■ Wedge-shaped opacity caused by collapsed lobe-specific pattern for each lobe.
The right upper lobe collapses against the mediastinum and thoracic apex with a broad-based opacity radiating from the hilum. If there is an outward bulge at the right hilum, this is good evidence that a hilar mass is responsible for the collapse (see Fig. 1.20). The lower lobe pulmonary artery is pulled upwards and outwards.
Left upper lobe collapse
This does not mirror right upper lobe collapse due to the absence of a middle lobe. The left upper lobe collapses forward against the anterior chest wall. The lower lobe expands behind it. The chest X-ray appearance is of a hazy density in the mid- and upper zones which fades away laterally and inferiorly (see Fig. 1.21). The collapsed lobe is adjacent to the left cardiac and mediastinal border, so this silhouette is completely lost. The aortic knuckle is lost (see Fig. 1.22) unless the lobar collapse is accompanied by overexpansion of the lower lobe with its superior segment occupying the apex.
Middle lobe collapse
This is easily missed on the frontal film and is often more obvious on a lateral projection. On the frontal projection, there is a vague increase in density seen in the right lower zone and the normally sharp right heart border is blurred. On a lateral projection the collapsed middle lobe forms a triangular opacity with its apex at the hilum and base projecting towards the sternum. (see Figs 1.23 and 1.24).
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