ELI5, why do MRIs not burn patients with titanium plates internally from eddy current?


Are the eddy currents induced by MRIs too insufficient to see rapid temperature increase? I understand MRIs are incredibly attracted to ferrous material, but what happens to non-ferrous metals in the body?

In: Physics

In short, for the same reason that they don’t ping at airports: it not 100% pure titanium but an alloy of several different things that are made to be as unobtrusive to the human body as possible, which means that both the alloy and the mri are made to “ignore” each other, for lack of a better word.


Source: got a few nails in my leg.

Had to look up the [answer](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6369045/), but yeah, it seems that eddy currents generally aren’t sufficient to cause sufficient heating concerns in most cases. (Although a few articles did have warnings)

They don’t give many details (nor does the citation), but it seems to be a combination of two things:

a) implants often don’t form a nice large conducting path. Interruptions in the path can reduce eddy current effects

b) the body in general (or saline solution) is generally pretty good at transferring heat. It’s actually an issue in things like heat treatments, things like blood dissipate local heat pretty effectively, especially near large arteries and the like.

Neat question, since it seems that contact burns are actually fairly common. From what i found, that’s because things like outside wires often do form large conducting paths that are nice for eddy currents, and are also fairly thermally isolated (being in air)

Probably something to do with the continuous and low total surface area of implants, but that’s a guess. Eddy currents are sufficient to heat things up to the point of burning the patient in the scanner. One example of that is wearing Lululemon’s clothing with Silverescent technology into an MRI, which has metallic fibers woven into the fabric c.f. https://www.marketwatch.com/story/people-wearing-yoga-pants-are-getting-burned-during-mris-2018-05-09 metallic fibers would have a much higher total surface area.

Also, the risk of heating from eddy currents likely has something to do with the specific scan sequence, and I would assume higher risks with those which use higher gradient angles, e.g. MPRAGE. This implies a case of greater oscillating changes in the angle of the magnetic field and are known to produce greater eddy currents and SAR (specific absorption rate). I believe these types of scan sequences are probably more common in research related MR scanning, but I don’t know much about typical medical scanning procedures in hospital environments.

Edit: added some words for better detail