Westermark sign

Overview
In chest radiography, the Westermark Sign, is a sign that represents a focus of oligemia (vasoconstriction) seen distal to a pulmonary embolus (PE). While the chest x-ray is abnormal in the majority of PE cases, the Westermark sign is seen in only 2% of patients.

The sign results from a combination of:
 * 1) the dilation of the pulmonary arteries proximal to the embolus and
 * 2) the collapse of the distal vasculature creating the appearance of a sharp cut off on chest radiography.

Sensitivity and specificity
The Westermark sign, like Hampton's hump (a wedge shaped, pleural based consolidation associated with pulmonary infarction), has a low sensitivity (11%) and high specificity (92%) for the diagnosis of pulmonary embolus.