Hilum overlay sign

The hilum overlay sign is useful in differentiating cardiac enlargement from a mediastinal mass. It refers to an abnormally dense hilum on frontal chest radiograph with preserved visualization of the hilar vessels.  

If a mass arises from the hilum, the normal pulmonary vessels (interlobar artery, upper lobe arteries, and left lower lobar artery) ² are in contact with the mass and their silhouette is obliterated. If the edges of the vessels are appreciated, this implies the mass is not in contact with the hilum and is, therefore, either anterior or posterior to it.

It can also be thought of in another way. If the right and/or left pulmonary arteries are visible,
more than a centimeter within the lateral edge of the mediastinal silhouette, then the lesion is not cardiac. 

Most of these masses are found to be in the anterior mediastinum.

History and etymology

Benjamin Felson (1913-1988) was the American radiologist who first described this sign ³. Dr Felson based this sign on the fact that, in more than 98% of the chest radiographs he reviewed, the visible proximal segment of the pulmonary artery was lateral to or just within the cardiac silhouette. Even when the heart is enlarged or there is pericardial effusion, this relationship remains true ⁴.