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dangus Mar30-12 03:35 PM

MRI and Electrons
 
MRI imaging uses primarily the magnetic fields of spinning hydrogen protons to generate images. I read somewhere that the magnetic fields generated by spinning electrons are stronger then those created by spinning protons. Why then are electrons not used in MRI imaging?

Thanks

Drakkith Mar30-12 05:28 PM

Re: MRI and Electrons
 
This article explains the physics behind how it works. Put simply, protons in hydrogen atoms take the right amount of energy for their spins to be flipped against the applied magnetic field with a radio frequency. The RF is then removed and the protons align themselves with the magnetic field again, releasing a radio frequency as they do, which is then detected by the machine. I'm guessing that the frequency required to flip the spin of an electron is probably much to low to be useful in this manner.

http://en.wikipedia.org/wiki/Magnetic_resonance_imaging

K^2 Mar30-12 05:40 PM

Re: MRI and Electrons
 
Other way around. EPR frequencies are too high. In 1 Tesla field, you get 28GHz for free electron resonance. For protons, it's closer to 40MHz.

Drakkith Mar30-12 05:45 PM

Re: MRI and Electrons
 
Quote:

Quote by K^2 (Post 3842266)
Other way around. EPR frequencies are too high. In 1 Tesla field, you get 28GHz for free electron resonance. For protons, it's closer to 40MHz.

Hmm. How do you calculate that? I figured electrons would be easier to flip since they are less massive. Obviously I was wrong!

DaleSpam Mar30-12 06:47 PM

Re: MRI and Electrons
 
Yes, the electron resonance is in the microwave range for typical field strengths.

K^2 Mar30-12 07:01 PM

Re: MRI and Electrons
 
Quote:

Quote by Drakkith (Post 3842273)
Hmm. How do you calculate that? I figured electrons would be easier to flip since they are less massive. Obviously I was wrong!

Resonance frequency is proportional to energy splitting in magnetic field. That energy is proportional to magnetic moment and strength of the field. Because angular momentum of proton and electron are the same, the magnetic field of an electron is a lot stronger. Classically, you expect electron's moment to be roughly 2,000 times greater due to being 2,000 times lighter, and if you look at the numbers I gave, it's not far off.

Here is a good article to put a bit more theory with this. Wikipedia: EPR

Drakkith Mar30-12 07:19 PM

Re: MRI and Electrons
 
Thanks. I'll have to look more into magnetic moments and such.

dangus Mar30-12 08:29 PM

Re: MRI and Electrons
 
Thanks guys. I dont know enough about how the RF is actually generated though. Is it not feasible to generate RF pulses at 28GHz?

K^2 Mar30-12 08:38 PM

Re: MRI and Electrons
 
Quote:

Quote by dangus (Post 3842491)
Thanks guys. I dont know enough about how the RF is actually generated though. Is it not feasible to generate RF pulses at 28GHz?

It's tricky enough by itself, but you also have to do some signal processing to do anything complex. EPR is done to study materials. It's just that pulse sequences for MRI get complex, and so doing them at these frequencies becomes complicated. Receiving the signal is even more complicated. For basic EPR, you don't need much. But for MRI, you need to get the whole spectrum and Fourier transform it.

DaleSpam Mar30-12 09:01 PM

Re: MRI and Electrons
 
Quote:

Quote by dangus (Post 3842491)
Thanks guys. I dont know enough about how the RF is actually generated though. Is it not feasible to generate RF pulses at 28GHz?

Besides what was mentioned above, the other problem is that these frequencies are good at cooking meat.

Vanadium 50 Mar30-12 10:01 PM

Re: MRI and Electrons
 
But that would only be a problem if we were made out of meat....oh....wait....

Since this already works with protons, what would be the point of making it work with electrons?

Drakkith Mar30-12 10:09 PM

Re: MRI and Electrons
 
Quote:

Quote by Vanadium 50 (Post 3842637)
But that would only be a problem if we were made out of meat....oh....wait....

Since this already works with protons, what would be the point of making it work with electrons?

I'm sure the OP is simply curious as to why we don't use the electrons, not suggesting we do.

K^2 Mar30-12 10:21 PM

Re: MRI and Electrons
 
Quote:

Quote by Vanadium 50 (Post 3842637)
Since this already works with protons, what would be the point of making it work with electrons?

Actually, there would be. Stronger magnetic moment would mean that you can reduce magnetic field for the same signal/noise ratio. One of the problems with human body is that it is filled with water, which is diamagnetic. That has tendency to distort magnetic field, and without a uniform field, it's very difficult to get a good MRI scan. The stronger the field, the more distorted the field becomes. It's not a huge obstacle in MRI, but it is in fMRI.

On the flip side, and thanks to DaleSpam of reminding me about this, microwaves have trouble penetrating human flesh due to conductivity. So even if you could reduce the RF power to the point where it won't cook the flesh, it won't penetrate very well. That might actually be the bigger problem than everything else mentioned above.

marcusl Mar30-12 11:31 PM

Re: MRI and Electrons
 
I don't think this answers Vanadium's, and the OP's, question. First of all, fMRI ("functional MRI" or brain function mapping) is just a flavor of MRI, which itself is a simple flavor of NMR. It really doesn't address electrons at all. Second and more importantly, NMR is a far richer phenomenon than EPR. Because of the couplings that spins in one nucleus have to its neighbors, and to the matrix (bone, interstitial fluid, or in the case of a solid, the crystal lattice), the nuclear resonance spectrum is extremely complex. It reflects chemical structure, coupling strengths, the distances to other nuclei, molecular conformation (shape and folding of a protein, e.g.), and the solidity or fluidity of the matrix, among other things. By contrast, the outer electron in an atom (which contributes to EPR) participate in chemical bonds with neighbors but otherwise do not reflect the same diversity seen in NMR. Only atoms with an unpaired electron can resonate, dramatically limiting the cases where EPR exists. These are the main reasons that EPR is a sideshow in the research world while NMR and MRI are ubiquitous.

BTW, EPR stands for electron paramagnetic resonance. It used to go by the name ESR or electron spin resonance.

Mike H Mar31-12 04:19 PM

Re: MRI and Electrons
 
My understanding of the in vivo EPR imaging that does go on is primarily focused on small animals (mice, in particular) and the occasional mention of (for example) human limbs that are placed in a resonator. Of course, tissue cultures and ex vivo samples are also possible with this sort of setup. They do, however, seem to be devised to work at lower field strengths. Clearly, if your interests involve the role of free radicals in biological systems, EPR imaging offers the ability to directly interrogate those species. (See, for example, here.) Insofar as to reasons for wanting to do EPR imaging, it's a lot like wanting to develop alternatives/complements to 1H MRI - you could work with something with a great deal less natural background signal (example - 19F MRI), or with a more biochemically informative nucleus (example - 23Na MRI).

I'm not sure I'd entirely agree with EPR being a sideshow - it really depends on what one's show is in the first place, after all. It's something of a specialized niche, I will agree, although I get the impression more and more people are becoming intrigued by its benefits. It's been invaluable in inorganic & solid state chemistry, and as I've alluded to earlier in this post, for understanding the nature and role of free radicals in a number of contexts. With the development of pulsed EPR methods, one can obtain longer distance constraints for materials and biological assemblies, typically between 20 to 80 Angstroms. Also, it's a natural choice for examining dynamics at the nanosecond time scale.

K^2 Mar31-12 05:57 PM

Re: MRI and Electrons
 
Quote:

Quote by marcusl (Post 3842729)
Second and more importantly, NMR is a far richer phenomenon than EPR. Because of the couplings that spins in one nucleus have to its neighbors, and to the matrix (bone, interstitial fluid, or in the case of a solid, the crystal lattice), the nuclear resonance spectrum is extremely complex.

And yet, very little of it is used in actual MRI. For the most part, what MRI systems are built to measure are water concentrations and water diffusion. A lot of the things you mention, like NMR on a crystal lattice, isn't even possible with MRI. If you've ever seen how solid NMR is done, you know why. If not, look it up.

EPR would be quite sufficient for medical uses, because everything a medical MRIs are built to measure can, in fact, be measured with EPR, and all the same imaging techniques apply to both.

dangus Mar31-12 07:19 PM

Re: MRI and Electrons
 
I have another question about MRI physics. I was going to make a new thread, but it seemed easier to just post it here since you all seem to know a lot about the topic.

When the perpendicular RF pulse is applied in a MRI machine the net magnetic moment tilts from the Z axis into the X-Y plane. What happens to the individual magnetic moments of all the protons though? Do the individual magnetic moments tip as well?

My previous understanding was that the individual magnetic moments dont change orientation, instead the tipping of the net magnetic moment is caused by the precession of all the protons in phase (which creates the X-Y component) and the elevation of some protons from the low energy state to the high energy state (thus shrinking the net Z component). Is this true?

K^2 Mar31-12 10:16 PM

Re: MRI and Electrons
 
No. If you apply a 90 pulse, individual moments align with XY plane, and precess in that plane with resonant frequency.


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