Chest ImagingLobar atelectasis
(also called lobar collapse), loss of lung volume with reduced inflation of a segment or a lobe. There are several mechanisms of lobar atelectasis, the most frequent being
bronchial obstruction. The major radiological signs of lobar atelectasis are opacity of the lobe and evidence of loss of volume of the lobe. Opacity results from the presence of intra-alveolar fluid in the case of
obstructive atelectasis or
passive atelectasis or from scarring or lung
fibrosis in the case of
cicatrization atelectasis. Occasionally, the affected lung may contain air and may appear relatively normal in transparency on the radiograph and in attenuation on
CT scans, especially in the presence of an incomplete fissure. The signs of loss of volume include direct signs such as displacement of fissures,
pulmonary blood vessels and major bronchi, and shift of other structures to compensate for the loss of volume. In the case of
obstructive atelectasis, the presence of a large
tumour mass located in a parahilar situation may produce a bulge in the contour of the collapsed lobe (golden S sign) (
Fig.1). Compensatory overinflation of the adjacent lobe results in spreading out of the vessels within that lobe. The
mediastinal shift accompanying lobar atelectasis is of variable degree. Generally, it is greatest with lower lobar atelectasis and with cicatrization upper lobe atelectasis, moderate with acute upper lobe atelectasis and absent in the case of right middle lobe atelectasis. Displacement of the anterior
mediastinal fat, and displacement of the trachea are by far the most reliable signs. Hemidiaphragm elevation is another sign of compensatory shift. This sign is of limited value. It is mainly recognizable in cases of atelectasis of the lower lobe or left upper lobe. Inward displacement of the chest wall causing narrowing of the spaces between the affected ribs, is only seen with a severe atelectatic lobe. It is much easier to recognize with
CT than the radiograph.
Right upper lobe
The right hilum becomes elevated; the major and minor fissures are displaced upwards and rotated towards the mediastinum. As a result, the collapsed lobe packs against the mediastinum and lung apex (Fig.1). Compensatory overinflation of the right lower lobe, especially of the superior segment, may lead to visualization of a sliver of lung invaginating between the mediastinum medially and the posterior aspect of the collapsed right upper lobe laterally. This results in a sharp lucent interface between the atelectatic lobe and the mediastinum on the chest radiograph, a phenomenon known as the Lufsichel sign, and a triangular configuration of the posteroinferior portion of the collapsed right upper lobe.
Left upper lobe
The hilum is displaced upwards and the major fissure forwards (Fig.2) (Fig.3). The lobe retains much of its original contact with the anterior chest wall. The Lufsichel sign is far more commonly seen than in right upper lobe collapse. Displacement of the anterior mediastinum fat and displacement of the trachea towards the left are commonly present. The left hemidiaphragm is moderately elevated (Fig.2).
Right middle lobe
The collapsed right middle lobe is easily and reliably recognized on the lateral chest radiograph. The major and minor fissures move towards one another and the collapsed lobe resembles a curved, elongated wedge. The wedge tapers in two directions: medial to lateral, and anterior to posterior. The right hilum is not displaced. The right hemidiaphragm and mediastinum are in a normal position.
Right lower lobe
The maj from different locations. The appearance is virtually identical to that seen with left upper lobe collapse.
Combined right lower and middle lobe collapse is seen with obstruction to the intermediate bronchus. The major and minor fissures are both displaced downward and backward creating an opacity that obliterates the dome of the right hemidiaphragm. Right hilum, mainstem bronchus and upper lobar bronchus are displaced downward. The anterior mediastinal fat is markedly shifted to the right side.
Whole lung collapse is characterized by a marked shift of the mediastinum and heart to the affected side. The hemidiaphragm is elevated upwards. This feature is recognized radiographically only on the left side by the high position of the stomach bubble. Compensatory overinflation of the opposite lung moves across the midline, particularly anteriorly behind the sternum creating a large retrosternal airspace on the lateral view.
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