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What is the best imaging modality for diagnosing acute aortic syndromes?
CT and MRI are the two best modalities for evaluating these conditions. CT is generally preferred because it is faster (important in patients with an unstable condition) and has better spatial resolution. MRI is often used for problem solving in patients with a stable condition or in patients with an iodinated contrast allergy. For patients with renal disease on permanent dialysis, CT is performed because of the risk of nephrogenic systemic fibrosis related to gadolinium-based MRI contrast agents. For patients with renal disease who are not on permanent dialysis, noncontrast MRA techniques can be used. Although these techniques generally have lower spatial resolution and take longer to acquire than contrast-enhanced MRA, they are usually sufficient to exclude an acute aortic syndrome. Ultrasonography (US) also has been used, but it cannot visualize the entire aorta and misses the extent of disease or associated complications. At this time, the main indication for use of catheter angiography for these conditions is treatment of complications.
How can these conditions be distinguished on axial CT or MR images?
A dissection flap most often appears as a linear or curvilinear focus across the vessel, separating the true lumen from the false lumen, which may be calcified. Intramural hematomas usually appear as a crescentic region of high attenuation or high T1-weighted signal intensity with smooth borders. The presence of an irregular luminal border with peripheral low attenuation may reflect ulcerated atherosclerotic plaque or thrombus. A limited intimal tear most often appears as a subtle contour abnormality of the aortic wall without history of trauma or presence of an intimal flap or intramural hematoma.
