The hallmark of airway obstruction on chest radiographs is the finding of air trapping. An exhalation-phase chest radiograph will show whether air trapping is present; this is manifested as failure of the lung to decrease in volume and failure of the lung to increase in opacity on exhalation compared with inhalation. In some cases, the mediastinum shifts to the side that is not trapping air on exhalational views. Air trapping occurs when an endobronchial lesion, usually a foreign body in a large airway, causes a check-valve type of obstruction. The foreign body does not completely obstruct the bronchus in which it is lodged. During inhalation, the bronchial diameter normally increases, allowing air to pass around the foreign body and enter the lung distal to the obstruction. During exhalation, the bronchial diameter normally decreases, and the air is trapped within the lung distal to the obstruction. This allows a foreign body that does not completely obstruct a bronchus during inhalation to do so during exhalation. As a result, the lung, lobe, or segment distal to the foreign body becomes increasingly distended until the pressure within it prevents more air from entering. In children, this type of check-valve obstruction usually results from the aspiration of food, commonly a peanut, or a toy or coin. In adults, this type of obstruction can result from aspiration of foreign bodies but also, more important, from an endobronchial tumor (Fig. 14-7). Extrinsic masses, such as
enlarged nodes or an enlarged heart, can obstruct a bronchus in a similar fashion.

Obliterative bronchiolitis is a syndrome of airflow limitation caused by bronchiolar and peribronchiolar inflammation and fibrosis, as was discussed in Chapter 13. In adults, it is most often idiopathic in etiology, but is also associated with lung and bone marrow transplantation as well as a variety of other insults to the lung. The Swyer-James, or MacLeod, syndrome is a form of obliterative bronchiolitis that occurs following an insult to the developing lung (1). In this syndrome, unlike in large central airway obstruction, small bronchi and bronchioles are affected, and the lung served by abnormal airways remains inflated by collateral air drift. By definition, the airway disease as assessed by the chest radiograph is predominantly unilateral, giving rise to the key finding of unilateral hyperlucent lung. In practice, obliterative bronchiolitis is often bilateral and patchy. The injury to the immature lung, which occurs during the first 8 years of life, commonly follows a viral infection. Bronchi and bronchioles from the fourth generation to the terminal bronchioles have submucosal fibrosis, which causes luminal irregularity and occlusion. Pulmonary tissue is hypoplastic, including the pulmonary artery and its branches, which are reduced in both size and number. Lung distal to diseased airways is hyperinflated and supplied by collateral air drift. Sometimes panacinar emphysematous changes are present. Patients are typically asymptomatic, often presenting as adults with an incidental abnormal chest radiograph.

Chest radiographs show unilateral hyperlucency because of reduced lung perfusion and air trapping (Fig. 14-8). The size and number of vessels in the middle and peripheral lung are reduced on the affected side. The hilum of the involved lung is small, but lung volumes are normal or only slightly decreased. Ipsilateral air trapping on exhalational chest radiography is a key finding of the condition. The air trapping can also be demonstrated on nuclear medicine ventilation studies or paired inhalation/exhalation computed tomography (CT).

The CT findings of Swyer-James syndrome include a patchwork of local low-density and hypovascular areas interspersed with lung of normal density (2). Air trapping can be confirmed on exhalation. Other changes that can be seen on CT include bronchiectasis, bronchiolectasis, atelectasis, and focal scarring (see Fig. 13-26) (3).

Pulmonary emphysema is a pathologic diagnosis that is defined as a condition of the lung characterized by abnormal permanent enlargement of airspaces distal to the terminal bronchiole and accompanied by the destruction of their walls and without obvious fibrosis. Although emphysema is usually a diffuse, bilateral process, it can on occasion be asymmetric, with one lung more severely involved than the other. This marked asymmetry in involvement can result in the more severely involved lung appearing hyperlucent compared with the opposite lung (Fig. 14-9). Emphysema is discussed in more detail in Chapter 13.

Congenital lobar emphysema (CLE) usually manifests in the neonatal period, but in some cases the presentation is delayed
until after the first month of life; it can also present in adulthood. Aplasia, hypoplasia, or dysplasia of bronchial supporting structures is postulated as the primary cause of CLE (4). The chest radiographic appearance of CLE is hyperexpansion of an isolated lobe in one lung, usually an upper or middle lobe. The expanded lobe may cause compressive atelectasis of the rest of the lung. Its appearance on chest radiography or CT can be similar to that of obliterative bronchiolitis.

Pulmonary vascular conditions may result in a hyperlucent lung on chest radiography that is indistinguishable from the lucency associated with airway obstruction. However, in primary vascular conditions, air trapping is generally not as severe as with airway obstruction.

One of the chest radiographic signs of pulmonary embolism is oligemia of the lung beyond the occluded vessel (Westermark sign). A large unilateral embolus, whether bland, septic, or neoplastic, can result in a unilateral hyperlucent lung (Fig. 14-10). When seen, the Westermark sign can be very helpful in suggesting further workup for pulmonary embolism in the appropriate patient population; however, this sign is not commonly seen. A more detailed discussion of radiographic and CT findings of pulmonary embolism is included in Chapter 17.

Unilateral absence or hypoplasia of a lung or a lobe is a congenital abnormality that, surprisingly, may cause few clinical problems when not accompanied by other congenital abnormalities. The chest radiographic findings are those of absent or decreased aeration of the affected side, signs of volume loss, and compensatory hyperaeration of the opposite lung. The decreased perfusion to the affected lung contributes to its relative hyperlucency. Radiographs, and especially CT, show the diminution or absence of a pulmonary artery. Fibrosing mediastinitis, when associated with encasement of a pulmonary artery by fibrous tissue, can produce a similar appearance. In this case, however, CT will show the abnormal fibrous tissue infiltrating the mediastinum and encasing the bronchi and vessels.