In a method or apparatus for identifying a region of interest in a medical image of a subject, at least one local maximum region of the image is determined for which a value of a given variable is a local maximum value. A user-selection of an initial voxel in the image is registered. As the region of interest, a connected set of voxels for which values of the given variable are greater than a threshold is selected, the selected set of voxels comprising a first local maximum region at a distance from the user-selected voxel, and the threshold being a given fraction of the local maximum value of the first local maximum region in the set.