Silhouette Sign
When examining the lung fields of a normal CXR, the silhouettes of the heart borders, the ascending and descending aorta, the aortic knob and the hemidiaphragms should be clear. Obliteration of any of these silhouettes by a water density can be caused by infection in the lung, blood, pus, etc and is known as the ‘silhouette sign’. All of these silhouettes, or structures, are in contact with a specific portion of the lung. Therefore, by determining exactly which structure is obliterated, you can determine where the lung pathology is located.
Silhouette/Structure |
Contact with Lung |
Upper right heart border/ascending aorta |
Anterior segment of RUL |
Right heart border |
RML (medial) |
Upper left heart border |
Anterior segment of LUL |
Left heart border |
Lingula (anterior) |
Aortic knob |
Apical portion of LUL (posterior) |
Anterior hemidiaphragms |
Lower lobes (anterior) |
**Remember! Anterior left hemidiaphragm is normally obliterated by the heart
Sihouette Sign #1: 45 y/o male
with cough, fever and pleuritic chest pain
Sihouette Sign #2: 30 y/o female with fever of unknown origin
Umm...where is the Silhouette Sign?: 15 y/o girl with cough and fever