1. A segmentation apparatus for interactively segmenting blood vessels in angiographic image data, wherein the segmentation apparatus comprises: a significant location determining unit for determining one or more locations of a current segmentation of the blood vessels in the angiographic image data as significant locations at which the current segmentation has a predetermined influence on a value of a blood flow parameter that is calculated based on the current segmentation, and a display unit for displaying the significant locations to an operator.