We read with interest the article by Alvarez-Linera et al (1) that evaluated the capacities of MR angiography, CT angiography, and digital subtraction angiography (DSA) to detect carotid artery stenosis. We agree that MR angiography is adequate to replace DSA in most patients. Nevertheless, it has been proved that CT angiography is highly accurate and can also replace DSA (2, 3). In contrast to DSA or MR angiography, CT angiography allows direct visualization of arterial wall and atheromatous plaque. Thus, the measurement of the stenosis is much easier. Alvarez-Linera (1) considered that calcified plaque could be a limitation of CT angiography. This limitation can be avoided with appropriate postprocessing. Removing calcification with sophisticated software is not a good technique, because the main risk is overestimation of the stenosis and it is time consuming. With multiplanar volume reconstruction, it is possible to visualize the entire bifurcation initially with a large-volume reconstruction. By reducing volume reconstruction, we clearly visualize the residual lumen at the maximal part of the stenosis, even when circumferential calcified plaques are present. If multiplanar volume reconstruction is not available, transverse oblique reconstruction can be used. Moreover, attenuation of intraluminal contrast and calcifications are not similar and CT angiography is able to differentiate mural calcifications and contrast material. Therefore, calcifications should not be considered limitations of CT angiography.
Concerning plaque morphology, detection of ulcerated plaques may prove to be important, because it has been suggested that the presence of plaque ulceration is a risk factor for embolism. There is, however, no clear consensus regarding the optimal imaging strategy for the analysis of carotid plaque morphology (4). The inability of DSA to depict plaque ulceration is well documented and cannot be a reference standard. The only way to evaluate CT angiography or MR angiography in the depiction of ulceration is the comparison with histologic correlation.
In contrast to the study of Alvarez-Linera (1), we believe that CT angiography is a highly accurate and precise technique for determining the percentage of stenosis. If it were not an ionizing technique by using iodinated contrast medium, CT angiography would be the first examination for carotid artery evaluation after Doppler sonography. Because of these disadvantages, at present CT angiography is proposed as an alternative to MR angiography for the demonstration of carotid disease.
Reply
We appreciate Randoux et al’s interest in our article. In our experience, spiral CT angiography has limitations in delineating the lumen of the artery with dense circumferential calcifications or with ill-defined patchy calcifications (1). In the first case, calcifications are the limiting factor on maximum intensity projection images because of the difficulty in differentiating mural calcifications and intramural contrast material. To minimize this limitation, analysis in conjunction with the transverse source images may be useful (2). Although transverse source images perpendicular to the vessel lumen, as well as multiplanar volume reconstruction images, were also analyzed in our study, dense circumferential calcification of the arterial wall caused artifacts that interfered with the evaluation of the degree of stenosis. The lack of definition between calcification and contrast material is determined in part by the mild hardening of the radiograph, which provokes artifacts, and above all by the gradual decrease of the plaque density on its surface, which may be similar to the contrast material density. Nevertheless, one limitation in our study was that we used a CT scanner with a single-detector row unit. For several months, we have been using a 16-detector-row multisection spiral CT scanner. Despite use of such a CT scanner, similar problems persist when there is an excess of calcium. Our preliminary results suggest that the presence of calcifications may be also a limitation even when multidetector row helical CT scanners are used. Furthermore, one should bear in mind the intrinsic disadvantages of spiral CT angiography, including the need for ionizing radiation, iodinated contrast material, and optimization of imaging delay time. Therefore, we think that elliptic centric MR angiography is the first noninvasive technique to replace conventional digital subtraction angiography. Spiral CT angiography may be considered the first alternative to evaluate carotid artery stenting, claustrophobic patients, or those with contraindications to MR.
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