There is a descending aortic dissection that begins beyond the brachiocephalic vessels (Stanford B) and only involves the descending aorta (DeBakey type III). Note also the presence of the aortic valve prosthesis and a small proximal endoleak related to the Dacron graft on ascending aorta (not seen on enhanced series).
The coeliac and superior mesenteric arteries both arise from the smaller true lumen.