What is the Westermark sign on imaging?

• A. Unilateral dilation of the pulmonary artery
• B. Pulmonary edema with interstitial markings
• C. Dilated pulmonary vasculature proximal with oligemia distally
• D. Pleural effusion

Answer: (C)

The Westermark sign reflects a perfusion deficit from an acute pulmonary embolus. It shows up on imaging as regional oligemia—reduced or scant pulmonary vascular markings—in the lung area distal to the obstructed vessel. Often you can also see relatively dilated vessels proximal to the blockage because of increased upstream flow or pressure. This combination—distal decreased markings with adjacent proximal vascular enlargement—is characteristic of the Westermark sign.

This pattern helps differentiate it from other findings: pulmonary edema would typically show interstitial markings or Kerley lines; a pleural effusion presents as fluid in the pleural space; simply dilated pulmonary arteries without distal oligaemia would not capture the perfusion deficit caused by an embolus.