Silhouette sign (x-rays)

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 4

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

Plain radiograph

Recognition of this sign is useful in localising 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 3, 4:

  • right paratracheal stripe: right upper lobe
  • right heart border: right middle lobe or medial right lower lobe
  • right hemidiaphragm: right lower lobe
  • aortic knuckle: left upper lobe
  • left heart border: lingula segments of the left upper lobe
  • left hemidiaphragm or descending aorta: left lower lobe

Sites of silhouette sign on the lateral chest radiograph include 3:

  • 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 2

Dr. Benjamin Felson (1913-1988) and Dr. Henry Felson were American radiologists who first described this sign 1.

The presence of a silhouette sign may not be due to intrapulmonary disease. For example 3, 4:

Chest x-ray
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Article Information

rID: 29979
System: Chest
Section: Signs
Tags: refs, cases
Synonyms or Alternate Spellings:
  • Silhouette sign of Felson
  • Loss of silhouette sign
  • Loss of outline sign

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Cases and Figures

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    Figure 1: normal cardiomediastinal contours
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    Figure 2: normal cardiomediastinal contours
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    Case 1: right middle lobe pnuemonia
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    Case 2: right lower lobe pneumonia
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    Case 3: right upper lobe pneumonia
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    Case 4: left lower lobe pnuemonia
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    Case 5: lingular segmental pneumonia
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    Case 6: left upper lobe pneumonia
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