RADIOLOGY: AORTA: Case# 42: AAA DISSECTION. This is a 46 year old male on peritoneal dialysis for end stage renal disease with fever and bilateral flank pain worse on the left. Patient has a known AAA and aortic dissection. Patient under- went a pericardial window yesterday and there is clinical question of ischemic bowel. Aneurysmal dilatation of the abdominal aorta with maximal AP and transverse dimensions of 6.0 and 4.8 cm, respectively. A vertically oriented dissection extends from the level of the right pulmonary artery into the common iliac arteries. Both the true and false lumen are opacified by contrast. The celiac artery and SMA are patent with their ostia arising just above the intimal flap, probably from the true lumen. A large amount of intraluminal thrombus is present throughout the lower thoracic and infrarenal AAA. Abdominal aortic aneurysms usually occur in the setting of atherosclerotic disease but may be caused by syphilis, by extension of a dissection from above, or by connective tissue disorders such as Takayasus arteritis. Complications of abdominal aortic aneurysm include rupture, peripheral embolization, thrombosis, and infection. The incidence of rupture increases with increasing aneurysm size above 4 cm. Rupture usually occurs into the left retroperitoneal space and rarely into the gastrointestinal tract and inferior vena cava. If a ruptured or leaking aneurysm is suspected, CT should be done with contrast enhancement. Typical findings include obscuration or anterior displacement of the aneurysm by an irregular high density mass or collection that extends into one or both perirenal spaces. The wall of the aneurysm may be identified by calcifications while the lumen enhances. Other findings include anterior displacement of the kidney by hematoma, enlargement or obscuration of the psoas muscle, and a focally indistinct aortic margin that corresponds to the site of rupture. In contrast, a chronic pseudoaneurysm appears as a well-defined, ususally round mass with attenuation similar or lower than that of the native aorta on noncontrasted images.