What is the silhouette sign?
The majority of structures visible on chest radiographs are due to the attenuation difference with air in the adjacent lung. Thus, the heart and mediastinal borders, the borders of the chest wall, and the vessels within the lungs are all seen because they attenuate x-rays to a greater extent than the adjacent air-filled lung. If the lung adjacent to one of these structures is consolidated, then there is no longer an attenuation difference between the lung and the structure, and so the structure and the normally visualized border disappear. This phenomenon has been termed the “silhouette” sign and is a specific marker of airspace lung disease.
This sign is useful to help localize sites of lung disease. For instance, if the diaphragm is obscured, this implies lower lobe lung involvement. If the right heart border is obscured, this denotes a right middle lobe lung involvement. If the left heart border is obscured, this implies left upper lobe (lingular) lung involvement. If the aortic knob or right paratracheal stripe is obscured, this implies upper lobe lung involvement.