Metal chair near MRI machine

http://www.simplyphysics.com/flying_objects/chair_4.html

by MikeCapone • 16 years ago

115 67 16 years ago

lutorm2• 16 years ago

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

jarin2• 16 years ago

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.

retube2• 16 years ago

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

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

mbateman2• 16 years ago

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.

india• 16 years ago

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

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

lutorm2• 16 years ago

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... ;-)

glabifrons2• 16 years ago

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.

mbateman2• 16 years ago

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.

ars5• 16 years ago

I'm quite sure you are wrong.

You can use a fiberglass, or aluminum stretcher.

URSpider94• 16 years ago

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.

js2• 16 years ago

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...

anthonyb2• 16 years ago

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...

jafl5272• 16 years ago

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

anthonyb2• 16 years ago

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.

joezydeco• 16 years ago

Here's a steel bottle getting sucked in

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

thunk• 16 years ago

nom nom nom

Edit: Oooh, yeah. Make it hurt.

naz• 16 years ago

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

gridspy2• 16 years ago

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

ars5• 16 years ago

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

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

derefr• 16 years ago

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.)

gridspy2• 16 years ago

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

Zak• 16 years ago

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.

bootload2• 16 years ago

"... 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?

ars5• 16 years ago

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.

LogicHoleFlaw• 16 years ago

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.

tocomment3• 16 years ago

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.

carterschonwald• 16 years ago

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

rbanffy• 16 years ago

Time machines, of course.

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

tocomment3• 16 years ago

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.

electromagnetic• 16 years ago

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

cheald• 16 years ago

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.

jarin2• 16 years ago

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!

zweben• 16 years ago

They should buy some plastic chairs.

aidenn0• 16 years ago

by "chair" they meant wheel-chair

jcl• 16 years ago

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

http://merlexi.com/mri.htm

kree10• 16 years ago

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.

mkramlich3• 16 years ago

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

mkramlich3• 16 years ago

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

kressler2• 16 years ago

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.

cgomez• 16 years ago

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

enntwo• 16 years ago

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.

techiferous• 16 years ago

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...

whatwhatwhat• 16 years ago

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

Confusion2• 16 years ago

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 :)

retube2• 16 years ago

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...

Confusion2• 16 years ago

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.

Rufus2• 16 years ago

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.

blahblahblah2• 16 years ago

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.

Rufus2• 16 years ago

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.

Metal chair near MRI machine

Related Stories