Lung atelectasis (plural: atelectases) refers to collapse or incomplete expansion of pulmonary parenchyma.


Atelectasis may be used synonymously with collapse, but some authors reserve the term "atelectasis" for partial collapse, not inclusive of total atelectasis of the affected part of lung or of whole lung collapse.


Atelectasis is a radiopathological sign which can be classified in many ways. The aim of each classification approach is to help identify possible underlying causes together with other accompanying radiological and clinical findings.

Atelectasis can be subcategorised based on underlying mechanism, as follows:

Atelectasis can also be subcategorised by morphology:

or by anatomical extent:

Radiographic features

Vary depending on the underlying mechanism and type of atelectasis

Plain radiograph / CT

Direct signs of atelectasis
  • displacement of interlobar fissures
  • crowding together of pulmonary vessels
  • crowded air bronchograms (does not apply to all types of atelectasis; can be seen in subsegmental atelectasis due to small peripheral bronchi obstruction, usually by secretions; if the cause of the atelectasis is central bronchial obstruction, there will usually be no air bronchograms)
Indirect signs of atelectasis
  • pulmonary opacification
  • shifting granuloma (or any other previously documented lesion, used as a reference for comparison)
  • compensatory hyperexpansion of the surrounding or contralateral lung
  • displacement of the heart, mediastinum, trachea, hilum
  • elevation of the diaphragm
  • propinquity of the ribs
Resorptive (obstructive) atelectasis
  • increased density (opacity) of the atelectatic portion of lung
  • displacement of the fissures toward the area of atelectasis
  • upward displacement of hemidiaphragm ipsilateral to the side of atelectasis
  • crowding of pulmonary vessels and bronchi in region of atelectasis
  • +/- compensatory overinflation of unaffected lung
  • +/- displacement of thoracic structures (if atelectasis is substantial)
Linear (plate, discoid, subsegmental) atelectasis
  • relatively thin, linear densities in the lung bases oriented parallel to the diaphragm (known as Fleischner lines) 

The sonographic morphology of atelectatic lung may resemble hepatic parenchyma, often referred to as  "tissue-like" or "hepatized" in appearance. Distinguishing features of atelectasis by etiology may appear as follows:

  • compressive atelectasis is most often visualized in the costophrenic recess bordered by a disproportionately large pleural effusion
    • low-level, homogenous echogenicity with few to no air bronchograms
    • margins are usually regular with a triangular shape
    • a shred sign may be present at the transition to aerated lung
  • obstructive atelectasis
    • early static air bronchograms due to distal air trapping
    • as the air is resorbed, bronchi may fill with fluid resulting in anechoic, tubular structures known as fluid bronchograms 
    • maybe differentiated from blood vessels with color flow Doppler 

History and etymology

Atelectasis comes from the Greek words 'ateles' and 'ektasis' translating to 'incomplete stretching or expansion'.