A widened mediastinum is often due to technical factors
Genuine widening is usually due to a vascular abnormality or a mediastinal mass
A lateral view may help determine the location of a mass
Widening of the mediastinum is most often due to technical factors such as patient positioning or the projection used. Rotation, incomplete inspiration, or an AP view, may all exaggerate the width of the mediastinum, as well as heart size.
In the setting of trauma, patients are positioned supine while a chest X-ray is acquired, very often causing the mediastinum to appear wide spuriously.
Mediastinal masses and vessels
If a PA standing chest X-ray has been taken with good inspiration and no rotation, any widening of the mediastinum is likely to be genuine. The main pathological causes to consider include masses and widening of vessels.