Silhouette sign

The silhouette sign describes the loss of a normally visible interface — such as a heart border or hemidiaphragm — when the lung directly next to it becomes airless. Two structures of the same density in contact cannot be told apart on a radiograph.

Radiographic features

The sign localises disease on a single frontal film:

• Right heart border lost: right middle lobe
• Left heart border lost: lingula
• Hemidiaphragm lost: lower lobe
• Aortic knuckle lost: left upper lobe (apicoposterior segment)
• Right paratracheal stripe lost: right upper lobe

Conversely, if a border remains crisp despite an overlying opacity, the abnormality lies in a different plane — for example a lower lobe process behind a preserved heart border.

Practical points

A poorly inspired or rotated film can blur borders and mimic the sign. Check technique before calling it.