The present invention relates a segmentation apparatus (10) for interactively segmenting blood vessels (2) in angiographic image data (3). The segmentation apparatus (10) comprises a significant location determining unit (11) for determining one or more locations of a current segmentation (4) of the blood vessels (2) in the angiographic image data (3) as significant locations (5) at which the current segmentation (4) has a predetermined influence on a value of a blood flow parameter that is calculated based on the current segmentation (4), and a display unit (12) for displaying the significant locations (5) to an operator. Thereby, the operator can be guided to focus his/her segmentation efforts to those locations of the current segmentation (4) that are most relevant for the accuracy of the calculation of the value of the blood flow parameter. This may ease the burden on the operator during the segmentation procedure.
