CT is the primary modality for evaluating abnormalities of the thoracic aorta. Multidetector CT, with its multiplanar capability, can be used to evaluate an aneurysm in any plane, determine its size and morphologic features, clarify its relationship to branch vessels, evaluate its effect on adjacent structures, and identify complications such as dissection and rupture. These options give multidetector CT a decided advantage over conventional angiography, which provides information primarily about the aortic lumen.
When an acute aortic syndrome is suspected (owing either to clinical signs and symptoms or to chest radiographic findings), nonenhanced CT is usually performed first to look for a high-attenuation acute intramural hematoma. The contrast-enhanced scanning that follows is the key part of the CT examination. The correct timing of the arrival of contrast material in the aorta is critical to image quality. Approaches to accomplishing this include a timing bolus or a bolus tracking technique. Electrocardiographic (ECG) gating is commonly used to reduce motion artifacts, which can mimic dissection or luminal irregularities; this is particularly important with higher heart rates and in areas that move the most with cardiac motion, such as the ascending aorta. In addition, ECG gating can facilitate evaluation of at least the proximal coronary arteries (if not the entire coronary artery system) if the specified acquisition parameters provide the appropriate spatial and temporal resolution. In cases of suspected aortic dissection, it may be useful for determining coronary artery involvement. We routinely use ECG gating for the thoracic portion of our CT examinations of the aorta, which are performed on either a 16- or 64-detector CT scanner.