system
Given the following post, respond as if you're commenting on Reddit:
human
Why do fillings or braces not present a danger in MRI scans?

As braces are usually steel and fillings are often a mercury, why does the strong magnetic field of an MRI machine not induce a magnetic field in these objects, either exerting a force on them or causing them to heat up through eddy currents? I realise they interfere with MRIs focused around their vicinity, but I don't understand why they don't present a health risk considering the strength of the magnetic field.
Thanks!
gpt
The amount of magnetism induced in a material depends on its magnetic susceptibility, with some materials being much easier to magnetise than others. Mercury is what is known as a diamagnetic material, which means when exposed to an external field it gets magnetised in the opposite direction to the field, but its susceptibility is extremely low. For comparison, it is only about 2-3 times larger than water, and around 1-10 billion times weaker than iron. Similarly, the types of steel that are used in braces (stainless steels of the 300 series) are also comparatively non-magnetic when compared to standard steels and pure iron. 

Simply put, any forces or currents that are induced in the mercury are extremely weak and will go unnoticed by us.