Computed tomography (CT) imaging of the chest is occasionally performed when diagnostic uncertainty exists— for example, in order to distinguish a pneumothorax from large bulla or when the lung field is obscured by surgical emphysema. It is also often carried out before a contemplated surgical procedure, or when an underlying lung abnormality— such as interstitial lung disease, lymphangioleiomyomatosis or histiocytosis—is considered a possibility.