The first radiographic sign of pulmonary venous hypeension is cephalization of the pulmonary vessels due to pulmonary vein and aery dilatation. In the normal individual, there is continuous passage of fluid from the pulmonary veins into adjacent interlobular lymphatics that return the fluid to the central mediastinal veins. If the lympatic reserve is overcome by increased transudate as a result of elevated pulmonary venous pressure, the interlobular septa are thickened and become visible radiographically. The first radiologic sign of pulmonary venous hypeension is cephalization of the pulmonary vessels. Ref: Atlas of Pulmonary Vascular Imaging By Conard Wittram, Page 51; Textbook of Radiology and Imaging By David Sutton, 5th Edition, Pages 288-89