(NY) MRI 'Disarms' Police officer

DocH

Thanks for the easy to informative, yet easy understand post on MRI technology!

I continue to be amazed at the real world variety and expertise present in the TFL membership.
 
ok...what am I missing? BB's and bullets are composed of copper and/or lead, right? As far as I know neither are magnetic...does the magnetic feild in a stronger latitude sudeenly make non-magnetic items magnetic?
 
The BBs from your Daisy Red Rider are copper plated STEEL. They bounce off of things like crazy, and are certainly magnetic.

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Get your 1911s and AR15s while you still can!
 
BB's are actually steel, IIRC. Some varieties of bullets have soft steel jacketing. After 6 weeks as a rule of thumb, even steel shrapnel is sufficiently scarred down to be ok in the magnet, with several caveats:

1) Intracranial metal from an unknown source (i.e. shrapnel) is a no-no. Brain doesn't form real scar tissue like the rest of the body, so a small piece of ferrous metal in the brain may twist around and wreck something important, like all your memories of 1972. :)

2) In or around the eye. The vitreous humor (the jelly inside the eye) and the retina are delicate, and any twisting could be catastrophic to vision. (Though lucrative to plaintiff's counsel)

3) Less critical is the artifact produced by ANY metal, not just ferrous. A tiny piece of copper jacket in the skin of your forehead, for example, might be perfectly safe for you to be in the magnet, but will cause an artifact that obscures part of the front half of your brain, depending on the magnet strength and the imaging sequences the tech plugs into the machine. This can make your very expensive diagnostic brain MR study less useful and precise.

HTH
 
Okay.. I think I learned more in this post than I did my Physics I class.. :D

At any rate.. what I want to know is:
"The weapon is out of service. Benwitz said firing the weapon was too risky because its molecular structure might have been altered."

[sarcasm]seriously?[/sarcasm]

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God, Guns and Guts made this country a great country!

oberkommando sez:
"We lost the first and third and now they are after the Second!(no pun intended)"
 
Weeeeellllll, I don't know about that...

Maybe, if the internal parts are themselves now magnetized. They might not function as intended as they now try to rearrange themselves along their new magnetic lines of force. Imagine a little box full of small odd-shaped magnets; now imagine the little odd shapes are a pistol sear, disconnector, striker/firing pin, etc. The gun might fire, but the firing pin might be magnetically attracted to the barrel, and remain protruding...Something like that. But certainly the metal would not be "more brittle". That sounds like B.S. to me.

[This message has been edited by DocH (edited September 18, 2000).]
 
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by KaMaKaZe:
At any rate.. what I want to know is:
"The weapon is out of service. Benwitz said firing the weapon was too risky because its molecular structure might have been altered."
[/quote]

I've been thinking about this one on and off all day now. I need to run this by DocH, but here goes my two (sort of combined) theories. (DocH, is the field in an MRI DC or does it alternate, that is, is the field constant or is it constantly collapsing and regenerating?)

The magnetic field was so powerful that it induced electric fields in the gun, causing eddy currents, these eddy currents disappating energy as heat, leading to heating of the metal to such a point that the temperature of the metal in the gun was high enough to change the preferred crystal structure (heat treatment, if you will) of the metal to a less desirable state.

My secondary theory, that I throw out here simply as a backup (I don't even like it myself), is that the magenetic field was so strong that the hysteresis retained magnetic field in the metal of the gun is strong enough to impede functioning.

The combined theory is that the heating from eddy currents and the magnetic hysteresis damaged the structure of the metal.

Maybe not so much to be dangerous, but certainly the risk is enough that you should not trust the gun anymore until it is properly tested/examined.
 
"Damnit Jim, I'm a country radiologist, not a materials scientist!"

Dizz: The "main magnet" is a constant field strength. However, in order to obtain images, an additional gradient field is applied by smaller, non-superconducting magnets in the x,y, and z planes. Protons spin at a frequency dependent on the magnetic field strength (the Larmor frequency fL; fL=(y/2pi)Bo where y/2pi is the gyromagnetic ratio of the isotope and Bo is the field strength) and the gradients are used to locate the signal from the protons in 3-dimensional space, thus allowing us to generate an image. Anyways....these smaller gradient fields are rapidly switched on and off during an exam. However, they are relatively weak compared to the main magnet.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>The magnetic field was so powerful that it induced electric fields in the gun, causing eddy currents, these eddy currents disappating energy as heat, leading to heating of the metal to such a point that the temperature of the metal in the gun was high enough to change the preferred crystal structure (heat treatment, if you will) of the metal to a less desirable state.[/quote]

I doubt the heating (if any) would have reached the hundreds of degrees necessary to alter the crystal structure of the steel/alloy in the gun. As I said before, the heating is a real phenomenon for retained orthopedic hardware, but not of any consequence in actual pratice. I think local heating effects are on the order of 1 degree C or less per cubic centimeter of tissue. I think that is insufficient to alter the structure of a metal object.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>My secondary theory, that I throw out here simply as a backup (I don't even like it myself), is that the magenetic field was so strong that the hysteresis retained magnetic field in the metal of the gun is strong enough to impede functioning. [/quote]

Actually, I think that this is the more likely (theoretical) possibility. In actuality, who knows without examining the firearm.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Maybe not so much to be dangerous, but certainly the risk is enough that you should not trust the gun anymore until it is properly tested/examined.[/quote]

On second thought, I agree. Prudence would not dictate otherwise.
 
From my work with magnetic particle testing machines, I would think that any residual magnetism could be easily removed. It seems to me that whomever made the statement about the gun being unsafe and thereby the implication that the gun would never be safe to fire again, was just uniformed about what really would happen to the weapon. I agree that the "molecular" properties of the gun are unaffected.
 
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