A device for segmenting an image of a subject (36), includes a data interface for receiving an image of the subject (36), which image depicts a structure of said subject (36). A translation unit translates a user-initiated motion of an image positioner into a first contour (38) surrounding said structure. A motion parameter registering unit registers a motion parameter of said user-initiated motion to said first contour (38). The motion parameter includes a speed and/or an acceleration of an image positioner. An image control point unit distributes a plurality of image control points (40) on the first contour with a density decreasing with the motion parameter. A segmentation unit segments the image by determining a second contour (44) within the first contour based on the plurality of image control points (40). The segmentation unit is configured to use one or more segmentation functions.