On Fri, 27 Jan 2006 11:07:58 -0700, Charles Spitzer wrote:

"Steve B" wrote in message
news:kctCf.14046$JT.13348@fed1read06...

One of the tekkies said that the magnets caused metal to spin or be pulled
out of the body.

The tekkie in question should stick to pushing buttons, because he's not
qualified to describe the physics involved.

He demonstrated how powerful it was by giving me a metal
object.

The force on an object, at a given distance, is proportional to it's
mass. First, he shouldn't be bringing ferrous objects into the scan
room, ever, for any reason. Second, he shouldn't be telling people that
metal could "spin or be pulled out of the body" because it's complete
and utter horse****.

I held it up at arms length about ten feet from the MRI, and it
felt like I was trying to twist a very stuck door knob.

Well, sure. It aligns with the flux lines of the field. Just like a
tiny little metal sliver...only thousands of times more forcefully.

my silver alloy wedding ring jumped around when i was in the mri tube last,
but the Ti plate in my neck didn't seem to move around, although they were
worried about it some when i told them i had one.

That is another example of a scan tech who doesn't understand their
technology. Titaniaum hand tools are what we use when _working on/in_
the magnets. I'm curious about your ring - does a magnet stick to it?
Could the moving around have been, instead, a _reluctance_ to move
in the field? A subtle difference.

One fun trick with an MRI magnet - get an empty soda can. Hold it up
right on two fingertips, and let it tip. If you're tipping the length
of the bore, it'll take maybe 5 seconds to tip over. If you go across
the bore, it'll fall over normally. The magnetic field is inducing eddy
currents in the can if it falls in a way that changes which flux lines
it's cutting (the long way) which gives you that resistance to movement.
Same effect with a coin on edge, tipped over. LOTS of force
(resistance to movement) on a 2'x2' aluminum door on the test systems -
those don't get a full screen room, you just enclose the bore on both
ends. The trapdoor, when the magnet is down, takes a good 5-10 seconds
to open, and it doesn't matter much how hard you pull on the sucker, it
moves at it's own speed. Really fun when you open the door on a magnet
that's not at field, you nearly fall on your ass because there's not the
resistance you expect...