Imaging description

Acute aortic syndrome is a term describing a group of emergent conditions of the aorta with similar clinical presentations. There are three types of acute aortic syndrome: aortic dissection, intramural hematoma, and penetrating atherosclerotic ulcer. In each of these entities, blood accumulates in the medial layer of the aortic wall. In aortic dissection, blood flows within a false lumen in the medial layer created by proximal and distal entry tears in the aortic intima. In intramural hematoma, high sheer forces result in bleeding of small vessels in the aortic media. Finally, penetrating ulcers result when atherosclerotic disease erodes through the intima into the media of the aortic wall. Contrast-enhanced CT is generally considered the test of choice for diagnosis of acute aortic syndrome. However, these screening exams are typically performed without electrocardiographic gating and the resultant images of the ascending aorta are limited by pulsatile motion during the cardiac cycle. This motion can produce images that mimic dissection or intramural hematoma, resulting in falsepositive diagnosis. Motion blurring can be recognized by an indistinct aortic wall, irregular outer contour of the aorta, a doubled appearance of the aorta, and blurring of adjacent cardiac structures. The appearance of a false intimal flap is common, and is most often seen in the left anterior or right posterior wall of the aortic root. Partially imaged aortic valve leaflets may also contribute to the false appearance of an intimal flap in this location. Motion blurring can also result in volume averaging of portions of the aortic wall with adjacent mediastinal fat, giving the false appearance of aortic wall thickening that can be mistaken for intramural hematoma (Figure 55.1). In most cases, these findings can be easily dismissed as motion artifacts; however, if there is uncertainty, ECG-gated CT angiogram or MR angiogram should be obtained for definitive evaluation (Figure 55.1). Additionally, acute aortic syndrome protocols should include precontrast images to evaluate for high-attenuation aortic wall thickening that is present in intramural hematoma.