levels (r = -0.66, P < .001) and elevated Ktrans measurements in smokers compared with nonsmokers (mean, 0.134 min(-1) vs 0.074 min(-1), respectively; P = .01).

Conclusion: The correlations between Ktrans and histologic markers of inflammation suggest that Ktrans is a quantitative and noninvasive marker of plaque inflammation, which is further supported by the correlation of Ktrans with proinflammatory cardiovascular risk factors, decreased high-density lipoprotein levels, and smoking.

Figure 1

Precontrast MR image (upper left) and dynamic contrast-enhanced MR images at bolus arrival (upper right) and 75 seconds later (lower left). On precontrast image, contours of the lumen (inner circle) and outer wall (outer circle) are identified. At 75 seconds, the outer contour has been placed just outside enhancing rim of adventitia (arrows). Strong contrast enhancement of the lumen and jugular vein (J) is seen at t = 0 sec. Graph shows increase in signal intensity (relative to precontrast signal intensity) at 15-second intervals. Kinetic model is fit to data (line).

Figure 2

Histologic identification of macrophages and endothelial cells. Double-labelled immunohistochemical slides show macrophages (red reaction product) and endothelial cells (black reaction product) at (a) low power and (b) higher power (magnification, × 200). (c) Macrophage and neovascular areas were quantified by using an image analysis program that yielded pseudocolor images for endothelial cells (green) and macrophages (red).

Figure 3

Scatter plots of v_p measured by using dynamic contrast-enhanced MR imaging versus histologic measurements exhibit significant or nearly significant correlations. Regression lines are drawn.

Figure 4

Scatter plots of K^trans measured by using dynamic contrast-enhanced MR imaging versus histologic measurements exhibit significant or nearly significant correlations. Regression lines are drawn.