The Fleischner sign is a prominent central artery that can be caused either by pulmonary hypertension  or by distension of the vessel by a large pulmonary embolus. 

In this case it is most evident on the right, although the left is also larger than normal.  This was due to pulmonary hypertension.

This case was investigated for what was misinterpreted as a right hilar mass - CT showed this to be merely and enlarged right pulmonary artery.