1. An apparatus for generating an attenuation correction map used for reconstructing an attenuation-corrected nuclear image of an object, the apparatus comprising: an image providing unit for providing an image of the object; a segmentation unit for applying a watershed segmentation to the image for generating a segmented image comprising image regions corresponding to the element classes; a feature determination unit for determining features of at least one of a) the image regions and b) boundaries between the image regions depending on image values of the image; an assigning unit for assigning attenuation values to the image regions based on the determined features for generating the attenuation correction map.