1. A method of identifying sub-objects of an object defined as a plurality of volume elements (voxels) in an image generated by an imaging apparatus, each voxel being associated with a feature value (F), comprising: a. determining a distribution of feature values of object voxels so as to define an F distribution; b. identifying peaks and separation thresholds within the F distribution; c. deciding whether an object is a potential compound object; d. based on the separation thresholds, for each object voxel, assigning a sub-object label corresponding to the feature value associated with each voxel so as to distinguish the voxels for each of the sub-objects; and e. refining the sub-object label assignment so as to resolve inconsistencies in sub-object boundaries.