Silhouette sign (x-rays)

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Silhouette sign is somewhat of a misnomer and in the true sense actually denotes the loss of a silhouette, thus, it is sometimes also known as loss of silhouette sign or loss of outline sign .

The differential attenuation of x-ray photons by two adjacent structures defines the silhouette, e.g. heart borders against the adjacent lung segments, and it is the pathological loss of this differentiation, which the silhouette sign refers to. In short, it denotes that a mediastinal border can only be obscured by pathology which is in direct anatomical contact.

Radiographic features

Plain radiograph

Recognition of this sign is useful in localizing areas of airspace opacities, atelectasis or mass within the lung, with the loss of these normal silhouettes on frontal chest radiographs being generally indicative of the site of pathology :

Sites of silhouette sign on the lateral chest radiograph include :

  • posterior border of the heart +/- posterior left hemidiaphragm: left lower lobe
  • anterior right hemidiaphragm: right middle lobe
  • posterior right hemidiaphragm: right lower lobe

The silhouette sign forms the basis of the hilum overlay sign, cervicothoracic sign and thoracoabdominal sign .

A caveat however is when pulmonary or mediastinal anatomy has been altered by treatment (such as surgery or radiotherapy) or disease, volume loss may affect these specific associations .

History and etymology

Dr Benjamin Felson (1913-1988) and, his brother Dr Henry Felson (1907-1998), were American radiologists who popularized this sign . However, it was the Felsons themselves who state that Dr H (Henry) Kennon Dunham (1872-1944), an American chest physician and radiologist, was the first to describe the sign 15 years earlier (late 1920s) .

Differential diagnosis

The presence of a silhouette sign may not be due to intrapulmonary disease. For example :