Westermark sign in pulmonary embolism (Image, NEJM)

Westermark sign: a focal area of decreased lucency on a plain chest film with an abrupt cut-off in the pulmonary artery, corresponding to a central occlusive pulmonary embolism.

A.S. Krishnan and Tristan Barrett share a nice example of the Westermark sign in the March 15 New England Journal.