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Metal chair near MRI machine

115 points| MikeCapone | 16 years ago |simplyphysics.com | reply

67 comments

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[+] Confusion|16 years ago|reply
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 :)
[+] ars|16 years ago|reply
A gun flew into a machine and dischared with the safety still on!

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

[+] gridspy|16 years ago|reply
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

[+] derefr|16 years ago|reply
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.)
[+] bootload|16 years ago|reply
"... 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?

[+] lutorm|16 years ago|reply
I didn't realize they were on all the time.
[+] mbateman|16 years ago|reply
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.
[+] jarin|16 years ago|reply
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.
[+] glabifrons|16 years ago|reply
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.
[+] naz|16 years ago|reply
It is refreshing to see such a candid writeup emerge from such a normally lawsuit-fearing industry.
[+] gridspy|16 years ago|reply
I guess it helps that no patients were injured in the accident.
[+] zweben|16 years ago|reply
They should buy some plastic chairs.
[+] aidenn0|16 years ago|reply
by "chair" they meant wheel-chair
[+] Rufus|16 years ago|reply
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.

[+] blahblahblah|16 years ago|reply
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.

[+] kree10|16 years ago|reply
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.

[+] mkramlich|16 years ago|reply
... they had to wipe the PC off the MRI? Or wipe the IT staff off the MRI/PC?
[+] Rufus|16 years ago|reply
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.

[+] whatwhatwhat|16 years ago|reply
This is a great link... My father is a Radiology Engineer and I will defiitely be sending this his way.
[+] mkramlich|16 years ago|reply
I hope the MRI's don't run Windows. Then they'd have two problems.
[+] kressler|16 years ago|reply
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.
[+] tocomment|16 years ago|reply
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|reply
what sort of devices are you thinking about? Pocket fusion reactors and other sorts of plasma bottling tech?
[+] electromagnetic|16 years ago|reply
We'll have a thousand new warning labels on devices like "People with piercings must stand clear" on their cellphone.
[+] jarin|16 years ago|reply
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!
[+] cheald|16 years ago|reply
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.
[+] thunk|16 years ago|reply
nom nom nom

Edit: Oooh, yeah. Make it hurt.