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It is said, that steel rod inside body, like funeral nail, can heat while magnetic resonance imaging (MRI).

But why? Isn't MRI scanner use constant magnetic field?

If the heating effect is caused by radio frequency (RF), then why titanium rods differ?

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  • $\begingroup$ Can you provide a link? A quick Google suggests that steel implants may cause minor problems with MRI scanners but that they certainly wouldn't melt. $\endgroup$
    – John Rennie
    Commented Oct 1, 2014 at 10:56
  • $\begingroup$ They won't melt, but allegedly will heat. $\endgroup$
    – Suzan Cioc
    Commented Oct 1, 2014 at 11:07
  • $\begingroup$ For example this document mentions heating only due the RF field, and the heating would be the same for any metallic conductor. Problems with ferromagnets are mentioned only in connection with the magnetic forces exerted on them. $\endgroup$
    – John Rennie
    Commented Oct 1, 2014 at 11:13

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As was mentioned before, MRI uses irradiation of radio-frequency waves to excite sample and produce a signal. A metal rod is basically a dipole antenna. Depending on its lengths it can absorb energy from the RF field, if the RF field satisfies a resonance condition (i.e. if (half) the wavelengths is comparable to the length of the rod.).

Since the RF-fields frequency is dependent upon the main magnetic field of the MRI device, the same rod may heat up differently in different MR machines. Hence, a fixed orthodontic retainer in your mouth may be a problem at one field strengths, but may be OK at another.

Hope that helps!

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There are some big RF fields used in NMR. The nuclear spins are polarized by the static field, but then spins are flipped (rotated) by a short burst of RF at the resonant frequency. (Search for Pulsed NMR to read more.)

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