Silhouette sign

Lingular Pneumonia
52 year old male presents with a cough and fever
Frontal CXR shows a lingular infiltrate with a positive silhouette sign. Both the superior and inferior lingular segments appear to be involved
Ashley Davidoff MD TheCommonVein.net
Lingular Pneumonia
52 year old male presents with a cough and fever
Lateral CXR shows a dense lingular infiltrate overlying the heart. Both the superior and inferior lingular segments appear to be involved
Ashley Davidoff MD TheCommonVein.net
Lingular Pneumonia
52 year old male presents with a cough and fever
CT scan in the axial plane shows a lingular consolidation with air bronchograms and a positive silhouette sign. Both the superior and inferior lingular segments are involved
Ashley Davidoff MD TheCommonVein.net
Silhouette sign, right middle lobe pneumonia. Initial frontal (A) and lateral (B) radiographs in a patient with clinical suspicion of pneumonia demonstrate obliteration of the right heart border. Follow-up radiographs the next day (C, D) illustrate dense opacification on the lateral view and persisting loss of the right heart border, confirming the presence of a right middle lobe pneumococcal pneumonia.
Source
Signs in Thoracic Imaging
Journal of Thoracic Imaging 21(1):76-90, March 2006.
  • This classic roentgenographic sign
      • first described by Felson in 1950
        •  “an intrathoracic lesion touching a border of the heart, aorta, or diaphragm will obliterate that border on the roentgenogram. An intrathoracic lesion not anatomically contiguous with a border of one of these structures will not obliterate that border.”
      • The right heart border silhouette
        • indicates right middle lobe process
      • The left heart border silhouette
    •  silhouette sign
      • cervicothoracic and hilum overlay sign.

Why We See Things

Contrasts Differences and Visibility
Ashley Davidoff thecommonvein.net
How many squares
Ashley Davidoff thecommonvein.net
How many squares
Ashley Davidoff thecommonvein.net

Silhouetting

In the Top Image

Why we see the left hemidiaphragm – Contrast of Black on White
Why we see the air – fluid level in the stomach – Black on White

In the Bottom image

Why we do not see the left hemidiaphragm

NORMAL AND SILHOUETTING OF THE LEFT DIAPHGRAGM

Because the alveoli are filled with pus from a left lower lobe pneumonia and we have white on white

When the alveoli are filled with air the thin walled smaller airways are not visualized (equivalent of silhouetted) However when they become filled with fluid eg pus blood malignant cells the contrast between the air and fluid or soft tissue created the air bronchogramWhen the Alveoli are Filled with Fluid

Filled and Half-Filled Alveoli – Differences in Appearance on CT
When the alveoli are fully filled with fluid, tumor, or pus for example, the overall net density will be white, and when adjacent to air filled airways, air bronchograms are visible (left side of image)
When the alveoli are only partially filled, the density of the fluid added to the density of the air results in an overall gray density, and when positioned next to air filled bronchi, there is insufficient contrast to create an air bronchogram and sufficient to enable visualization of the blood vessels. This is called ground glass opacification
Ashley Davidoff
TheCommonVein.net lungs-00681

On the Other Hand….
When there is consolidation and we do not see the silhouette sign it implies the airways are filled with a soft tissue density – like in aspiration pneumonia

Aspiration Pneumonia
74 year old male alcoholic with bilateral basilar lobar atelectasis caused by bilateral aspiration
CT scan shows airless lower lobes with small bilateral effusions.
Ashley Davidoff MD TheCommonVein.net RnD image 236Lu
Aspiration Pneumonia
74 year old male alcoholic with bilateral basilar lobar atelectasis caused by bilateral aspiration
CT scan shows airless lower lobes with small bilateral effusions. 3D reconstruction shows total obstruction of the right mainstem bronchus, and patent proximal mainstem bronchus
Ashley Davidoff MD TheCommonVein.net 236Lu

 

Links and References