1. A computing system for interactively segmenting blood vessels in angiographic image data, wherein the segmentation apparatus comprising: a processor; and a computer readable storage medium with computer readable executable instructions, including a significant location determiner, wherein the processor is configured to execute the significant location determiner instructions, which causes the significant location determiner to determine for 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 for displaying the significant locations to an operator.