Silhouette Sign

Cardiac margins are clearly seen because there is contrast between the fluid density of the heart and the adjacent air filled alveoli. Both being of fluid density, you cannot visualize the partition of the right and left ventricle because there is no contrast between them. If the adjacent lung is devoid of air, the clarity of the silhouette will be lost. The silhouette sign is extremely useful in localizing lung lesions.

To utilize the silhouette sign you must know what structures are adjacent to each silhouette.


Silhouette  Adjacent Lobe/Segment
Right diaphragm  RLL/Basal segments
Right heart margin  RML/Medial segment
Ascending aorta  RUL/Anterior segment
Aortic knob LUL/Posterior segment
Left heart margin Lingula/Inferior segment
Descending aorta LLL/Superior and medial segments
Left diaphragm  LLL/Basal segments

You should know that the pleura encircles the lung and diseases of  the pleura can also obliterate silhouettes. The same is true for mediastinal masses.

A: Ascending aorta

B: Left heart margin

C: Left diaphragm

D: Aortic knob

E: Right heart margin

F: Right diaphragm