Figure 4. Schematics show two cross sections of a vessel, one normal (left) and one with a stenosis (right). The horizontal lines denote the selected image plane; the small circles denote the user-selected points within the vessel. False-positive stenoses can be introduced by incorrect placement of the point within the vessel; on the left, the top point is incorrectly placed and results in a smaller vessel diameter than the bottom point. False-negative stenoses can be introduced by incorrect orientation of the plane; on the right, a plane perpendicular to the current one should be selected to visualize the stenosis.