When a radiologist examines an organ angiographically, he injects a radiopaque contrast material into selected vessels and takes a series of radiographs of the opacified vascular network. Then he searches the vessel images for abnormalities that indicate either pathology or pathophysiology within the organ. Some of his diagnostic criteria for an abnormality are (1) displacement and deformity of vessel complexes or individual vessels, (2) intrinsic changes within the vessels such as variations in caliber and contour, and (3) alteration of the circulation time or the direction of the flow of contrast material within arteries and veins. In cases of tumors he also searches for clues as to the nature of the growth, i.e., whether benign or malignant. Obviously his ability to record small vessels clearly has a decided effect upon the accuracy of his diagnosis.