1. A three-dimensional (3D) imaging system comprising: a processor for generating a volume-rendered 3D image on a display using a plurality of voxels from a 3D image dataset; and a view selector, responsive to user inputs, for determining a first set of voxels corresponding to a boundary in the volume-rendered 3D image using a predetermined boundary-specifying criteria applied to a boundary function of the opacity of the plurality of voxels.