Why do these metal objects bounce around in an MRI?

[Machine_Elf]

Video here of some people playing with the magnetic fields in an MRI machine. At t=4 seconds, he throws a steel shackle into the tube, and it bounces around violently before coming to a stop. At t=13 seconds, the same thing happens to a stapler.

Question: why do these things bounce around so violently? In all my tinkering with magnets and metal, the two items clap together once and that’s it. What’s going on in there?
Noteworthy (but not a question): later in the video they dangle an office chair in the magnetic field, and observe over 2000 pounds of pull. :eek:

[beowulff]

The magnetic field in the MRI is mostly Axial. So, when an object is thrown into it, it gets accelerated rapidly down the length of the tube, overshoots, then gets accelerated back the way it came.
Since it was probably not thrown exactly down the centerline, it bounces around until it comes to rest on a wall, someplace where the field diverges from perfectly parallel.

[Tim_T-Bonham.net]

Also, the magnetic field is changing constantly & rapidly inside an MRI (that’s what it needs to do to produce the images), so that would toss the metal objects around.

[Francis_Vaughan]

Only when actually imaging are the gradient coils energised and the magnetic field changing - and it changes only by a very tiny amount. The main field is so dominant that the gradient field is imperceptible in comparison.

That is a really serious magnet. 4 Tesla means it is intended for imaging phosphorus as well as hydrogen. That isn’t your usual MRI machine.

[Chronos]

Whoa, these folks are throwing around staplers, in a 4 T magnet? They’re braver than I, that’s for sure.

[beowulff]

Recent MRI fatality.

Note that the article says that the victim was assured that the machine was “turned off.” This shows a surprising level of ignorance about how an MRI works. In normal operation, the main (superconducting) magnet is never turned off.
To do so, and then to have to re-start it is a big deal, and so it is avoided unless the power fails, or the machine needs service.
So, an MRI can be assumed to be never safe to have ferrous metals near it.

[Napier]

There is, or at least was, a rule of thumb that says MRI machines cost about a million dollars per tesla. They’re throwing projectiles around in the heart of a four million dollar machine, it appears. I wonder why this is OK? Maybe the inner lining of the magnet was going to be replaced anyway?

[scr4]

@Napier

There is, or at least was, a rule of thumb that says MRI machines cost about a million dollars per tesla. They’re throwing projectiles around in the heart of a four million dollar machine, it appears. I wonder why this is OK? Maybe the inner lining of the magnet was going to be replaced anyway?

The description points to this video (in which they do something even more destructive), where they say:

The magnet was a decade old and no longer used for research. Weighing four tons and requiring expensive rigging to remove/ship, and with a scrap value only in the tens of thousands (there’s not much of a market for old 4 T magnets), it was better for us to convert it into a “mock scanner,” for practice functional MRI sessions. So the specialty equipment is still in use, just not as a scanner.