I’ve never really learned the conversion from Tesla to MHz (the units chemists are interested in for NMR) though I see an 800 MHz NMR has a field of 18.8 Tesla. Any NMR above about 100 MHz uses a superconducting electromagnet (liquid helium is used to cool liquid nitrogen to get the metal to superconduct) to generate the field and you just don’t go and turn off one of those. Properly shutting down an NMR takes lots of time and is rather expensive. What field strength do they normally use for MRI (and could you put it into MHz for me, even though the mechanisms are somewhat different?) As it is, I spend a fair amount of time around a 200 MHz NMR (getting well inside the 5-gauss line to insert samples, after all) and I’ve yet to wipe one of my magnetic strips when I forget to take my wallet out of my pants. I saw a picture, pulled from somewhere on the internet, where a gas cylinder cart got attracted to the magnet and there’s a tank of gas, chained to the cart, basically stuck to the NMR.
I’m not familar enough with MRIs to say for sure, but I can’t immediately think of any reason there should be a correlation between operating frequency and magnetic field strength. I’d be interested in hearing from anyone who does know, though.
The 5 gauss figure is the minimum field strength known to be capable of demagnetizing low-coercivity card stripes, but most such cards can withstand up to a 300 gauss field, typically. Most credit cards issued these days have high-coercivity stripes, and would need a field strength of at least 1000 gauss, with most able to stand as much as a 4000 gauss field.
I could not quickly find any cite online, but within the last couple years a young boy was killed at Memorial Hospital in Belleville, IL when a steel tank was left in the room accidentally when they started up the MRI machine with him in it. The tank flew into the machine so hard that it struck him in the head and killed him. They’re only supposed to use aluminum tanks in the MRI treatment rooms.
-But I am another who would not have thought that MRI machines had strong magnetic fields all the time. I was under the impression that it was just when they were in use.
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The field strength may vary depending on when they’re actually scanning (maybe - I’m not that familar with the details of how they work) but given that you’re using magnets super-cooled by liquid helium, starting up and shutting down are rather involved, time-consuming, and expensive. So the machine “idles” when not being actively used.
Potentially.
Or, if it can’t get the gun out of the holster, and assuming the holster/belt are strong enough, will bodily drag the wearer to the magnet.
Two words: human error.
It happens.
The field extends a surprising distance. Space restrictions may make it very difficult to place hallways far enough away to avoid the major field effect entirely. MRI’s going into spaces designed specifically for modern imaging needs may have this, but not necessarially ones fit into hospitals that have been standing for decades, particularly in urban areas where space is at a premium.
All particles with spin (such as a proton, with spin = 1/2) have what’s called a gyromagnetic ratio, which is the ratio of the magnetic dipole moment to the angular momentum of the nucleus. Experimentally, the gyromagnetic ratio is the frequency of precession of the nucleus (or the frequency of applied electromagnetic radiation, in MHz) over the magnetic field strength (Tesla).
For hydrogen, the nucleus involved w/MRI, the gyromagnetic ratio is 42.58 MHz / T.
No body has yet splained why a cop with a gun was in an MRI room? Criminal needs a head zap, go stand behind the wall with the rest of the folks. ::: sheesh ::::
Don’t they also warn away possible patients with surgical pins and screws, Vets with shrapnel, machine tool operators (who may have microscopic flecks of iron or steel dispersed in their bodies)?
And pacemakers. There’s a fairly large NMR (I think it’s a 500-MHz NMR for the biochemists) in the basement of the chemistry building at NMSU where the field extends far enough that they have magnetic field warnings on the men’s restroom nearby. And on the other side is one of the 200-MHz NMRs.
Mk VII, I’m surprised they let you into the room with any metal on you. I’ve only had my head examined by MRI, so they didn’t make me strip down to a hospital gown or anything, but the only metal I was allowed to take in was the button and zipper of my jeans. And, of course, only ferromagnetic items are really dangerous to the instrument itself. The field could conceivably damage a watch, but it’s not like the watch is going to be sucked off your wrist and onto the instrument.
The example I believe was a burglar alarm and an unsecured door. The police had to clear the building in case it was a burglary. Not all MRIs are in hospitals. We have a private MRI facility in the town I work in. In the official email that came down it said they are dangerous even when they are “off”. Maybe an expert can clarify.
There is no evidence that a police officer had his pistol pulled out of it’s holster by an MRI other than the anecdotal one on the MRI website linked to by Exapno. Given the details of that incident that dovetail almost perfectly with the news story which was also from Rochester, someone added the bit about the holster when they put that on the website.
I read the full story in two different papers and there was no mention of such a thing happening, only that an off-duty policeman had gone to have an MRI done, told a “clinic worker” that he had his handgun with him, and the worker told him that it was OK.
One could surmise that the cop had his gun out to place it somewhere while he was getting the MRI.
