This case illustrates thoracic aortic aneurysm presenting as a mediastinal mass. 

The case also illustrates that arterial phase imaging is not required to make the diagnosis - it wasn't performed in this case because of allergy. Arterial imaging helps with diagnosis of complications and characterization. The aneurysm is probably a pseudoaneurysm resulting either from prior trauma or a penetrating atherosclerotic ulcer. 

Case courtesy of Bob Cook, MD. Western Memorial Regional Hospital Corner Brook, Newfoundland.