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Analytical Chemistry

Quenching NMRs, Accidentally and On Purpose

You may remember the MRI accident in India, where someone brought an oxygen tank into the imaging room. The consequence were not good ones – not only did two people get pinned to the machine by said tank, it took hours for them to kill the magnet because the emergency quench circuit was deactivated.
A longtime reader sends along the news that GE (maker of the MRI machine involved) has asked for all its installed machines to be checked to make sure that the quench circuits are operational. One would assume that this is being done both for safety and for legal/liability purposes (being able to come into a court and say that every other GE machine in the world has a working quench circuit, so how come this one didn’t, and to forestall any other such problems). It’s basically a short instruction list – press this button, make sure that this light comes on, and so on.
If you’re not an NMR user, it might come as a surprise to find that the quench circuit is really little more than a heater. All these NMR magnets are superconductors; it’s the only practical way to get that kind of magnetic field. (Note that “practical”, for these purposes, means finely machining a bunch of expensive rare-metal alloy and cooling it down with liquid helium – that should tell you what the other options must be like). Also note that “MRI” (magnetic resonance imaging) is merely the consumer-friendly acronym tacked onto what the rest of the scientific world calls an NMR. The phrase “nuclear magnetic resonance”, though, gave some patients the willies, since they thought it involved nuclear isotopes and radiation, so a less threatening phrase was quickly dreamed up.
So you have a big round superconducting magnet, with a ridiculous amount of electric current ripping around it, forever. Well, or until it warms up, whichever comes first. Sometimes, when you’re getting these things running for the first time, some anomaly will cause a small domain in the magnet to decide that it’s tired of the superconducting lifestyle (I am bypassing the more technical explanation). It finds itself, then, suddenly exhibiting electrical resistance, and it has plenty of current to exhibit it with, too. That part of the magnet heats up very quickly, and that heating throws the regions around it out of superconductivity, and they heat up just as fast, and then. . .well, what you get, as a bystander, is a loud, clanging thump, followed traditionally by a very expensive geyser of helium vapor. No one is pleased.
Unless they’re pinned to the damned magnet by an oxygen cylinder, of course. Then you definitely want to be able to slap a big red knob, artificially heat the magnet, and have all the larger stray ferromagnetic objects suddenly drop to the floor. These knobs are generally mounted behind a cover of some sort, to prevent someone from leaning up against them or putting a philodendron on top of them, because (1) that aforementioned geyser can represent thousands to tens of thousands of dollars of helium these days, and (2) quenching is Not Good for the integrity of the magnet, and in the worse case you might find yourself with a lot of high-quality scrap metal. (I’ve long thought that a magnet housing itself could be turned into a real conversation piece of a barbecue pit, perfect for departmental get-togethers).
Helium has become a lot more expensive over time, with some rather large price fluctuations along the way due to the small number of suppliers. This has led the market to supply various sorts of helium-recycling devices for NMR facilities, which were once too expensive to make sense, but now pay for themselves pretty handily – unless someone stumbles on another big helium-rich natural gas field, anyway.

43 comments on “Quenching NMRs, Accidentally and On Purpose”

  1. RM says:

    since they thought it involved nuclear isotopes and radiation,
    (Pushes Poindexter glasses up on nose.) Well, actually it *does* involve nuclear isotopes and radiation. (Stable and radio-frequency, respectively)

  2. Random says:

    I actually never saw an emergency quenching button on any NMR spectrometer. I didn’t really look for them, but I don’t think every NMR spectrometer has them.

  3. Anonymous says:

    @2. Random: I would guess that is because Scientists, who operate NMR spectrometers, are usually expected to know how not to get stuck on the magnet and no one else gets access to those instruments. MRIs are standing in hospitals, however, with all kinds of patients, medical personal and other people not really aware of the dangers walking around. That’s why MRIs do have emergency quench circuits.

  4. MA says:

    @2: I’ve only really heard of them on big instruments, 600+ MHz.
    My facility’s instrument (300 MHz “R2D2” model) has a quench circuit but no “Big Red Button”

  5. Anonymous says:

    Is there no way to quench by forcing the orientation of two or more magnets against each other so that their fields cancel?
    That would be a lot cheaper (and safer) than allowing them to burn out…

  6. Anonymous says:

    … and just add some really strong tension springs to counter the force of magnetic repulsion, so that a small additional force will be enough to flip orientation of the magnets either way.

  7. Anonymous says:

    Come on, someone tell me how clever I am! 🙂

  8. Wage_Slave says:

    When you can do this on purpose however it is visually very exciting. We quenched our old 400MHz this week – all it took was 50ml of gaseous helium in the vacuum space. If you have not seen an NMR quench there are a few on YouTube 🙂

  9. Anonymous says:

    My brother is a research radiologist and got to witness a MRI quenching incident the day that the janitor forgot himself and instead of using the special plastic kit, wheeled his conventional mop and galvanized bucket into the MRI room. They flew across the room and attached themselves to the side of the magnet loop. The resulting eddy currents were enough to cause local loss of superconductivity and to trigger a quench.

  10. Anonymous says:

    @4: You don’t need a quench button on Mercury magnets; they’re call quenchomatics actually 😉

  11. Anonymous says:

    Do NMR machines really wipe credit/debit cards, or is that just an urban myth (and a geeky one at that).

  12. JAB says:

    @9 we had a similar incident many years ago with a janitor and a bucket – in our case it didn’t quite quench or damage the magnet permanently. NMR magnets are now much better shielded, so it’s no longer necessary to remove your credit cards before entering the NMR lab. Also it makes it possible to put two big magnets in the same space.

  13. Algirdas says:

    @ Wage slave,
    can you give more details on your quench? Are you saying that you introduced gas into the evacuated jacket of magnet’s helium dewar? Why helium? I know it has excellent (the best?) thermal conductivity for a gas, but wouldn’t pretty much any gas do the trick if you bleed in enough of it?

  14. Anonymous says:

    @11 absolutely. I once wiped all of my bank/credit cards by leaving my wallet in my pocket when putting a sample in a 600 MHz magnet. Trying to pay for fuel on the way home was interesting… I learned that it is possible to enter your card number manually.

  15. Kyle Wilson says:

    I was principal software engineer for a company that designed and sold MRI systems for ten years or so. The amount of current (at least in our small ones at 1.5T) wasn’t that huge…about 100 amps if I remember correctly…of course the winding design feeds into this. When life or serious injury is on the line, I was surprised that no one involved in that incident tried to quench the system by compromising the cryogenic integrity of the system. Finding any fitting that would allow air into the cryostat and loosening it (or in a pinch breaking it) should cause a quench pretty quickly…probably voids you warranty, but at that point you’ve got bigger issues…

  16. a.nonymaus says:

    Re: 11
    They will also damage wrist- (and presumably, pocket-) watches, as I have discovered to my annoyance.
    Re: 13
    Helium is used because it won’t be condensed by the liquid helium in the Dewar. So, it breaks the insulation of the vacuum, allowing the helium to rapidly boil off.

  17. Peter Kasting says:

    If you want even more excitement than a loud thump and a helium geyser, try this video for the results of an unexplained pressure buildup in a unit that was apparently being decommissioned:

  18. SBW says:

    For the most spectacular quench of all, read up on the LCH incident on 19 September 2008. The LHC magnets have so much energy stored (about a gigajoule total) that they have active quench detection circuits which route the stored energy to multiple external refrigerator-sized dump resistors, which heat up by a few hundred degrees C in a second or so. The S34 incident used about 200 megajoules to warm those resistors, vaporize a lot of helium, and bend and move a bunch of metal. (The standard equivalent for TNT is 1.9 MJ per pound, so this incident was comparable to the detonation of 100 pounds of TNT.) It was caused by about 200 nanoohms of ‘excess resistance’ in an electrical splice.

  19. SP says:

    The phrase “nuclear magnetic resonance”, though, gave some patients the willies, since they thought it involved nuclear isotopes and radiation, so a less threatening phrase was quickly dreamed up.
    No, the joke is that the technique was pioneered at MGH in Boston. So technicians would come in to patients and say, in their best Boston accent, “I’m here to take yah for yah En Em Ah.” The resulting patient panic required quick invention of a new acronym.

  20. TeddyZ says:

    There are actually high field high temperature supercon magnets. Liquid He is for losers.

  21. milkshake says:

    You don’t really need a new helium-rich gas field. You need He separation unit at the natural gas company. Helium separation is expensive, requiring major investment, and US government ruined the helium market by selling off below market price its giant helium surplus stored in helium strategic reserve (yes, there used to be helium strategic reserve, since WWI, for military balloons). It will take few years but if the price is right companies will start separating helium again

  22. Derek Lowe says:

    #20 Teddy Z – those are pretty neat. But that’s for a value of “high temperature” that still calls for liquid nitrogen. Admittedly, that’s a heck of a lot easier to deal with (practically and financially) than liquid helium. . .

  23. MJ says:

    Strictly speaking, it’s not quite forever for a supercon magnet – the weld spots inside generally impose some sort of geological decay time for the magnet. But quench circuits (“the big red button”) seem to mostly be an MRI thing – it could be that my memory is going at last, but I don’t recall seeing them with NMR systems. NMR spectrometers, after being charged & cryo-shimmed, have a plug installed that locks into the circuit lead at the top of the coil.
    FWIW, the link to the HTS magnets just seem to be using a (pulse tube) cold head for refrigeration, not liquid nitrogen, based on the documents at the site. I am kind of curious as to what their “uniformity” specification of

  24. MJ says:

    Not sure why my previous message didn’t go through all the way –
    I am kind of curious as what their “uniformity” specification of under 1 ppm translates into spectroscopically, e.g., is it a linewidth one would get before or after one would cryoshim a supercon magnet on a suitable standard sample?

  25. Anonymous says:

    Since the topic is NMR/MRI –
    I just had a MRI of my brain and cervical spine, and I could not understand why the MRI machine was so loud. They gave me ear plugs to wear, but during the scan it still seemed like a jack hammer was pounding away 2 inches from my ear. At one point, it sounded like 2 jack hammers, each set at a different pitch. Other times it was like a hammer pounding, or like a police siren. Does anyone know what makes this happen? The machine was so loud, I could hear it outside the building, when I was in the parking lot.
    I’ve been in labs with NMR instruments, and I don’t recall them making any noise at all.
    I must say, 0.5 mg of Xanax takes away nearly all the panic of being in a close space with jack hammers nearby.

  26. gogarden?? says:

    What’s going with Pipeline? As I’m reading, it automatically redirects to either or myspace or something. I barely have time to type a comment. Am I the only one experiencing this?
    It’s with Pipeline only, not other websites.

  27. Gordonjcp says:

    At the local vet school, there’s an NMR big enough to feed a cow through. They had to block off part of the car park because there might be interactions with the field from it.
    One interesting thing is the sign in the security gatehouse for the campus which says (and I’m paraphrasing because I can’t find the photo I took) “Notice to Emergency Services – no matter how bad the situation gets, no-one goes into the Magnet Room without phoning and getting authorisation to quench”

  28. Anonymous BMS Researcher says:

    Chasing pointers from the MRI blog cited in Derek’s first post about the Mumbai incident, I came across this tweet about an issue — of increasing importance for MRI safety — that had not occurred to me:
    Suffice it to say, should I need an MRI this will not be a concern for me!

  29. Anonymous says:

    @20: Neat, especially the MRI models, but the device shown seems a little small for humans as a whole.
    BTW, does anybody know why HTS magnets are not more common in such applications, are the materials more prone to lose superconductivity in a magnetic field? The temperature difference in Kelvins might not be that large but to my knowledge the difference in operating costs and hassle is. At universities LN2 is stored in big outside tanks which are refilled by tanker trucks, at some instiutes rather frequently (IIRC weekly) indicating a considerable consumption and a quite bearable price.

  30. Kyle Wilson says:

    @25 The loud noises when imaging is being done are the result of varying field gradients being established on top of the main field of the magnet to select the bits of tissue to be imaged. Each row of ‘voxels’ involves one gradient set to select the plane in which the imaging will occur followed by a flip to another play-out gradient to select a plane at right angles. The tissue that will emit the RF that results in the final row of voxels/pixels in the image is at the intersection of these two planes and the final resolution along that line is due to the different frequencies at which the nuclei (pretty much only hydrogen for MRI) emit.
    I would guess that for chemical NMR the whole sample is held in a uniform field and excited with RF so there is no change in the field over time and thus no reaction force in the magnet windings to make noises.

  31. Anonymous BMS Researcher says:

    I dunno why they still use helium instead of nitrogen for MRI/NMR, but there must be a good reason since liquid nitrogen is much cheaper and easier to get (the air we breathe is mostly nitrogen, so it’s basically just a matter of cooling and compresding air; thus anybody with the right equipment can make LN2).

  32. Anonymous BMS Researcher says:

    Here’s an explanation of how three gradients are used to image voxels in three spatial dimensions:

  33. gippgig says:

    #29: Making HTS wires that can carry high currents is extremely difficult. One problem is that current flow tends to make magnetic flux lines move, dissipating power as if the wire had resistance.

  34. a-non says:

    @25, @30: I always thought that the noise inside an MRI was due to needing to “spin” the magnets rather than the sample. Most laboratory NMR requires spinning the NMR tube in the magnetic field; whereas, spinning the patient in the magnetic field would likely cause a bigger headache than the “jackhammers”.

  35. Anonymous says:

    why would you want to spin the sample in mri – totally counterproductive

  36. aairfccha says:

    @35: to average out emission anisotropies and get a better NMR spectrum? I don’t know *why* myself, but my father worked at Bruker and the sample capsules he showed me had a spinning mechanism (air turbine at the side of the lid, similar in principle to a dentist drill).

  37. Wage_Slave says:

    @13 – Interesting question.
    Yes it has to be helium – OK it could be hydrogen but we do not want to be playing with a hydrogen cylinder!
    Why helium – well, the central core of the magnet is helium cooled (just over 4K for a 400, 2K for bigger magnets) so every other ‘gas’in normal air is a solid at that temperature. You have to introduce much more air before you actually reduce the vacuum by which time you have lots of solids in the vacuum space considerably increasing the risk of ports getting blocked leading to dangerous over pressure.

  38. Anonymous says:

    I’ve never been more impressed by magnetic fields than when I saw the aftermath of a gas company trying to top off the He a 3T research MRI with a ferromagnetic cylinder…..

  39. a. nonymaus says:

    To add to what #30 said, these gradients are added to the main field by putting current pulses through room-temperature coils within the tube through the big magnet. When you put current pulses through coils in a big magnetic field, what you get is a loudspeaker. Although it does nothing good for the image, you can put arbitrary signals through some models of MRI scanners. Apparently, “Dark Side of the Moon” is a good choice for such an arbitrary signal.

  40. Matt says:

    @25, @30, @34, @39. When you image an object the gradients are activated with extremely fast switching rf pulses. The gradients are nothing but wire coils around the main B0 field (which is nothing but a giant solenoid) that can create their own magnetic fields. When running, the wire begins to feel a Lorentz force (Or Laplace force) acting on it. The force begins to exert torque on the wires and sound that you are actually hearing is the extremely fast vibrating gradient coils moving due to the extremely fast torque forces acting on them. The gradient coils are usually epoxied down but the torque is quite strong.

  41. Anonymous says:

    Put a big red button on something and someone is going to push it, even if there is a huge sign in several different languages explaining the purpose of the button. This was my experience providing analytical support to medicinal chemists for close to 25 years. As some have noted, most magnets are now shielded, except for the bore which is vertical on analytical NMRs and the field strengths are weaker than those found on MRIs. I have heard of people coming in the morning finding things like floor polishers attached to the bottom of a NMR. I have never heard of any injures occurring from analytical NMRs, but people have been killed by MRIs. Recently one person died and another was seriously injured in NJ while trying to remove veterinarian’s office MRI. I think that the blow-out disk was blocked. There are blow-out disks or plugs on the magnets to allow the gases to escape in the case of a quench. Without a good way to release the gasses these instruments become bombs.

  42. AlphaGamma says:

    @36- surely in MRI, anisotropy is what you care about? The precise chemical composition of the patient doesn’t matter, you’re trying to find out where things are!
    The “dentist’s drill” type devices are capsules for MAS-NMR (Magic Angle Spinning), which is the only way to get a decent NMR spectrum of a solid sample. It averages out any orientation-dependent interactions.

  43. Anonymous says:

    @26. gogarden??
    Malware, most likely. Something like “” which is *really* hard to get rid off properly.
    It is about time you learned to use the useful and sadly necessary, Windows(?) function System Restore! Maybe you can see a recently installed application in “My Programs” and apply a restore point sometime before that?
    Anything from “Resoft LLC” is Evil!!

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