Obstructive atelectasis is caused by complete bronchial intraluminal obstruction, extraluminal compression, or contraction of scar tissue in the lung, predominantly the former. When the bronchus is suddenly and completely obstructed (bronchial foreign body or blood clot), the air in the alveoli is mostly resorbed within 18 - 24 hours, and the corresponding lung tissue collapses. The imaging manifestations of obstructive atelectasis are related to the location and time of the obstruction, as well as preexisting lesions in the lung. The obstruction can occur in the main bronchi, lobar or segmental bronchi, and bronchioles, resulting in corresponding lateral, lobar, segmental, and lobular atelectasis.
Figure 1 Schematic diagram of lobar atelectasis (dark areas)
a. Atelectasis of the right upper lobe; b. atelectasis of the left upper lobe; c. atelectasis of the right middle lobe; d. atelectasis of the right lower lobe
X-ray
In unilateral atelectasis, the affected lung is homogeneously hyperdense, intercostal space narrows, mediastinum shifts to the affected side, diaphragm rises, and healthy side has compensatory emphysema.
In lobar atelectasis, the atelectatic lobe is reduced in size and is homogeneously hyperdense, adjacent interlobar fissures are centripetally displaced, mediastinum and hilum may shift to the affected side, and adjacent lobes may develop compensatory emphysema.
In segmental atelectasis, simple segmental atelectasis is less common, triangular dense opacity may be on the frontal chest radiograph, with the base outward and the tip pointing to the hilum, and the segmental volume is reduced.
Lobular atelectasis is caused by mucous obstruction of terminal bronchioles, mostly in bronchopneumonia, and is manifested by some small patchy dense opacities. It is difficult to distinguish from adjacent inflammation.
CT
In unilateral atelectasis, the volume of the involved lung is reduced, and homogeneous opacity of soft tissue density can be seen. Contrast-enhanced CT can show obvious enhancement, and the location and cause of main bronchial obstruction can be found.
In lobar atelectasis, atelectasis of the right upper lobe is manifested by triangular or zonate opacity of soft tissue density on the right side of the upper mediastinum, with the tip pointing to the hilum and clear edges; atelectasis of the left upper lobe is manifested by triangular opacity of soft tissue density, with bottom connected to the anterolateral chest wall, tip points to the hilum, and the posterior outer edge is concave towards the anterior inner side; atelectasis of the right middle lobe is more common, and is manifested by triangular opacity of soft tissue density beside the right heart edge, with tip pointing outward; atelectasis of the lower lobe is manifested by triangular opacity soft tissue density beside the spine, with tip pointing to the hilum, its anterior outer edge is sharp, the diaphragm on the affected side is elevated, and the hilum is displaced downward
Segmental atelectasis is common in the inner and outer segments of the right middle lobe, and is manifested by triangular opacity of soft tissue density beside the right heart edge, with concave edges.
In lobular atelectasis, CT manifestations resemble x-ray manifestations.
Figure 2 Lobar atelectasis
a. On chest plain film, atelectasis of the right upper lobe shows inverted triangle opacity (↗); b. CT mediastinal window shows atelectasis of the left upper lobe (↗) with massive opacity at the left upper hilum.
MRI
Atelectasis of the lung lobe or segment is predominately hyperintense on T1WI and slightly hyperintense T2WI. On contrast-enhanced CT, the hilar mass that causes atelectasis can sometimes be distinguished.