
Metal chair near MRI machine - MikeCapone
http://www.simplyphysics.com/flying_objects/chair_4.html
======
Confusion
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 :)

~~~
retube
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...](http://public.web.cern.ch/public/en/spotlight/SpotlightATLAS-
en.html)

~~~
Confusion
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.

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

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

~~~
bootload
_"... 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?

~~~
LogicHoleFlaw
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.

~~~
fr0sty
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'.

------
anthonyb
Lots of fun pictures on that site, eg.
[http://www.simplyphysics.com/flying_objects/FloorPolisher.JP...](http://www.simplyphysics.com/flying_objects/FloorPolisher.JPG)

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

~~~
jafl5272
[http://www.simplyphysics.com/flying_objects/acetylenetorch.h...](http://www.simplyphysics.com/flying_objects/acetylenetorch.html)

~~~
anthonyb
and
[http://www.simplyphysics.com/flying_objects/Chair_Removal.fl...](http://www.simplyphysics.com/flying_objects/Chair_Removal.flv)
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.

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

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

~~~
glabifrons
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.

~~~
mbateman
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.

~~~
ars
> 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.

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

~~~
ars
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...](http://www.exo.net/~pauld/activities/magnetism/forcebetweenmagnets.html)

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

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

------
zweben
They should buy some plastic chairs.

~~~
aidenn0
by "chair" they meant wheel-chair

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

<http://merlexi.com/mri.htm>

------
techiferous
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...](http://cgi.ebay.com/350-lb-Rare-Earth-Neodymium-Magnet-INDUSTRIAL-
FORCE-
New-/220615966201?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item335dbc7df9#ht_3753wt_1242)

------
Rufus
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...](http://mrimetaldetector.com/blog/2010/02/mri-projectile-
accidents-one-exemplar/) and the other posts.

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

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

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

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

~~~
mkramlich
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. ;)

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

~~~
tocomment
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.

~~~
tocomment
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?

~~~
hugh3
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.

------
thunk
nom nom nom

Edit: Oooh, yeah. Make it hurt.

