If you have objects embedded in your body with a significant fraction of iron, cobalt or nickel (ie ferromagnetic elements), they will be attracted strongly to the magnet, and can cause severe tissue damage. (Ex-)Servicemen who may have small pieces of shrapnel in their eyes would be an example.Just like MRI,are dental X-ray's are harmful for your health.
However, iron and nickel tend to corrode in the body, so metallic components put into the body tend to avoid these elements
- titanium alloys for bone supports and pacemaker cases.
- In the past, dental fillings were made of gold or amalgum (mercury, copper, tin and silver), but today they tend to be made of more ceramic materials.
- Hip replacement joints are made of titanium alloys, or chrome-cobalt alloys. Depending on the alloy, this cobalt may have various responses to magnetic field.
Joint replacements are firmly anchored in bone, so small forces won't cause damage in the way that a piece of metal would in the eye.
It would be wise to remove body piercings and earrings before having an MRI - as much because they have induced currents that could degrade the image quality.
...but alancalverd is the expert here!