Figure 2. 61-year-old man with marked left upper lobe atelectasis due to a carcinoid tumour in left upper lobe bronchus. (a) Posteroanterior radiograph shows a small left perihilar opacity. The left hilum is elevated and the left upper lobe pulmonary artery is invisible. (b) Anteroposterior scout radiograph on CT shows a radiolucent stripe (arrowheads) between the atelectatic left upper lobe and the aortic arch (Luftsichel sign). (c) CT shows the atelectatic left upper lobe as a triangular opacity with its apex directed posteriorly. The hyperinflated superior segment of the left lower lobe extends medially to the atelectatic lobe (arrow), producing the Luftsichel sign.