Atelectasis refers to the collapse of lung tissue. It may be sub-segmental, segmental, lobar or it may involve an entire lung. Atelectasis may be due to airway obstruction or it may be secondary to external compression of lung tissue. Causes of airway obstruction include tumors, foreign body, mucus plugging, broncolith, and right middle lobe syndrome.
 Types of atelectasis
 Lobar collaspe
 Right upper lobe collapse
- Collapsed RUL shifts medially and superiorly, resulting in elevation of the right hilum and the minor fissure.
- Minor fissure in RUL collapse is usually convex at its superior aspect, but it may appear concave because of an underlying mass lesion. This is called the S sign of Golden .
- Tenting of the diaphragmatic pleura, called juxtaphrenic peak sign, is another helpful sign of RUL atelectasis
 Right middle lobe collapse
- RML collapse obscures the right heart border on a PA image.
- Lateral view shows a triangular opacity overlying the heart because the major fissure shifts upward and the minor fissure shift downward.
 Right lower lobe collapse
- Collapsed RLL shifts posteriorly and inferiorly resulting in a triangular opacity that obscures the RLL pulmonary artery.
- The major fissure, normally not visible on a PA radiograph, is evident with RLL collapse.
- Superior mediastinal structure shifts to the right, causing a superior triangle sign.
- Laterally, the collapsed RLL obliterates the posterior one third of the right hemidiaphragm and projects as an opacity over the normally lucent area.
 Left upper lobe collapse
- Atelectatic LUL shifts anteriorly and superiorly.
- In one half of the cases, a hyperexpanded superior segment of the LLL is positioned between the atelectatic upper lobe and the aortic arch. This gives the appearance of a crescent of the aerated lung (air cresent) called the luftsichel sign .
- On PA views, an atelectatic LUL produces a faint opacity in the left upper hemithorax, obliterating the left heart border.
- On lateral views, the major fissure is displaced anteriorly behind the sternum.
 Left lower lobe collapse
- On frontal views, increased retrocardiac opacity obliterates the LLL pulmonary artery and the left hemidiaphragm. The hilar structures shift downward, and the rotation of the heart produces flattening of the cardiac waist, which is known as the flat-waist sign.
- The superior mediastinum may shift and obliterate the aortic arch; this is the top of the aortic-knob sign.
- On the lateral radiographs, an opacity silhouettes the posterior third of the left diaphragm, and an opacity is projected over the normally lucent area.