The primary imaging findings of AAA rupture often are readily apparent and include a retroperitoneal hematoma extending directly from the aneurysm and active extravasation of contrast material (Fig. 7.1) [7]. The vast majority of ruptured AAA patients present with hemorrhage that extends into the retroperitoneum, and intraperitoneal hemorrhage is far less common. Primary AAA rupture into the inferior vena cava as aortocaval fistula or gastrointestinal tract as aortoenteric fistula is also rare. Although contained AAA rupture can also present with the abovementioned findings, impending rupture may have much more subtle findings, including perianeurysmal soft tissue stranding. Additional CT findings associated with AAA rupture or impending rupture include the hyperattenuating crescent, draped aorta, and tangential calcium signs. Rapidly increasing aneurysm diameter and focal discontinuity of intimal calcification within the wall of the aneurysm also can be indications of aneurysm instability.

A “hyperattenuating crescent” refers to a crescentic area of high attenuation within the wall or mural thrombus of an aneurysm secondary to penetration of blood from the aneurysm lumen into the aortic wall or mural thrombus (Fig. 7.2) [8]. By definition, a “hyperattenuating crescent” has a higher CT attenuation than the luminal blood on unenhanced scans and a higher attenuation than the skeletal muscle on postcontrast scans but is not as high as the attenuation of calcium [9]. Although the reported prevalence of this finding in patients with AAA rupture ranges from 21 to 77 %, the specificity has been reported to be as high as 93% [9, 10].

A “draped aorta” describes the appearance of an AAA wall that bulges posteriorly, either unilaterally or bilaterally, and has lost its fat plane with the adjacent psoas muscle and vertebra (Fig. 7.3). This finding is a sign of aortic wall insufficiency and impending rupture, but also can be seen with a contained aortic leak [11, 12].

Focal discontinuity of intimal calcification within an AAA wall, particularly when shown to be a new finding in comparison with prior studies, has been shown to be a useful sign of acute or impending rupture [9]. The “tangential calcium” sign, defined as calcified atherosclerotic plaque adjacent to an area of focal calcium discontinuity that is divergent from the normal arc of the aorta, is another finding highly associated with AAA rupture (Fig. 7.1) [13].

Aortic intramural hematoma (IMH) represents blood within the aortic wall without persistent flow or an intimomedial flap. Whether this blood arises due to rupture of the vasa vasorum or thrombosis of a dissection is somewhat controversial but probably irrelevant from the perspective of the radiologist as both potential etiologies result in the endpoint of IMH, a condition which is managed similarly to dissection [14, 15]. IMH usually resolves spontaneously, but it may progress to classic dissection, focal aortic ulceration, aortic aneurysm, aortic dissection, and rarely rupture [15, 16]. Factors that predict progression of an IMH to dissection or rupture include the presence of an aortic aneurysm, an ulcer-like projection, persistent symptoms after treatment, and progressive increase in aortic wall thickness and/or aneurysm diameter [17, 18]. IMH most commonly occurs in the descending thoracic aorta, rarely in the abdomen alone.

IMH is seen on imaging as an eccentric, crescent-shaped collection of blood in the aortic wall. On CT, this feature is best seen as hyperattenuation (40–70 HU) on precontrast images, particularly with the use of narrow window settings (Fig. 7.4) [19]. On MRI, precontrast T1-weighted or black blood images may be useful to demonstrate intramural blood products, which can be iso- or hyperintense to the skeletal muscle, depending on the age of the hematoma [20]. Interpretation based on viewing postconstrast images alone may be problematic as the intramural hematoma is less conspicuous when adjacent to the brightly enhancing aortic lumen, thus potentially being confused for mural thrombus. Imaging features that support mural thrombus rather than an IMH include an irregular intraluminal surface (IMH typically is smoothly contoured), isolation to a dilated portion of the aorta (IMH is more common in a normal diameter aorta), and the presence of multiple, interrupted lesions [15, 20]. Imaging features that help distinguish IMH from aortic dissection include the lack of enhancement of IMH, whereas delayed images generally depict some flow in the false lumen of a classic dissection. Fixed, medial displacement of intimal atherosclerotic calcification rather than a spiraled appearance of displaced intimal calcium also favors an IMH over a classic aortic dissection [21].