Left upper lobe atelectasis

Posteroanterior (PA) view of left upper lobe atelectasis due to an endobronchial squamous cell carcinoma. An ill-defined opacity is superimposed on the upper half of the left hemithorax, making it appear more opaque than the comparable region in the right hemithorax. The trachea is deviated to the left, the left hilum is retracted cephalad, and the left pulmonary artery fills the aortopulmonary window. Vascular branches to the superior segment of the left lower lobe form an array of linear and tubular opacities (ALTO). A vertical lucency separates the aortic arch from the medial margin of the collapsed lobe and forms an air crescent ("Luftsichel") (arrow). This finding probably represents an incomplete left major fissure with parts of the lower lobe pulled forward by the upper lobe, which collapses anteriorly.