Whenever working with contour VOIs, the user can interactively add or remove individual pixels by just clicking at them. To do so, the corresponding tool has to be selected as illustrated below.
The example below illustrates the operation. A first Contour 1 was generated using iso-contouring. Then with Click at individual voxel to add to VOI the pixel represented by Contour 2 was added. Finally with Click at individual voxel to remove from VOI the pixel represented by the "minus" Contour 3 was removed. The resulting ROI is the shaded area.