An increase in the transradiancy of a part of a lung may be due to a giant cyst or bulla, compensatory or obstructive emphysema, or a decrease in the pulmonary blood flow. Such a change in blood flow may result from pulmonary embolism and less frequently from thrombosis of the pulmonary artery. On rare occasions, congenital absence or hypoplasia of the pulmonary artery with or without intracardiac anomaly and compression of the artery by a mediastinal mass can cause diminished perfusion of the lung.

In 1953, Swyer and James¹ reported, under the partial title "Unilateral Pulmonary Emphysema," the case of a 6-year-old boy with recurrent bronchitis and bronchiolitis and a hyperlucent lung. Pulmonary angiograms revealed a hypoplastic right pulmonary artery, but hypoplasia was not demonstrable in the resected lung. There was, however, widespread obliteration of the peripheral lung capillaries and bronchitis with bronchiolitis. The authors presumed a functional in