Can a root canal shows as an artifact on a head mri?

Yes but minor: Root canal can show up as a black spot around the area where the signal drops out due to interference from the magnet. The artifact is usually small and doesn't effect a great deal.

MRI Artifacts?: It depends on what was used to fill the canal. MRIs use magnetic fields to look for soft tissue changes. Most dental materials are non-magnetic and so should not be an issue. Some canals were filled with silver points and may be an issue. All the metal can be an issue for CAT scans.

Definitely, yes: Root canal can show up as a black spot around the area where the signal drops out because to interference from the magnet field.

MRI aftifacts: Here's a good reference for your question: https://www.wemakespokanesmile.com/dental-materials-mri-compatible/ In my opinion most RCT fillings are non metallic and should not be a problem. If it was filled with a silver point which is hardly used anymore, it might produce an artifact.

Yes: A root canal can show up on an mri but the doctors will know what it is. This will not be an unknown or questionable finding.

Yes: If the root canal is completed the rc filling is radiopaque; meaning it will show up as a white line on any imaging.

Doubtful: The common root canal fillings are not metallic. Even if it was it would be to small to effect an MRI. You could have silver fillings in your mouth, and they also would not effect the MRI. So your answer is NO!

Artifact maybe: Some older root canals were filled with a silver point that was placed into the canal. This would be more likely to show up as an artifact than the standard gutta percha, which is used for root canals. It is an inert substance.