1. A method for automatic or semi-automatic segmentation of a 3D image data set, comprising: providing a computerized processor with a 3D image data set, acquired with a medical imaging apparatus, representing an examination region of an examination subject, said examination region comprising an anatomical organ; providing information to said computerized processor designating a type of said organ in said examination region; in said computerized processor, using the designated type of said organ to obtain a model-based segmentation algorithm wherein the designated type of said organ is a basis of the model in the model-based segmentation algorithm; in said computerized processor, automatically segmenting the 3D image data set by executing said model-based segmentation algorithm based on said designated type of said organ, to produce a segmentation result; after executing said model-based segmentation algorithm, automatically or semi-automatically segmenting said 3D image data set in said computerized processor by executing a greyscale value-based segmentation algorithm that uses said segmentation result produced by said model, to produce another segmentation result; and at a display screen in communication with said computerized processor, visually displaying at least one of said segmentation results.