1. An apparatus for post-processing 2D cross-sectional slice images defining a 3D image volume data set, the apparatus comprising: a graphical user interface controller; a 2D segmenter; and a 3D segmenter; wherein the graphical user interface controller is configured to generate for display on a screen a graphical user interface, the user interface when so displayed comprising an interactive first window widget displaying on the screen an initial slice of the 2D slices, the slice showing a cross section of an object of interest, the window widget configured to allow a user to specify by a pointer tool position data of a single initial point anywhere on the displayed initial slice; wherein the 2D segmenter is configured to 2D segment, in response to the user specifying the position data, the displayed initial slice by calculating across the slice a curvilinear segmentation line passing through or starting at the specified single initial point; wherein the graphical user interface controller is further configured to generate for display in the first window widget the calculated segmentation line overlaid on the initial slice; wherein the 3D segmenter is configured to calculate, in response to a user request, a 3D segmentation of the object across a plurality of the slices by using the calculated segmentation line as a geometric constraint for the 3D segmentation; and wherein the graphical user interface controller is further configured to generate for display on the screen at least a part of the so calculated 3D segmentation.