The left lower lobe collapses medially and posteriorly to lie behind the heart. It classically displays a triangular opacity which may be visible through the cardiac shadow, or may overlie it, giving the heart an unusually straight lateral border. The hemidiaphragm may be obscured where the opacity lies against it. It is also easily missed, especially on an underpenetrated film, where no detail is seen behind the heart.
On the lateral film there is abnormally increased density over the lower thoracic spine due to the triangular opacity of the collapsed lobe. In this example this is not obvious, but the sloping anterior sharp margin is seen just above the diaphragm and anterior to the spine.
The left upper lobe collapses forwards. It thus presents no sharp margins on the frontal film, unlike that due to the horizontal fissure on the right. Instead it casts a veil like opacity over the left hemithorax, normally more dense towards the apex.
The lower lobe expands behind it, and so the aortic knuckle, which lies towards the back of the chest, characteristically remains clearly visible. This crescent of air adjacent to the aortic knuckle is known as the Luftsichel sign (German; luft=air, sichel=crescent).
On a lateral film the collapsed lobe is visible as a band of soft tissue density retrosternally, its sharp posterior margin produced by the almost vertical oblique fissure.
