Findings
-The CXR demonstrates a widened mediastinum.
-The CT demonstrates the intimal flap in the ascending and descending thoracic aorta.
Discussion
-Aortic dissection is the separation of the layers of the aortic wall, usually caused by a tear in the weakened intima . Aortic dissection is the most common fatal condition involving the aorta. The Stanford Classification has 2 types: Type A (60%) involves the ascending aorta. It may involve the arch or descending aorta to varying degrees. Type B (40%) involves the arch and/or descending aorta but not the ascending aorta. This is a Stanford type A aortic dissection, involving the origins of most of the aortic arch vessels. Notice that the aortic knob is still distinct and there is no mediastinal hematoma which differentiates this from an aortic transection.


http://eradiology.bidmc.harvard.edu/Classics/item.aspx?section=Emergency+Radiology&labelpk=33f0adab-853f-4010-a0e2-0756b3f1eac5&pk=f754f9fb-33f0-4a73-9afd-47845e2c601a