1. A method of segmenting a target organ of the body in medical images, the method comprising: a) providing a probabilistic atlas with probabilities for a presence of the target organ at different locations relative to a bounding region of the target organ; b) providing one or more medical test images; c) for each of the one or more medical test images, identifying, at least provisionally, a location of a bounding region for the target organ in the test image; and d) for each of the one or more medical test images, generating a segmentation mask of the target organ in the test image using the probabilities from the probabilistic atlas, wherein generating the segmentation mask is performed by a data processor, and wherein generating the segmentation mask of the target organ comprises finding a core region of voxels that have locations within the bounding region where the target organ is considered likely to be located according to the probabilistic atlas, and most of which have intensity values expected for the target organ.