The 'silhouette' sign on Chest X Ray

Two substances of the same density, cannot be differentiated from each other on X-ray. This phenomenon, the loss of the normal radiographic silhouette (contour),is called the Silhouette sign.

The silhouette sign is a misnomer! It should be called the 'loss of silhouette' sign. Normal adjacent anatomical structures of differing densities form a crisp 'silhouette,' or contour. Loss of a specific contour can help determine the position of a disease process.

For example, the heart (a soft tissue density structure - near white) lies adjacent to lung tissue (near air density - near black). A crisp contour or 'silhouette' is formed at the interface of these two tissue densities. Loss of clarity of the right heart contour (formed by the right atrium) implies disease of the right middle lobe which lies next to the right atrium. Loss of distinction of the left heart contour indicates an abnormality of the lingula (part of the left upper lobe which wraps over the left ventricle).

Salient Features of Silhouette Sign : 

1) A density in RML or Lingula can produce sign on the heart or ascending aorta, but one in lower lobe cannot.
2) Involvement of apico-posterior segment of the LUL almost always obliterates the aortic knob.
3) Lesion in the anterior segment of Upper lobe shows this sign on heart or ascending aorta, whereas the posterior segment lesion does not.
4) An anterior mediastinal mass causes a silhouette sign on heart or aorta, but a posterior one does not, although it may obliterate the aortic knob.
5) Posterior encapsulation of pleural fluid does not produce the sign on the heart or ascending aorta, but anteromedial encapsulation or free pleural effusion does.

The right heart border is silhouetted out.  
This is caused by a pneumonia

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