Superior mediastinum:

Should have a width

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Note added at 7 mins (2023-03-24 12:22:04 GMT)
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the smaller than sign cause problems

again:

Superior mediastinum:

Should have a width smaller than 8 cm on a PA CXR.
A widened mediastinum can be associated with:
AP CXR view, which magnifies the heart and mediastinal structures
Unfolded aortic arch (not pathological) or a thoracic aortic aneurysm

https://litfl.com/abc-of-cxr-interpretation/

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Note added at 9 mins (2023-03-24 12:23:51 GMT)
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Aortic unfolding is a term used to describe the radiological abnormality on chest radiographs seen as widening of the mediastinum. This change occurs with aging and generally reflects proximal aortic dilation, aortic arch widening, and decreased curvature.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3995952/#:~:tex...

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Note added at 11 mins (2023-03-24 12:25:38 GMT)
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Aortic unfolding is an abnormality visible on a chest X-ray, that shows widening of the mediastinum which may mimic the appearance of a thoracic aortic aneurysm.[1]

With aging, the ascending portion of the thoracic aorta increases in length by approximately 12% per decade, whereas the diameter increases by just 3% per decade. This elongation causes the ascending aorta to appear as a vertical shadow on the left heart border. Unfolding is often associated with aortic calcification which implies aortic degeneration and hypertension. [2] Aortic unfolding, though not serious, should be differentiated from the more severe dissection of the aorta.

https://en.wikipedia.org/wiki/Aortic_unfolding