hilum overlay sign

This PA chest
radiograph demonstrates an abnormal contour in the right hilar region, with visualization of the pulmonary vessels through the mass (the hilar overlay sign) indicating its posterior mediastinal location. On resection this was found to be a benign solitary fibrous tumor of the pleura.
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The hilum overlay sign is useful in differentiating whether an opacity on a frontal chest radiograph in the region of the lung hilum is located within the hilum versus anterior or posterior to it. The sign refers to preserved visualization of the hilar vessels, excluding abnormalities that localize to the middle mediastinum.

If the cause of the opacity arises from the hilum, the silhouette of the normal pulmonary vessels (interlobar artery, upper lobe arteries, and left lower lobar artery)  is obliterated. Causes of these opacities include middle mediastinal tumors, hilar adenopathy, pericardial effusion, vascular enlargement, and cardiac enlargement.

If the edges of the vessels are appreciated, this implies the cause of the opacity is not in contact with the hilum and is, therefore, either anterior or posterior to it. Most of these opacities are masses in the anterior mediastinum.

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.

History and etymology

Benjamin Felson (1913-1988) was the American radiologist who first described this sign . 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 .

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