Hello. I am come here with a simple question: how can I know the voxel size of a tooth from a vtkVolume (dentition CT scan) ? I don’t know from where to take this value … I need it for computing the size range in order to use it on vtkImageConnectivityFilter method.
Thank you for your patience 
You can get voxel size from vtkImageData’s GetSpacing() method.
Usually teeth touch each other, so vtkImageConnectivityFilter will not be able to segment individual tooth. vtkImageThresholdConnectivity can break small bridges (see NeighborhoodRadius property) but that also removes sharper edges from the teeth.
There are several semi-automatic tools in 3D Slicer’s Segment Editor, which makes it relatively easy to segment teeth from CBCT. If the image quality is very good then it may just need a single click. If you need help getting started then you can post your specific questions on the Slicer forum, where there are a number of teeth segmentation experts who can help you.