Luftsichel sign
From Radipedia
Lucency adjacent to the aortic arch on frontal view indicating left upper lobe collapse, literally "air sickle". The lucency occurs as a result of the superior segment of the left lower lobe migrating superiorly and abutting the aortic knob due to volume loss of the left upper lobe.
Right upper lobe collapse give the S-sign of Golden. Both a/w bronchogenic carcinoma, either endobronchial or extrinsic compression.
