Man wearing heavy metallic necklace dies after being sucked into MRI machine

Umm, yes, they can. An emergency shutdown is very expensive...

I saw that headline. Didn't read it.

How long does it take?

Joe

You should. It's short and free. And not so many folk realize that those warnings are not just safety nuts nattering.

Joe

You should. It's short and free. And not so many folk realize that those warnings are not just safety nuts nattering.

Joe

---------------------------------- I used that tech in an NMR machine while a chemist. The magnetic field was of similar strength, 1.4 Teslas, and would bend the blade of a knife held near it.

NMR is Nuclear Magnetic Resonance, a name that had to be changed for medical use to avoid frightening patients.

Someone else forgot to remove his wristwatch when changing samples and magnetized it. He took it to a jeweler to be demagnetized, which normally makes the watch vibrate a little at first. In this case it flew off the machine and landed on the floor, breaking its crystal.

.. And not so many folk realize that those warnings are not just safety nuts nattering.

Joe

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I did an emergency discharge on a 3 tesla 6" room temperature bore supercon once in grad school. There was a vacuum leak in the cryostat and the boiloff rates had gotten high enough that it had to be shut down and we didn't have a power supply to do it. We were trying to borrow a supply from the system vendor but couldn't get one in time. Oxford had an emergency procedure in the manual that used cross coupled stud diodes rated for the main coil current (30 or 40 amps?) on a heat sink along with a little dc supply and clip leads to energize all the shim and main coil superconducting switches. Took us an hour to scrounge the parts and a few minutes to get the charging wand inserted and connected. The actual discharge took about 20 minutes for the field to be weak enough that you couldn't feel it tug on a screwdriver held against the side of the cryostat (not inside the bore). We let it go 2 or 3 hours just to be safe, then burped a small hit of helium gas into the cryostat to spoil the vacuum and boil off the rest of the LN2 and LHe. It was all room temp a week or two later when we tore it down to replace the magnet.

That was back in the early 1980's. I've seen pics of MRI machines with the diodes on a heat sink hung on the cryostat along with a discharge cable for the charging wand that already had all the superconducting switch leads tied together and a 5 volt power supply for the switch heaters so it was all ready to go in case of an emergency. That plus the normal charging wand and down it comes. An MRI magnet would have a lot more stored energy than our little 3 T magnet so I'd guess at least an hour or two to get the field down to where you could remove a large iron object like a fire extinguisher (or a person with a large ferrous necklace :-)) and 24 hours to be sure it was completely discharged.

----------------------------------- The superconducting coil is a large inductor that stores up to a few kilowatt-hours of energy in its magnetic field, like a spinning flywheel. De-energizing an MRI magnet requires dissipating energy comparable to discharging a trolling motor battery or cooking a meal. Also inserting resistance into the coil creates an IR drop voltage the coil's insulation must withstand.

The closest I've come was building a machine for GM that simulated releasing the energy in an alternator rotor's magnetic field if the battery cable disconnected. The model was a corroded connection and a bumpy road that caused rapidly repeating load dumps.

That second part is interesting. I have one titanium rod, four titanium pins, maybe some brass fragments in my leg (mostly removed), and had a couple pins in my two front teeth, but they may have been mostly removed when I had porcelain crowns done.

I have been inside an MRI machine, but the order of things escapes me. Time and temporal juxtaposition is a bit of a blur for some things in my life.

That second part is interesting. I have one titanium rod, four titanium pins, maybe some brass fragments in my leg (mostly removed), and had a couple pins in my two front teeth, but they may have been mostly removed when I had porcelain crowns done.

I have been inside an MRI machine, but the order of things escapes me. Time and temporal juxtaposition is a bit of a blur for some things in my life.

Bob La Londe

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The MRI radio frequencies are between 50MHz and 300MHz. The shortest wavelength, at 300MHz, is 1 meter so your metal parts were likely too small to be efficient dipole antennas.

THANK YOU for using the proper name!

THANK YOU for using the proper name!

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The choices were to change the scary word, or try to calm patients by explaining an interaction between Quantum Mechanics and electromagnetic field theory, notice that I didn't try here. It's still NMR to chemists who understand how it works.