The frontal chest x-ray shows dilated central pulmonary arteries with abrupt distal cut off.

Given the history of acute dyspnoea and recent leg surgery, the findings are likely to be due to a central pulmonary embolism (PE).

The patient went onto have a CTPA, which confirmed central PE.

In a patient presenting with chest pain or dyspnoea, chest x-ray is performed not to diagnose PE but to rule out alternative diagnosis like pneumonia, pneumothorax etc. This is because the chest x-ray may be normal or may have few non specific signs even in the presence of extensive PE.

Some of the chest x-ray findings suggestive of PE are:

• Hampton’s hump – pleural based wedge shaped opacity with apex towards the hilum. It indicates pulmonary infarction and usually seen 24 hours after onset of symptoms.
• Westermark sign – oligaemia of the involved lung field.
• Knuckle or Sausage sign – due to dilatation of the central pulmonary artery due to occlusion by the embolus.
• Other findings may be – plate like atelectasis, pleural effusion, elevation of the affected side diaphragm.

Reference: Grainger & Allison’s Diagnostic Radiology, A Textbook of Medical Imaging, 6th Edition.