What are the imaging features of atelectasis?
Atelectasis typically appears as a well-defined homogeneous area of lung opacification, sometimes with air bronchograms, that is often confined to a pulmonary segment or lobe, although it can range from a minimal linear opacity to complete lung opacification. This is frequently associated with decreased lung volume as well as abnormal shift of pleural fissures, bronchi, and pulmonary vessels toward the atelectatic lung parenchyma. As atelectasis represents normal but nonaerated lung tissue, it will enhance uniformly on CT or magnetic resonance imaging (MRI) after intravenous contrast administration. This is in contrast to other airspace opacities such as by pneumonia and tumors which tend to have more heterogenous and variable enhancement. Indirect signs of atelectasis may include ipsilateral diaphragmatic or cardiomediastinal shift toward the atelectatic lung parenchyma, ipsilateral rib space narrowing, and compensatory hyperinflation of unaffected portions of the lungs.