Conventional CT of the aorta.

The advent of CT has changed the way thoracic aortic aneurysms and dissections are evaluated. In many cases, CT is the only roentgenographic examination needed beyond plain radiograph. In evaluating aneurysm. CT's advantage over aortography is that it shows the wall and the mural thrombus, not just the contrast column. In some cases of aneurysm, however, particularly those arising near the aortic arch, aortography may be necessary to establish the precise location of the lesion with respect to the brachiocephalic arteries. In evaluating dissection, CT is less invasive and at least as accurate as aortography. As with aortography, the classic findings are contrast-filled multiple channels with intervening intimal flap(s). CT's particular advantage over aortography is that it permits diagnosis of dissection when blood in the false channel is clotted rather than free-flowing. In this case, the diagnosis can be made by demonstrating displaced intimal calcifications or the high-attenuation thrombus in the aortic wall or periaortic tissues. In cases of acute dissection for which emergency surgery is considered, however, aortography is preferred to CT because CT cannot provide information about aortic insufficiency or the condition of critical arterial branches such as the coronary, brachiocephalic, mesenteric, and renal arteries. In follow-up of treated aortic dissection CT is preferred over aortography because it is noninvasive and more informative. Compared with MRI, CT has the advantages of greater reliability and of compatibility with life-support apparatus. MRI is not able to depict calcification, so any displacement of calcification cannot be detected. MRI is most suitable for follow-up of treated patients or for acute patients who are stable or who cannot tolerate intravenous contrast agents. A flexible approach is essential in choosing the appropriate radiographic tests in the work-up of aortic dissection. In this article, a workable strategy is outlined, but the radiologist should not hesitate to change the order of tests or use a second test if results from one test are confusing or inconsistent with other clinical data.