Hacker News Reader

Comment by koliber | Hacker News Reader

Completely tangential story. A few months back I was getting an MRI. I stashed my belt, coins, keys, and phone. The machine started its clicking and thumping when I realized I had my wedding band on my ring finger. Immediately my mind raced to a video I once saw of an MRI machine propelling a fire extinguisher across the room. I braced for my finger to be torn off while I slowly took the wedding band off using only one hand. Luckily it stayed put through the whole scan and nothing happened.

View on HN

If it was ferromagnetic you would likely feel it tugging as you got close to the magnet. Gold, silver, titanium are ok in MRI. However, next time, the second you realize you have metal on your person you should immediately inform the techs rather than try to just hold onto it! Aside from the risk of injury, it can be a real pain to get stuck items off the machine.

Taking off metal jewellery in the scanner is about the worst thing one can do - it’s then loose. It would be very unlikely to move from a clenched fist, even if ferromagnetic. However having it escape your hand isn’t that difficult.

I’ve seen most these things play out. And as you say, it’s exactly what the call bell is for.

I work on software for MRI machines, and one of the first things they do is high powered magnet safety training which is mandatory for everybody.

Even non-ferromagnetic materials react to the high field strength, and to show that, they let me hold a ring of aluminum just outside the bore. You can feel it "snap" to either parallel or perpendicular to the table when you try to turn it. It was a surreal experience.

I forgot to take my tungsten carbide wedding band off as well for an MRI, nor did the MRI techs say anything. It was in the middle of the MRI scan that I realized it was still on and then my fingers on the ring hand kind of started to feel fairly warm, but not certain it that was actually the ring picking up magnetic energy or if it was psychosomatic, but no harm became of it.

I looked it up afterwards and tungsten apparently as little to no magnetic effects, but depending on the amount of carbon used in it, it can.

Tungsten carbide jewelry is a mixture of tungsten carbide powder and a metal binder, typically cobalt or nickel. The metal binder is electrically conductive and thus susceptible to the induction heating you felt.

What you were experiencing was a) probably real and b) due to the conductive properties of the ring , not magnetic.

Oversimplification: Moving a conductor in a magnetic field or vice versa indices current in the conductor , resistance in the conductor results in heat.

The main field in an MRI is static but there are a lot of other fields moving around…

Similar happens on your body also (eddy currents) and deeper tissue gets energy which has to be controlled for - it can cause stimulation in peripheral nerves and heating .

> little to no magnetic effects

Whether or not it’s a conductor is probably more of a concern. The loop likely wouldn’t be large enough to cause any drama though.

Fun fact: metal being ripped away isn't the only negative effect possible, it can also heat up and burn you! (Why did I think this was fun again?)

https://www.sciencedirect.com/science/article/pii/S193004331...

https://www.washingtonpost.com/wellness/2023/07/26/mri-yoga-...

I got my third MRI after having a titanium plate put in my neck, and wow was I more nervous that time than the earlier two times. I made the tech let me test the panic button to prove it worked. Of course there was no problem with the MRI.

I asked about mine, and they said it was fine for the 1.5-tesla machine but they probably would have had me remove it for the 3-tesla one. I did feel it pulsing, but not noticebly warmer.

One of the things MRI do to create images is to pulse magnetic fields during imaging. These pulses are far weaker than the main field but will cause vibration of metals due to Lenz's and Faraday's laws. As the magnetic field changes it induces current in the ring, current in the ring interacts with the magnetic field to produce a (small) force on the ring. If you were getting a head/brain scan in a typical MRI your hand will lie near the locations within the scanner that see the largest swings in magnetic field. Beyond the vibration, rings are generally too small to be a heating issue even at 3T.

Best practice for at least a decade is to always remove all rings and all jewelry and failure to detect rings or other jewelry is generally seen to indicate a problem in screening. That is... if a radiologist sees evidence of a ring on the images there better be an explanation. The reality is that particularly older people have not removed their rings in decades and their joint disease may have expanded so much that it simply cannot be removed and the risk/benefit doesn't justify damaging the ring nor denying them the benefits of a scan. But if the patient can't take the ring off, the magnet wont either.

Just for reference, people get head scans with braces pretty regularly and it's not considered a safety issue. Braces and rings can affect image quality though so that's usually the concern. So if the ring is near the body part being imaged you'd probably be asked to remove it because they'll easily cause undesired issues (in, say, roughly a 3-6inch radius) that can result in images that radiologists deem unusable for making a diagnosis.

Reply on news.ycombinator.com