MRI & White Gold: Is Heating a Risk?

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razidan
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Hi,
So I recently had to get an MRI. I forgot to take off my wedding ring, but the technicians said it's ok "if it's real gold".
I told them it was white gold (which is an alloy of 58.5% gold and the rest is other metals). They said it was fine, but later told me they didn't really know what white gold was.

Anyway, the second the machine started blasting radio waves, I felt a... Response... from the ring. It felt kind of like vibrations that matched the frequency of the machine.
It started heating, so I took it off.

So, here's my question. I'm assuming metals heat up because of induced current. But, then, why would gold be fine? I think it's because it has very low resistance, and therefore would not heat up as much?

And another question, they let me have earphones on and played music while the machine was going. How did the machine not interfere with the current in the earphones?

Thanks,
R.
 
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razidan said:
Anyway, the second the machine started blasting radio waves, I felt a... Response... from the ring. It felt kind of like vibrations that matched the frequency of the machine.
It started heating, so I took it off.

So, here's my question. I'm assuming metals heat up because of induced current. But, then, why would gold be fine? I think it's because it has very low resistance, and therefore would not heat up as much?
I would recommend taking all jewelry off before entering the MRI room. I know that certain metals are OK, but since you typically don’t have metallurgical study data on your jewelry, I feel it is safer just to take it off.

Given that you felt forces on the ring then it is likely that some of the alloy was iron, nickel, or cobalt which are the most common ferromagnetic metals.

The pulsing you heard was the magnetic gradient fields, and those would have been trying to get the ferromagnetic domains in the metal to align with a changing field. The hysteresis in doing so would heat up the ring.

razidan said:
And another question, they let me have earphones on and played music while the machine was going. How did the machine not interfere with the current in the earphones?
Usually the earphones do not have any current. The sound is piped in through a pneumatic tube. It degrades the sound quality quite a bit, but preserves the image quality.
 
razidan said:
... but the technicians said it's ok...
Search the web for MRI accidents, to find some gruesome stuff where some technician though it was OK. Taking off a ring is not really that much trouble.
 
ZapperZ said:
I'm not sure how these technicians were trained in safety, but the two times that I've accompanied someone to have an MRI done, the instruction explicitly indicated to not have ANY metallic item on the person
The technologists will do what they are trained to do, but unfortunately some places vary in their safety standards. I think that all patients should remove all clothing and jewelry and change into hospital scrubs. Too many items of clothing contain metal, and too many items of jewelry also. Removable dental items and prosthetics also.
 
Dale said:
Given that you felt forces on the ring then it is likely that some of the alloy was iron, nickel, or cobalt which are the most common ferromagnetic metals.

The pulsing you heard was the magnetic gradient fields, and those would have been trying to get the ferromagnetic domains in the metal to align with a changing field. The hysteresis in doing so would heat up the ring.
.
Thanks, would a non-ferromagnetic metal react at all?

A.T. said:
Search the web for MRI accidents, to find some gruesome stuff where some technician though it was OK. Taking off a ring is not really that much trouble.

ZapperZ said:
Just don't volunteer to carry an oxygen canister into an MRI room...

I'm not sure how these technicians were trained in safety, but the two times that I've accompanied someone to have an MRI done, the instruction explicitly indicated to not have ANY metallic item on the person.

Zz.

Dale said:
The technologists will do what they are trained to do, but unfortunately some places vary in their safety standards. I think that all patients should remove all clothing and jewelry and change into hospital scrubs. Too many items of clothing contain metal, and too many items of jewelry also. Removable dental items and prosthetics also.

Thanks for the responses, and no disrespect, but my question was about the physics and not safety issues. next time i'll remember to remove the ring.
 
razidan said:
Thanks, would a non-ferromagnetic metal react at all?
You can get eddy currents in non ferromagnetic metal, but that would not be significant for something the size of a ring.

razidan said:
I told them it was white gold (which is an alloy of 58.5% gold and the rest is other metals)
It looks like nickel is commonly used in white gold, so that is probably the source of the force.
 
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You were probably feeling the effects of eddy currents induced in your ring by the pulsed magnetic fields in the gradient coils. Dissipation will cause heating, and Lenz's Law tells you that your ring will experience a force as well.