Imaging of Abdomen and Pelvis: Uncommon Acute Pathologies Rathachai Kaewlai, MD, Divya Kurup, MD, and Ajay Singh, MD A cute abdomen is the most common cause of presentation to the emergency room. Although conditions such as acute appendicitis, cholecystitis, pancreatitis, bowel obstruc- tion, and acute gynecologic pathologies are more common causes of acute abdominal and pelvic pain, there are a num- ber of uncommon acute pathologies that are important to diagnose because of either the high morbidity and mortality associated with them or their ability to mimic surgical abdo- men. We discuss the clinical findings, common imaging meth- ods performed for diagnosis, and radiologic imaging features of uncommon abdominal pathologies that can present with acute abdomen. The conditions include ruptured abdominal aortic aneurysm (AAA), acute mesenteric ischemia, perfo- rated peptic ulcer, volvulus, intussusception, internal hernia, acute epiploic appendagitis, and spontaneous splenic hem- orrhage. Ruptured Abdominal Aortic Aneurysm AAA is a permanent and irreversible localized dilatation of the abdominal aorta by  1.5 times the expected normal diameter. 1 Conventionally, the diagnosis is made when the diameter of the abdominal aorta is  3 cm. They account for approximately 15,000 deaths per year and is the 13th leading cause of death in the United States. 2 The patients typically present with the triad of sudden abdominal or flank pain, shock, and pulsatile abdominal mass. Most patients arriving at the emergency department usually have ruptures of the posterolateral wall, which leak into the retroperitoneal space. The aortic rupture of the an- terolateral wall is often associated with sudden death. In rare circumstances, rupture into the duodenum or inferior vena cava may occur, resulting in aortoduodenal or aortocaval fistula, respectively. Computed tomography (CT) is the modality of choice for the diagnosis of ruptured AAA 3-5 because of its high accuracy, wide availability, and short scan time. In addition, informa- tion from CT can be helpful for surgeons to determine treat- ment options. Ultrasound (US) is considered less useful in the diagnosis of this condition due to its limited ability to evaluate the entire abdominal aorta and operator depen- dency. Although magnetic resonance imaging can depict ruptured AAA, it is not commonly used because of long im- aging time, difficulty in scanning an ill patient, and limited availability in the emergency department. The most common CT finding of ruptured AAA are retro- peritoneal hematoma in the vicinity of AAA (Fig. 1). 3,4 Ret- roperitoneal hematoma appears on CT as hyperattenuating collection predominantly in the retroperitoneal space, in- cluding the perirenal and pararenal spaces. Retroperitoneal hematoma from ruptured AAA almost always involves mul- tiple retroperitoneal compartments. 3 If CT is performed with intravenous contrast administration, contrast extravasation may be visualized and is considered the most direct sign of aortic rupture. In most cases, even without the use of intra- venous contrast medium, aortic aneurysms are apparent. Calcification of the aneurysm wall should be traced for evi- dence of discontinuity that can signify the site of rupture. Disruption of calcification is a specific but uncommon find- ing in ruptured AAA. 6 Mural thrombi are common within the aneurysm sac and can have homogeneous or heterogeneous low attenuation on CT. The crescent sign represents crescentic hyperattenuation within the luminal thrombus or plaque in the aneurysm. A radiologic-pathologic correlation study has shown that cres- cent sign may represent hemorrhage in the mural thrombus or in the aneurysmal wall that tracks from the lumen through a cleft in the thrombus within an aneurysm. 7 It may be seen in ruptured or impending-ruptured AAA on CT. The sensitivity and specificity of crescent sign for the diagnosis of compli- cated AAA is 77% and 93%, respectively. 8 “Draped aorta sign” refers to an abnormal contour of the posterior aortic wall, which is ill-defined and drapes around the anterior vertebral body instead on axial CT images. It represents a deficient posterior aortic wall that can be seen with a con- tained rupture of AAA. 9 Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, MA. Address reprint requests to Ajay Singh, MD, Massachusetts General Hospi- tal, Harvard Medical School, 10 Museum Way, # 524, Cambridge, MA- 02141. E-mail: mghpartners@yahoo.com 228 0037-198X/09/$-see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1053/j.ro.2009.05.004