As a result of pathological studies carried out on plaques, a classification (also approved by the American Heart Association) was developed. There are eight types of atherosclerotic lesions according to this classification. Fibrotic lesions are characterized by fibrous tissue and do not contain a lipid structure. Fibrotic and calcific plaques are more stable plaques and their probability to rupture is very low; however, they exhibit a symptom of hampering blood flow from the carotid artery 15
The main aim in radiological imaging of a patient with atherosclerotic carotid artery disease is to detect the degree of carotid artery stenosis caused by the plaque. As the percentage of stenosis increases, the possibility of becoming symptomatic increases. The generally accepted view is that plaques that cause 70% or more stenosis should be treated 16.
De Weert et al. 17 suggested that the surface structure of the atherosclerotic plaque can be examined by an MDCT angio. Adraktas et al. 18 and Van Gils et al. 19 were able to reveal the risk factors showing temporal alterations on the plaques of the patients with carotid stenosis who were followed up with MDCT. The use of ionized radiation and the artifacts resulting from the calcifications and metallic implants limit in general the utilization of CT. Furthermore, it is not able to show the plaques soft components such as a hemorrhage 20. A diagnostic improvement in the discrimination between soft tissues through a dual energy CT has been reported; in the future this technique could be used more frequently in carotid plaque examination 21.
In our study, a significant difference was found between the unenhanced and enhanced contrast plaque densities, the plaque densities increasing after the contrast medium administration on MDCT. The retention of the contrast material means that the plaques contain inflammation and neo-angiogenesis. Generally vulnerable plaques which are likely to become symptomatic contain plenty of inflammation cells and microvascular angiogenesis. In light of this information, theoretically vulnerable plaques (i.e. symptomatic plaques) are expected to retain more contrast material than stable, asymptomatic plaques. In the literature, there are only a few studies on this topic which use the same technique presented here (i.e., MDCT) with inconsistent results. Saba et al. have found statistically significant density increases in symptomatic plaques and suggested that the discrimination of vulnerable plaque could be made in MDCT angio examinations where unenhanced and enhanced contrast imaging is performed 22,23. On the contrary, we could not find any significant difference between the asymptomatic and symptomatic plaques in terms of density increase on the unenhanced and enhanced contrast images in MDCT. Ha et al. have found similar results with a lack of significant correlation in the averaged HU value between the pre-contrast and post-contrast early arterial phase and between the symptomatic and asymptomatic sides, in MDCT. However, they had found a statistically significant difference in the post-contrast HU values and difference of pre-contrast and post-contrast HU values between the symptomatic and asymptomatic sides 27.
Our study demonstrated that there was a statistical difference among the plaque densities between the bulbar and post-bulbar locations; the plaques at the post-bulbar location having lower density values. Lower densities imply that the plaque contains more fat and/or less micro-calcification. These characteristics are likely to belong to the vulnerable plaque. Besides the degree of plaque stenosis among the symptomatic patients were also found to be significantly higher in our study, which is also consistent with literature 16.
In the density measurement on MDCT cross-sections, some of the limitations of this study that we thought influenced the results negatively are given below;
1. There may be changes in the locations of unenhanced and contrast-enhanced examinations due to breathing, swallowing and other causes. This could result in an error in measuring location.
2. Beam heartening artifact affects density measurements directly. Although plaques containing calcification were not included, the effects of these artifacts could originate from other soft tissue and bony structures and could not be completely eliminated.
3. Since contrast-enhanced images were taken at the early arterial phase to prevent venous contamination for ideal MDCT angiography, intra-plaque contrast retention (density increase) might not have been sufficiently established; thus imaging at a later phase might give more correct results.