General Considerations

• All types of atelectasis involve loss of volume in some or all of a lung with resultant increased density of the involved lung
• The atelectasis referred to here is that caused by bronchial obstruction, usually a tumor (i.e. a bronchogenic carcinoma), a foreign body or a mucus plug 

Signs of atelectasis 

• Increase in density of the affected lung
• Displacement of the fissures or the mediastinum towards the atelectasis
• Crowding of the vessels and bronchial tree in the area of volume loss
• Elevation of the hemidiaphragm
• Overaeration of the opposite lung 

Lower Lobe Atelectasis

• Lower lobe is usually tethered to the hemidiaphragm and mediastinum by the inferior pulmonary ligament (IPL)
  • The IPL attaches the lower lobe to the mediastinum and may be seen normally on CT scans of the chest
• The inferior pulmonary ligament is actually not a ligament but reflections of the pleura (visceral and parietal) inferior to the hilum extending downward to the diaphragmatic pleura
• It is on the medial border of the left lower lobe
• There is no superior pulmonary ligament as a counterpart
• With collapse, there is rotation of the major fissure posteriorly and medially 

Imaging signs of Left Lower Lobe Atelectasis

• Wedge-like density behind heart with base of triangle on the left hemidiaphragm and the apex at the hilum on frontal view
• Silhouetting of all or just the medial portion of the left hemidiaphragm
• Posterior displacement of the major fissure on the lateral view
• Hilum may be displaced inferiorly
• Descending branch of the pulmonary artery may disappear
• There may be a shift of the heart towards the left which means less of the right heart border or none of the right heart border may project to the right of the spine
• There may be a a spine sign due to superimposition of the non-aerated lower lobe and the spine on the lateral view
• In severe atelectasis of the lower lobe, the atelectatic lobe is so small it may not be recognizable on a conventional radiograph
  • The hemidiaphragm may be seen throughout its length. i.e. there is no silhouette sign
• The normal contour at the top left of the aortic knob may become invisible
• Appearance of left lower lobe atelectasis is similar to right lower lobe atelectasis

Associations

• Following cardiac surgery, especially in which there is topical hypothermia of the heart and phrenic nerve

British Journal of Radiology 74 (2001),89-97 © 2001 Lobar atelectasis: diagnostic pitfalls on chest radiography K Ashizawa, MD, K Hayashi, MD, N Aso, MD and K Minami, MD 

Radiology, Vol 148, 479-483, Copyright © 1983 by Radiological Society of North America   Inferior pulmonary ligament: computed tomographic appearance   RC Rost Jr and AV Proto