In 1938 we first described our method of visualizing the chambers of the heart, the pulmonary circulation and the great blood vessels and later published a detailed account of the technic and a summary of the results obtained in the first 133 cases.1Before that time there had been no practical way of visualizing the interior of the heart and of the intrathoracic blood vessels. Although the roentgen ray had become almost indispensable to the accurate diagnosis of heart and lung diseases, it gave an incomplete picture of the anatomy of the cardiovascular system, because the four separate chambers of the heart and their component parts were represented on roentgenogram and fluoroscopic screen by one shadow, and the intrathoracic blood vessels were seen indistinctly if at all. Roentgenologic diagnosis, therefore, had to rely on such indirect evidences of disease as alteration in the size, shape and pulsation of the