CT - Contrast CT
Axial C+ arterial phase
There is aortic dissection arising at the level of an aberrant right subclavian artery (which courses behind both trachea and oesophagus).
No dissection of the ascending aorta can be identified (thus this represents a DeBakey type III, Stanford type B dissection).
The true lumen in this instance is easily traced as the smaller circular lumen located posteriorly on inferior slices. Calcification on the luminal surface confirms this.