These are all doable using semi-automatic segmentation tools in 3D Slicer (VTK-based medical image analysis application). Difficulty depends on the quality of the image (presence of metal artifacts, resolution, and noise) and the clinical need (accuracy and detail of the segmentation).
These topics are regularly discussed on the Slicer forum, so you can find useful information by searching there - and if you cannot find answers to any of your questions then feel free to post them.
For example, teeth segmentation:
- Teeth Segmentation - #19 by Reham_Hassan - Support - 3D Slicer Community
- segmentation different structure of individual tooth - Support - 3D Slicer Community
Nerve segmentation in 3D Slicer:
