In chest radiography, the Westermark sign is a sign that represents a focus of oligemia (hypovolemia) (leading to collapse of vessel) seen distal to a pulmonary embolism (PE). While the chest x-ray is normal in the majority of PE cases, the Westermark sign is seen in 2% of patients.
Essentially, this is a plain X-ray version of a filling defect as seen on computed tomography pulmonary arteriogram.
The sign results from a combination of: