An anatomical explanation of the formation of butterfly shadows.

Butterfly or batswing shadows are perihilar opacities of a particular shape. They were observed by early authors in cases of pulmonary oedema (Day, Sisson, and Vogt, 1929; Coe and Otell, 1932; Roubier and Plauchu, 1933). Later details of the radiological appearances are described, variations noted, and several explanations of the pathogenesis suggested (Zdansky, 1933; Roubier and Plauchu, 1934; Lelong and Bernard, 1937; Roubier, 1938; Werkenthin, 1939, and others). The term " butterfly" density was introduced by Nessa and Rigler (1941) and the term " batswing" shadow by Hodson (1950).