Westermark’s Sign

In chest radiography, the Westermark Sign, is a sign that represents a focus of oligemia due to vasoconstriction seen distal to a pulmonary embolus (PE).

Pulmonary embolism (PE) is an important cause of mortality, particularly among hospitalized patients. Because of the difficulty in diagnosing PE on the basis of clinical and laboratory data, imaging studies play a central role in establishing or excluding this diagnosis. While the chest x-ray is normal  in the majority of PE cases, the Westermark sign is seen in 2% of patients.

Westermark's Sign. Frontal radiograph of a patient with pulmonary embolism showing  increased radiolucency in the upper and middle zones in the left lung due to decreased vascularization.

The sign results from a combination of:

1. The dilation of the pulmonary arteries proximal to the embolus.
2. The collapse of the distal vasculature creating the appearance of a sharp cut off on chest radiography.