The Silhouette Sign



Figure 1a: Normal chest x-ray showing demonstrating the 'Silhouette Sign'.

(Click on image to enlarge)

Figure 1b: Annotated normal chest x-ray highlighting (red line) the crisp right heart border silhouette.

(Click on image to enlarge)



One of the greatest radiologists of all time, Benjamin Felson, in his seminal works from the 1950s coined the term the silhouette sign.


It is a founding and core principle of chest radiograph interpretation used to identify the lobe in which consolidation or collapse occurs.


On a normal chest radiograph the aerated lung lies adjacent to other structures of different densities (and so different degrees of x-ray absorption occur).  This creates a silhouette, which is observed on the chest radiograph.


If the density of the structure is altered for example the aerated lobe of the lung becomes consolidated or it collapses its density will increase, it will them absorb more x-rays, and then it will be indistinguishable from the adjacent structure.


For example, a crisp silhouette is observed between the normal right middle lobe and the right heart border, which is formed from the right atrium (Figure 1).  When consolidation occurs, like in a pneumonia, the silhouette is lost (Figure 2).


Figure 2a: Chest x-ray of a woman with right middle lobe pneumonia/consolidation, obliterating the normal crisp silhouette sign.

(Click on image to enlarge)

Figure 2b: Close up view of the right middle lobe pneumonia/consolidation with the absence of clear silhouette sign.

(Click on image to enlarge)

Figure 2c: Annotated close up view of the right middle lobe pneumonia/consolidation with the absence of clear silhouette sign .

(Click on image to enlarge)



The same principle applies for all the lobes of the lung.

Left upper lobe: aortic knuckle

Lingula: left heart border

Left lower lobe: left hemidiaphragm

Right upper lobe: right paratracheal stripe

Right middle lobe: right heart border

Right lower lobe: right hemidiaphragm.



Felson B, Felson H. Localization of intrathoracic lesions by means of the postero-anterior roentgenogram; the silhouette sign. Radiology 1950;55:363-374






Images and text contributed and prepared by

Dr Ian Bickle, Department of Radiology,RIPAS Hospital

All images are copyrighted and property of RIPAS Hospital.