MRI and Electrons

  1. 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
     
  2. jcsd
  3. Drakkith

    Staff: Mentor

    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
     
  4. K^2

    K^2 2,470
    Science Advisor

    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.
     
  5. Drakkith

    Staff: Mentor

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

    Staff: Mentor

    Yes, the electron resonance is in the microwave range for typical field strengths.
     
  7. K^2

    K^2 2,470
    Science Advisor

    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
     
  8. Drakkith

    Staff: Mentor

    Thanks. I'll have to look more into magnetic moments and such.
     
  9. Thanks guys. I dont know enough about how the RF is actually generated though. Is it not feasible to generate RF pulses at 28GHz?
     
  10. K^2

    K^2 2,470
    Science Advisor

    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.
     
  11. DaleSpam

    Staff: Mentor

    Besides what was mentioned above, the other problem is that these frequencies are good at cooking meat.
     
  12. Vanadium 50

    Vanadium 50 17,581
    Staff Emeritus
    Science Advisor
    Education Advisor

    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?
     
  13. Drakkith

    Staff: Mentor

    I'm sure the OP is simply curious as to why we don't use the electrons, not suggesting we do.
     
  14. K^2

    K^2 2,470
    Science Advisor

    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.
     
  15. marcusl

    marcusl 2,112
    Science Advisor
    Gold Member

    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.
     
    Last edited: Mar 31, 2012
  16. 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.
     
  17. K^2

    K^2 2,470
    Science Advisor

    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.
     
  18. 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?
     
  19. K^2

    K^2 2,470
    Science Advisor

    No. If you apply a 90° pulse, individual moments align with XY plane, and precess in that plane with resonant frequency.
     
  20. marcusl

    marcusl 2,112
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    Gold Member

    Not so. Virtually all clinical diagnosis is based on T1- and T2-weighted contrast enhancement, arising from spin-lattice and spin-spin couplings, respectively. To oversimplify and exemplify: T1-weighted images particularly highlight differences in "solidity" between tumor and tissue, meniscus and bone, etc. while T2 weighting emphasizes fluid due to inflammation and is used in spinal imaging because it highlights cerebral spinal fluid so that disks, impingements and bulges are visible.
     
  21. marcusl

    marcusl 2,112
    Science Advisor
    Gold Member

    The classical and quantum mechanical pictures of NMR are sufficiently different to cause confusion if you mix and match the concepts. In the classical view, the spins are tipped away from the z axis by the RF field, whereupon they precess. The longer the field is applied, the greater the polar angle from the z axis. A CW field will send the spins from z to the xy plane (90) to -z (180) and back up to 360 or 0, and then it continues on. To tip them into the xy plane, the RF is turned off.

    In the quantum picture, the spins are a two-state quantum system (spin up or down) with an energy gap (the Zeeman splitting). Transitions accompany the absorption or emission of a photon of correct energy. The spins in this view are always aligned or anti-aligned with B0.
     
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