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A few years ago in the biohacking community there was a phase where people (mostly engineers) were having small magnets embedded in a fingertip in order to sense magnetic fields. I was slightly tempted at the time but never had the procedure. Since that time I have had an MRI scan. The question is the obvious one - what would happen when finger meets a 2T field?

Depending on alignment the finger would be either repelled, attracted or possibly rotated into alignment. I imagine it would be rather unpleasant, but how unpleasant? Just a tugging sensation or the finger magnet doing an impression of "Alien" and bursting out of the skin?

And what of the RF excitation? The magnet would probably not make a good antenna, but I'm not sure. Do we add "cooked" to the menu?

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The tugging of a tiny magnet will be uncomfortable and perhaps painful, but the magnet shouldn't get ripped out of the finger. There may be internal bleeding due to the magnet motion ripping the nearby capillaries. Wrapping the finger tightly would reduce the range of motion of the magnet, and thus discomfort.

How tiny should the magnet be? That requires experimentation - yes, unfortunately. People's pain tolerance varies, so one person may experience way more discomfort than another with a magnet of the same size. It's a tradeoff between sensitivity to magnetic fields and magnetic fields making one feel pain.

Cooked is off the menu, it'll be served raw. There's not enough RF energy density for a tiny magnet to do much RF absorption.

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    \$\begingroup\$ RF no, but I'd be worried about induction heating from the gradient coils, Won't take much, the heat dissipation of a little piece of metal in peripheral tissue won't be great. \$\endgroup\$ Feb 12 at 20:40
  • \$\begingroup\$ We also have to bear in mind that fingertips have a lot of pain (and other) receptors, which is why the magnet is implanted there. \$\endgroup\$ Feb 13 at 9:40
  • \$\begingroup\$ The question isn't about the tugging of a small magnet. It is about the tugging of an enormously strong magnet on a piece of metal (which also happens to be a magnet itself, but that is basically irrelevant). You can't have an MRI if you have any ferromagnetic material in or on your body. You can search on google for images where MRI machines have picked up metal chairs and beds and thrown them across the room. \$\endgroup\$
    – Tom V
    Feb 13 at 23:23
  • \$\begingroup\$ @TomV Tugging of a small magnet by the big magnet is indeed what I was implying. MRI magnets will nicely pick up large metal objects. They also pick small steel metal pins as you'd use in sewing, but they won't pull them out of your fingers if you hold them tight. I tried that several times, just because pins vibrating in the magnet bore is good entertainment. You can't have an MRI if you have any ferromagnetic material in or on your body. It's not a binary criterion. It all depends on the quantity. Blood itself is paramagnetic and exerts additional force on the vessels in strong fields. \$\endgroup\$ Feb 14 at 17:45
  • \$\begingroup\$ @Kubahasn'tforgottenMonica thanks for the extra detail. This seems really counter-intuitive, but there is no substitute for personal experience! \$\endgroup\$
    – Tom V
    Feb 14 at 21:35
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Any kind of ferromagnetic material embedded under the skin would be forcibly ripped out by the main magnet if the embedded material is close enough to it. That would include embedded magnets.

Even up to that point, the magnet will want to rotate and align with the main field, which itself will cause some damage, again as it gets close enough.

At any rate MRI safety guidelines would likely contraindicate a patient with ferromagnetic implants getting near the machine, let alone getting a scan. At the very least, the implant would need to be stabilized with a tape / bandage wrap to limit movement and tissue damage. In some cases an ice pack might be applied to counteract heating from the RF coils.

More here: http://www.mrisafety.com/SafetyInformation_view.php?editid1=239

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  • \$\begingroup\$ Perhaps, but skin is very tough. Much worse than holding it against a strong Nd magnet? \$\endgroup\$ Feb 12 at 17:27
  • \$\begingroup\$ It depends on how close you are to the static magnet. \$\endgroup\$ Feb 12 at 17:31
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    \$\begingroup\$ I've brought small pieces of iron into an MRI machine. There is noticably stronger pull than gravity, but it's not overwhelming and you can hold them in your hand. I'd expect a small magnet in your skin to feel strange but not be ripped out unless it was already barely holding on. Same way metal in your hands doesn't rip its way out of your grip. \$\endgroup\$ Feb 12 at 19:44

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