go figger

Martin H. Eastburn wrote:


Thank you, Sir.
richard
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YEP! They covered that on Mythbusters just the other night. The "Exploding Tattoo". :-)
--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
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Why did my friend get his body wanded and picked through - finding all before ?
Maybe the term isn't MRI - might be a different function.
Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder
Dave Hinz wrote:

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On Fri, 27 Jan 2006 21:32:18 -0600, Martin H. Eastburn

If it's big enough, yeah, it's a concern. But slivers? They're not going to spin, they're not going to "catch fire". They'll cause image artifacts, sure.

No, probably was MRI. Metal isn't desirable, but it won't catch fire or throw you across the room.
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I have had several MRIs done over the years. Last one was last week, and was the 3 Tesla variety, the latest and strongest MRI.
As with all, I checked the boxes where they asked if I welded or worked with metal.
They were very concerned over my heart stent, but went ahead with the MRI when they saw that the stent did not appear on a chest x ray, meaning it had been removed during my heart surgery, or it was not metal.
But, I figured they would do x rays of my head because of the welding/metalworking history. They didn't.
One of the tekkies said that the magnets caused metal to spin or be pulled out of the body. He demonstrated how powerful it was by giving me a metal object. 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.
Anyway, that's my MRI story. I go on the 8th for results. It was a brain scan, and I hope they find some functioning tissue.
Steve
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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.
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On Fri, 27 Jan 2006 11:07:58 -0700, Charles Spitzer

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

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 horseshit.

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

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...
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well, i'm not sure it's pure Ti, nor the screws used to hold it to the bone. on an xray, the screws look like 1" fine thread drywall screws.
no, a magnet, even a super magnet doesn't stick to the ring. i thought it was reluctance too, as it seemed to try to lineup a certain way in the tube. it almost had enough force to move my finger if i relaxed my hand. do the force lines move in an mri? i wondered if they'd generate eddy currents in the ring at the time.
a neat trick: suspend a large horseshoe magnet on a string centered over an Al plate that is on a low friction turntable. rotating the magnet will cause the Al plate to rotate.
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On Fri, 27 Jan 2006 12:07:42 -0700, Charles Spitzer

I can't speak for your surgeons, of course, but the whole point of going to titanium was to provide MRI-compatible implants. Even stainless normally thought of as non-magnetic is magnetic at 1.5 or 3T.

Yup.
Well, there's the static field, which is very very (to a few parts per billion) even within the imaging area. Then there's the dynamic (Gradient) fields, in the x,y, and z directions. Those are at mostly audio frequencies, which is why the scans are as noisy as they are.
So. Ring on a finger, dynamic magnetic field, inducing current into...

I can see how that could create _torque_, if the ring was at an angle to the flux lines. Hm. If I still worked there, I'd do a little experiment. Ah well.

Sounds like the same eddy current effect I was describing with the coin, can, and aluminum plate, yup. If that was more portable, it'd be a great bar-bet...
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A really neat show is to slide a supermagnet down an aluminum, or any non-magnetic metal, and see how it creeps down. Dropping a magnet down an aluminum tube is neat too. If you slide a washer down the aluminum first it really shows the difference. ERS
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Actually - all elements are responding to the field. You are flipping electrons in the outer shell. Add enough energy and you might get 'light'! when an electron is ripped off. People don't work well in that level for very long.
Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder
Dave Hinz wrote:

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On Fri, 27 Jan 2006 21:45:44 -0600, Martin H. Eastburn

Close. You're aligning any nucleii which have a magnetic moment (hydrogen is a great one because it's just one proton) with the field. An RF pulse then tweaks _that_, which then cause an electron to jump to the next energy level and back down, giving off that energy in a predictable way. You're not ionizing anything, you're just bumping the leectron up a shell and then back donw.
No electrons are being ripped off, though, it's not that kind of a pulse.
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Come on you guys - set up a magnetic field. Hold a sheet of metal or better a ring. If the field changes - as in frequency or intensity or direction - flux lines will be cut and current will pass. A ring will have a short current. It will solenoid off the finger or just get hot.
It is the things that transformers are made from. It is door bell theory... Martin
Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder
Charles Spitzer wrote:

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Charles Spitzer wrote: ...

Ah - that's the phenomenon used in mechanical speedometers. A magnet rotated by the cable, an aluminum disk fastened to the needle, & restrained by a spring.
Bob
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Better to be danged than dangled
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