In a method and apparatus for identifying regions of interest in medical images of a subject, in particular images captured by a medical imaging apparatus, such as a PET scanner, a list of voxels of the image is obtained, and sorted according to a first variable. A user-selection of an initial voxel in the image is then registered, and at least one voxel or group of voxels from the sorted list is selected as a region of interest, according to a property of the at least one voxel or group in relation to the user-selected initial voxel. The image may be pre-processed to generate the sorted list of voxels, following which the user selection of the initial voxel is registered.