Air trapping, also called gas trapping, is an abnormal retention of air in the lungs after expiration. It is observed in obstructive lung diseases such as asthma, bronchiolitis obliterans syndrome and chronic obstructive pulmonary disease. The cause is obstruction such that the patient is unable to expel air completely. Air trapping is a cause of obstructive pattern spirometry results, leading to an elevated residual volume. Air trapping is often incidentally diagnosed on computed tomography (CT) scanning. The only way to absolutely differentiate air trapping from emphysema is by taking expiratory images...