Diamagnetism and MRI

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Jorpho
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Diamagnetism and MRI

Postby Jorpho » Tue Oct 20, 2015 1:37 pm UTC

I stumbled upon this video, in which a man demonstrates diamagnetism by showing that tomatoes, by virtue of the water within, can be repelled by a magnet.

But then he says this principle is also demonstrated in MRI machines. Isn't that at best a tremendous exaggeration? MRIs rely on the splitting of degenerate energy states of hydrogen nuclei in a magnetic field, but any magnetic repulsion would strictly be a property of the electrons, right?

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BlackSails
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Re: Diamagnetism and MRI

Postby BlackSails » Mon Nov 02, 2015 3:55 am UTC

Honestly, that video seems pretty fishy. That looks like a staggering force for a handheld magnet. Levitating a frog (probably around 10^3-10^4 more force), takes a 100 T/m gradient field.

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Jakell
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Re: Diamagnetism and MRI

Postby Jakell » Wed Nov 04, 2015 1:38 am UTC

I've done the same thing with ketchup packets; with a zesty neodymium magnet that has a pretty strongly divergent magnetic field, you can push around a well balanced (and low friction) bit of water. If you want numbers, I've tied a ketchup packet with fishing line, and hung it from the ceiling. Pushing on it with a 1 inch long, 1 inch diameter magnet gets the string to an angle of a few degrees, and so for a 10ish gram packet, I was applying a force of about 0.005 N, or about 1/20th the packet;s weight.

A 100T/m field is 0.1 T/cm, and according to this calculator the field near my magnet is plenty divergent near it's surface to diamagnetically repel a mostly-water thing. By no means will it levitate a ketchup packet, but a bit of pushing is not unreasonable.
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Copper Bezel
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Re: Diamagnetism and MRI

Postby Copper Bezel » Wed Nov 04, 2015 5:49 pm UTC

Jorpho wrote:But then he says this principle is also demonstrated in MRI machines. Isn't that at best a tremendous exaggeration? MRIs rely on the splitting of degenerate energy states of hydrogen nuclei in a magnetic field, but any magnetic repulsion would strictly be a property of the electrons, right?


From the Wikipedia page on MRI and on its receiving coils, it does sound as if the magnetic resonance in play is a property of the nuclei, not the electrons. More importantly, since MRI acts primarily on water, it's definitely a demonstration of diamagnetism rather than ferromagnetism (or paramagnetism), which was the main point the video was making. Are those statements correct?
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Minerva
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Re: Diamagnetism and MRI

Postby Minerva » Sat Nov 07, 2015 1:34 am UTC

I guess you could say that paramagnetism is important in the context of gadolinium MRI contrast compounds - but that's paramagnetism, not diamagnetism.
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Re: Diamagnetism and MRI

Postby stianhat » Thu Dec 03, 2015 11:19 am UTC

I may be a bit late to the ball here but...

An MRI does not rely on diamagnetism, per se - it relies on paramagnetism in the nuclei of whatever you are studying. The MRI machines of today are strong enough to make paramagnets of most superconductors, ferromagnets, ferrimagnets and diamagnets. Electrons are no match for it =P. Though, I would not recommend putting a ferromagnet inside (or near) a live MRI machine, because they will most certainly destroy each other. And possibly everyone else present at the time.

In any case, the MRI aligns the spins of protons and neutrons in a nucleus (but most usually they just look at the proton in hydrogen atoms) and then relaxes and they radiate their energy after a tiny latency period. Oscillation between these states will separate the stuff that resonate at that particular frequency. Very cool machine. In the words of Jean-Luc Picard - "quite hypnotic".


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