The invention relates to an apparatus for segmenting a series of 2D or 3D images obtained by monitoring a patient's organ or other body part, wherein a first segmentation is carried out on a first image of the series of images and wherein the first segmentation is used for the subsequent segmentation of the remainder of images of the series of images. A series of transformations are carried out wherein each separate transformation embodies a fitting operation between two images of the series of images, and wherein substantially all images of the series of images are subject of such a transformation. The first segmentation on the first image of the series of images is modified and subsequently applied to any further image of the series of images according to the transformation or sequence of transformations that fits the first image to a further image of the series of images.