Abdominal aortic aneurysms that have ruptured or are at imminent risk of rupture

The rupture of an abdominal aortic aneurysm (AAA) is considered a high-risk surgical emergency, given the catastrophic consequences and high mortality rate. The objective of this pictorial essay is to illustrate the radiological signs that indicate rupture or imminent rupture. To that end, we describe cases treated at our facility and present a brief review of the literature on the topic. The clinical diagnosis of imminent AAA rupture can be difficult, because patients are usually asymptomatic or have nonspecific pain complaints. In the subsequent follow-up, it is possible to identify radiological signs that indicate instability or rupture itself and thus change the prognosis. Computed tomography is the modality of choice for evaluating an AAA and abdominal pain in the emergency setting. It is therefore essential that the radiologist immediately identify the imaging findings that indicate AAA rupture or the imminent risk of such rupture.

The objective of this pictorial essay is to familiarize radiologists with the imaging findings that indicate rupture or imminent rupture of an AAA, using images related to patients examined at our facility. All examinations were performed in multidetector CT scanners: a 160-slice scanner (Aquilion Prime; Toshiba Medical Systems, Otawara, Japan); or a 128-slice scanner (Optima; GE Healthcare, Milwaukee, WI, USA).
A total of 50 adult patients received intravenous injection of low-osmolar iodinated contrast medium, with the aid of an injection pump, at a flow rate of 4.0-4.5 mL/s and an approximate overall dose of 1.5 mg/kg of body weight. Volumetric images were acquired in axial and multiplanar reconstructions, before and after the

INTRODUCTION
In most cases, abdominal aortic aneurysms (AAAs) are asymptomatic and are diagnosed incidentally during imaging examinations performed for other indications. The natural history of an AAA consists in a progressive increase in its diameter and its potential rupture, a medical emergency associated with extremely high mortality and therefore requiring immediate surgical treatment (1) . Early identification of imaging findings indicating the rupture or imminent risk of rupture of an AAA can change the prognosis and ensure more appropriate treatment, making it fundamental that radiologists recognize such changes (1,2) . Because it is a widely available, rapid imaging method, computed tomography (CT) angiography is the exam of choice in such cases (1,3,4) . 0100-3984 © Colégio Brasileiro de Radiologia e Diagnóstico por Imagem administration of the contrast, with an emphasis on arterial and venous studies.

FINDINGS THAT INDICATE AAA RUPTURE
The imaging finding most commonly associated with AAA rupture is retroperitoneal hematoma adjacent to the affected aortic segment (5) . That finding translates to a loss of aneurysmal wall integrity and appears on CT as a periaortic focus of soft-tissue density. The hematoma can extend into the pararenal and perirenal spaces (Figure 1), as well as to the psoas muscle ( Figure 1) and into the intraperitoneal space. In contrast-enhanced images, active extravasation of the contrast agent can be seen.
In the case of a ruptured aneurysm contained, neighboring structures such as the vertebral bodies or adjacent retroperitoneal tissues buffer the hemorrhage and the patient may remain hemodynamically stable (1) . A CT scan of a contained rupture can show the draped aorta sign, in which neither the posterior wall of the aorta nor the periaortic fat plane is distinguishable (1,5) (Figure 2).

FINDINGS THAT INDICATE IMMINENT RUPTURE OF AN AAA
The maximum diameter and growth rate of an aneurysm are the most common predictors of its rupture, underscoring the importance of serial imaging in the followup of patients with an AAA (5,6) . In most cases of typical fusiform aneurysms, a surgical approach is indicated if the aneurysm diameter is > 5.4 cm (6) or the aneurysm grows by more than 5 mm over a six-month period (6,7) (Figure 3).   The hyperattenuating crescent sign corresponds to a hyperattenuating peripheral area within the wall of the aorta or within a mural thrombus, indicating infiltration of blood from the lumen of the aneurysm into those structures, with consequent weakening of the wall of the aneurysm (1,5) . The hyperattenuating crescent sign is best visualized on unenhanced CT scans ( Figure 4) and is characterized by attenuation greater than that of intraluminal blood.
A focal discontinuity of the parietal circumferential calcification of the abdominal aorta can indicate that an aneurysm is unstable. That is especially relevant when the discontinuity is new or there are new outpouchings (1,5) .
Although less common in AAAs than in thoracic aortic aneurysms, the development of penetrating atherosclerotic ulcers ( Figure 5) also indicates that an aneurysm is unstable. The expansion of such ulcers increases the risk of outpouching and rupture (1) .  A B