Metal chair near MRI machine | Hacker News Reader

Metal chair near MRI machine

simplyphysics.com

115 pointsMikeCapone67 comments

I've had this kind of thing happen to me when I was graduating in physics. I was working with a liquid helium based refrigerator that had a superconducting magnet inside it, pretty much the same technology used in MRI. The setup could reach fields up to 13T and the old CRT display that accompanied the setup would visibly skew above ~6T (it was pretty close to the magnets). I was sitting on a small wheeled stool of which two varieties floated around the labs: one with metallic wheels (red) and one for use near these setups (blue). Unfortunately, one of the wrong ones had found its way into this lab, but do you normally pay attention to the color of stools? I sat down, probably around a meter from the setup, cranked up the magnetic field, stood up to adjust something and BANG, the stool collided with the fridge. Luckily nothing was damaged, but the reason for these two types of stools was suddenly very clear to me :)

13 Tesla? That sounds a lot. would make it just about one of the most powerful magnets in the world. I would have thought 6T would vaporise a TV set, not just distort the image.

E.g the ATLAS magnet is 4T and requires 21,000 amps to power it.... http://public.web.cern.ch/public/en/spotlight/SpotlightATLAS...

In 2001 it would have been a world record, but by 2005 those kinds of superconducting magnets had become pretty regular pieces of equipment, at least for the kind of nanophysics lab I was in.

You should realize it creates that field only inside a small space with a diameter of a few cm. The entire thing is suspended in liquid helium, including the samples on which we did measurements. I believe we estimated that, at a meter from the magnet, the field had dropped to something like 3T. I'm not sure how much power it drew, but I seem to recall it was plugged into a regular socket.

A gun flew into a machine and dischared with the safety still on!

http://www.simplyphysics.com/flying_objects/GuninMagnet.html

Now that's something I'd like to see on MythBusters. (Though not necessarily with an MRI machine—those things cost more than their entire budget. A large ferromagnetic coil would do nicely.)

Awesome explanation of the physics of why the gun discharged here (http://www.ajronline.org/cgi/content/full/178/5/1092)

Short summary:

- A round was chambered in a safety locked gun.

- The "Sear" at the top of the gun that normally moves when the gun is fired was locked, and the empty cartridge was still in the gun, confirming that the gun was physically locked

- The pin was pulled into the unlocked firing position by the magnetic field

- The impact of the gun when it hit the CT caused the firing pin to move against the spring and hit the back of the loaded bullet, causing it to fire. There is white paint on the front of the gun where the gun hit the CT confirming the impact point

Good summary, but it doesn't quite make it clear what safety device failed. The firing pin block exists to ensure that an impact in-line with the firing pin cannot move it when the trigger is not depressed. It too could be moved by inertia, but its movement is perpendicular to the firing pin itself, so an impact cannot move both at once. In this case, the magnetic field moved the firing pin block, while inertia pushed the firing pin forward relative to the rest of the gun.

This is the only time I've ever heard of a firing pin block failure resulting in an accidental discharge.

> This is the only time I've ever heard of a firing pin block failure resulting in an accidental discharge.

The is the only time I heard of a gun encountering an extremely powerful magnetic field.

"... At the time the weapon discharged, it was reportedly in a cocked and locked position; that is, the hammer was cocked and the thumb safety was engaged to prevent the hammer from striking the firing pin. A live round was in the chamber ..."

Round chambered, weapons discipline, not! It was reported the safety was on. I wonder if it wasn't?

Please read the actual article, it answers all those questions.

They proved that the safety was on because the empty shell was still in the chamber, since the safety prevented the slide from moving and ejecting it.

"... They proved that the safety was on because the empty shell was still in the chamber ..."

Is this the first time a round has been discharged with safety on?

No, it's pretty standard to have a round chambered with the safety on. For some models having the hammer cocked is also correct general carrying procedure, depending on the firing mechanism the particular gun uses. I do still find it worrying that a firearm would discharge in this manner. Most have multiple internal safeties to prevent accidental discharge during things like a drop from a height.

I am not familiar enough with the details of the Colt firing pin safety, but I believe 'dropped from a height' is not necessarily equivalent to 'pulled from 3ft into a 1.5T magnet'.

Lots of fun pictures on that site, eg. http://www.simplyphysics.com/flying_objects/FloorPolisher.JP...

Which looks like it might be a bit harder to remove...

http://www.simplyphysics.com/flying_objects/acetylenetorch.h...

and http://www.simplyphysics.com/flying_objects/Chair_Removal.fl... to give you some idea of how hard they "suck" - that's two guys (one with a lever) plus some sort of strapping so that people in another room can pull too.

Takes an iron man to do it. He is in the other room ;-)

Here's a steel bottle getting sucked in

http://www.youtube.com/watch?v=7g5UVrOt2CI

I didn't realize they were on all the time.

You know how it takes months to power up and down the Large Hadron Collider? MRI machines are kind of like a mini version of that.

and it takes weeks to power them down too, else you get the dreaded quench...

http://en.wikipedia.org/wiki/Superconducting_magnet#Magnet_q...

Yes, you can't even turn them off by unplugging them, as it were. A certain kind of emergency shutdown dumps the helium needed to allow superconductivity (and thus maintain the magnetic field). It costs like $20,000 to do this and is quite dangerous in itself.

It's called MRI quenching and dangerous it can be[1].

[1] http://www.youtube.com/watch?v=1R7KsfosV-o

Modern high-field superconducting MRI magnets can hold tens of megajoules of stored magnetic field energy, equivalent to the explosive energy in several kilograms of TNT.

Yeah, I should have thought this through. I knew you kept them cool, and with a bit of thinking it should have been clear that you don't turn a superconducting coil off, since nothing's powering it... ;-)

They're not. They're only magnetic when they are in operation. I've been in them, I've been with people who've been in them, and my family is in the medical field. If they were fully energized all the time, you wouldn't be able to bring people in and out of the room on a stretcher.

I'm not an MRI tech but I work on MRI image analysis academically, so I know a bit more than usual by hearsay and I see them used all the time. Why are there "MRI magnet is always on" signs in the medical imaging areas of the med school here? Are you sure they don't use titanium stretchers near the field? Why do they perpetually supply the MRI with liquid helium?

My understanding is that even when they are not used there is a field of between 1 and 3 teslas, depending on the scanner. (A quick google search for things like "mri magnet is always on" and "mri safety" and such seems to confirm this.)

Edit: Also, you can get closish to the scanner with a ferromagnetic object without a problem. The field doesn't encompass the whole room or anything. 10 or 15 feet away will be fine.

I apologize, I should clarify my point.

They're not magnetic enough to pull a chair off the floor or things across the room (as the story indicates) when they are not in operation. You can walk around the room with your watch on (for example), but if they fire it up and you're in there with it, it's destroyed.

Just look how close they have to get the steel oxygen bottle to the center (in the video linked above) before it moves. It's almost inside it already.

There are low field scanners that use conventional electromagnets, which aren't always on. However, the scanner shown is a 1.5 Tesla (T) or 3T scanner (difficult to say which from just the housing), and the main magnetic field is always on. The main field is supplied by a superconducting ring immersed in liquid helium. As a previous poster noted, the only way to shut one down is to dump the helium, and the cost to bring the scanner back up is in the tens of thousands of dollars. There are smaller electromagnets involved to apply field gradients across the bore of the scanner (on the order of 50 mT/m) which are off when the scanner is not operating, but these are quite small compared to the main field.

My Ph.D. dissertation was on data acquisition and reconstruction techniques for MRI. I've logged hundreds of hours operating high field scanners like the one shown, and dozens of hours being scanned for various research studies. I've also (carefully) hauled a variety of strange things in and out of scanner rooms. Most of the tools we used were non-ferrous, and we had to be extremely careful with the few ferrous pieces of equipment we had to use. The magnetic field does drop off rapidly as you move away from the scanner, so objects more than 10 feet or so away are unlikely to be pulled in.

Sorry, but you're wrong. The main magnetic field is always on. It's produced by superconducting rings that are "ramped up" when the magnet is initially installed and remains on at all times, even when the operator console is powered off. Basically, the ramp up procedure sets up a current loop in the superconducting coil via induction in a controlled manner. After the initial ramp up, the field can only be shut off by bringing in an engineer and special equipment to perform a ramp down procedure, the operator executing an emergency quench, or an accidental quench occuring due to a failure of the refrigeration system. Basically, a quench happens if the temperature of the superconductive rings exceeds the temperature threshold required to maintain superconductivity, the resistivity of the ring material becomes non-zero and runaway heating occurs because of the high current in the ring (lots of amperes), the runaway heating of the ring causes the surrounding liquid helium to go supercritical and rapid (and potentially explosive) boil off occurs.

The electromagnets which are only turned on during operation (used for the gradient fields and for shimming the main field) are insignificant in magnitude compared with the main field. The main field is powerful enough to lift a ferromagnetic chair off the floor if the chair gets close enough to the bore without the gradients being active.

> The field doesn't encompass the whole room or anything.

Well, technically it encompasses the entire universe - or least the part that is in the light cone since the field was energized. :)

But magnetic fields drop off by distance^3 or ^5, so the strength drops rapidly with distance.

What do you mean ^3 or ^5? Is there some funky physics I'm misunderstanding here?

Because magnets have two poles, the force drops off by varying amounts, depending on the shape and size of the magnet. (The closer pole attracts, while the farther one repels. The other magnet also has two poles, which makes things even more complicated. For example in a short magnet the second pole has a greater influence than it does in a long one.)

It also depends on if you are interacting with another magnet, or with unmagnetized iron. ^3 or ^5 is just an approximation - it can go to ^7, and it's not a definite number, it varies.

See: http://www.exo.net/~pauld/activities/magnetism/forcebetweenm...

>> The field doesn't encompass the whole room or anything.

>Well, technically it encompasses the entire universe - or least the part that is in the light cone since the field was energized. :)

As permittivity is generally a function of frequency doesn't this mean that the progress of a magnetic field through space can differ to that of a [theoretical] light cone (which bounds the volume of causal connectedness)?

Magnetism gives me headaches.

I'm quite sure you are wrong.

You can use a fiberglass, or aluminum stretcher.

Sorry, but you're (mostly) incorrect. The field for all modern MRI machines is provided by a superconducting magnet, this magnet is "on" all the time, by virtue of the superconducting ring current. The additional fields applied during analysis are inconsequential.

Where you are right is that the fields decay quite quickly (exponentially, in fact) as you move away from the magnet. For the metal chair in the picture to be "sucked in" to the field, I would assume that someone would have had to basically insert it into the bore.

Are you sure about the exponential decay? My first guess would be something closer to inverse quadratic, but that's probably wrong.

It's 1/r^3 for a dipole magnet.

Equally in all directions?

Depends on the MRI magnet type apparently, but with modern superconducting magnets, yes:

http://health.howstuffworks.com/mri.htm/printable

http://en.wikipedia.org/wiki/Superconducting_magnet#Persiste...

It is refreshing to see such a candid writeup emerge from such a normally lawsuit-fearing industry.

I guess it helps that no patients were injured in the accident.

They should buy some plastic chairs.

by "chair" they meant wheel-chair

Perhaps they should buy some plastic "chairs", then. :)

http://merlexi.com/mri.htm

Reminds me of neodymium magnets, which can be so strong that you have to plan your route through a room when transporting them:

http://www.wiredpakistan.com/forums/viewtopic.php?id=4895

http://cgi.ebay.com/350-lb-Rare-Earth-Neodymium-Magnet-INDUS...

The dueling speculation with regard to how close (or far) you need to be from an MRI scanner ignores a few points. First, the attractive (translational) effect is not strictly a result of the strength of the magnet, it is a product of the magnet strength and the magnetic spatial gradient (or, by analogy, the 'steepness' of the magnetic field).

Contemporary MRI scanners use what's referred to as 'active magnetic shielding' which means that the magnetic field is 'girdled' and held closer to the scanner than it would be if we just let it follow the cube-of-the-distance drop-off rate. This increases the 'steepness' of the magnetic field, but pulls it closer to the body of the instrument.

One of the major factors regarding attractive force is the object that is being pulled. The longer it is (not so much the 'bigness' but the length), the greater the potential pull. The attractive force is a result of the difference between the magnetic field as experienced by one end of an object and the field at the other end. The greater the difference (a product of the steepness of the field and length of the object), the stronger the attractive force.

So, theoretically you could take a 2D ferromagnetic filament and, if you turned it so that it didn't cross any of the magnetic flux lines, the attractive force would = 0. Keeping the center in the same location but rotating the filament so that it crossed flux lines, presto, attractive force!

If you're looking for more information about magnetic projectile accidents, I suggest you check out http://mrimetaldetector.com/blog/2010/02/mri-projectile-acci... and the other posts.

Rufus knows what he's talking about. A conventional 1.5T or 3T clinical scanner with active shielding makes the danger zone of the fringe field more compact, but also makes the transition zone between where a ferromagnetic object experiences little force and where it experiences intense force that makes it a dangerous projectile quite small in space (and therefore quite difficult for a human being to predict). If you've got a ferromagnetic object within the 5 gauss line on a modern clinical system, you are playing a dangerous game of chicken. On a system without active shielding, that transition zone is more gradual.

The quoted field strength of a magnet can be very deceptive. That number indicates the strength of the field at the magnet's isocenter. It says absolutely nothing about the characteristics of the fringe field. There are small 9T animal magnets that you can work on with a steel crescent wrench and 1.5T magnets that you wouldn't dare to even consider walking in the room with ferromagnetic tools.

Reminds me of a story I read years ago (can't find a working link) of how the IT staff at a hospital would routinely wipe hard drives by just taking them into the MRI room.

Once, one of them decided to save time by not removing the drive from the PC first, and you can probably guess the rest.

... they had to wipe the PC off the MRI? Or wipe the IT staff off the MRI/PC?

BTW: A few companies that make MR spectrometers for laboratory research sell 900 MHz systems, which are = 21 Tesla (and change).

People MRI's are a whole lot weaker than many research magnets out there, today.

This is a great link... My father is a Radiology Engineer and I will defiitely be sending this his way.

I hope the MRI's don't run Windows. Then they'd have two problems.

Some of them do run Windows on the console the operator the uses. Some run Linux, and some of the older generation scanners used SGI workstations. They run various RTOS's on the systems that actually control the scanner.

I thoroughly enjoyed your reply as it offered useful information while also making the original commenter look silly.

I did not enjoy your reply as it did not add anything useful, and overlooked the fact that the comment you considered silly was intended as a joke. Dismissing something funny as silly is a bit like dismissing water as being moist. ;)

On a Windows/Hospital related note, I heard this antecdote in a class, any confirmation if true or source? Seems very plausible and likely.

During an operation, one of the OR monitoring devices went down because it was running on Windows and was neglected for some period of time, causing Windows to do an automatic reboot for a "Critical Windows Update."

Of course, seeing a necessary piece of OR equipment go down due to a Windows setting didn't make anyone happy, so the response was to remove automatic update from all of the Windows boxes in the Hospital. Not just mission critical ones, all of them: Secratary's machies, Nurse's machines, IT machines, etc. Fast forwood some months/years, no manual updates were done either, and with automatic ones turned off, a virus came across through a worker's computer and wiped out everything.

True or not, I still think it is a good story for how not to handle security issues.

It's weird to think that in 20 to 30 years well have magnetic fields this strong in every day devices. I wonder how we will deal with issues like this.

what sort of devices are you thinking about? Pocket fusion reactors and other sorts of plasma bottling tech?

Time machines, of course.

http://en.wikipedia.org/wiki/John_Titor#Time_machine

How about a portable MRI for starters? You could possibly use it a brain computer interface. You could also perhaps use transcranial magnetic stimulation to send data into your brain? (Just some wild thoughts).

On the more mundane side, you could use the diamagnetic effect of high magnetic fields to deflect water from windshields, keep snow off your driveway, give your tires traction on ice. You could even hover over water.

I'm sure there are lots of other applications I'm overlooking.

Why all the down votes? If these ideas are flawed why not explain it? How are ideas for the application of magnetic fields not in the spirit of hn?

I think the downvotes are for the unsupported assertion that in 20-30 years we'll have magnets this strong in everyday devices. This seems unlikely, for various reasons.

We'll have a thousand new warning labels on devices like "People with piercings must stand clear" on their cellphone.

I don't think that we're ever going to see supermagnets in common consumer devices, just because they can be so ridiculously destructive. The layman tends to think of magnets as fun toys, but the high-end ones are exceptionally dangerous. Walking around with a neodymium magnet in your pocket would get someone killed before the day was out.

Of course! As the trend toward faster processors, more storage, and better battery life in personal electronics begins to reach its limits in the next few decades, a new trend toward destructively powerful superconducting magnets will naturally arise. Take that, Kurzweil!

nom nom nom

Edit: Oooh, yeah. Make it hurt.

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