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MRI question: Production of EM Waves using magnets

  1. Aug 11, 2014 #1
    Hi all,
    I've got a quick clarification question. Generally when the production of electromagnetic waves are discussed, it's done in the context of an accelerating electric charge. Is it possible to generate EM waves by instead accelerating something like a permanent magnet? I.e. if I wiggle a permanent magnet back and forth, are EM waves produced? I'm pretty sure that the answer is yes, but I see this case discussed so infrequently that I'm doubting myself.

    The motivation for this question comes from reading into the Physics of MRI scans, which involve generating a net magnetization vector (from alignment of nuclear magnetic moments) within a patient which is made to rotate. The scanner ultimately picks up the signal from this rotating magnetization vector. I've seen detection process described as reading a voltage signal induced in pickup coils due to the changing magnetic field from the magnetization vector as per Faraday's law. Alternatively, some references describe the rotating magnetization vector as giving off radio-frequency EM waves which are picked up by the coils.

    My feeling is that these are both two aspects of the same phenomenon: there is an alternating voltage induced in the coils as they interact with the magnetic field component of the EM waves produced by the rotating magnetization vector within a patient. This lead me to think about simple experiments involving oscillating a magnet near a coil of wire and observing the current that results in the coil. This is usually described as an example of EM induction in textbooks, but I think it too would be an example of EM wave production. Is this correct?

    Alex
     
  2. jcsd
  3. Aug 11, 2014 #2

    mfb

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    Staff: Mentor

    Yes. It is a very ineffective way, however, and you cannot reach relevant frequencies.
    Right.
    This is not an effect of radiation. It is a near-field effect, and you don't see the contribution from radiation at all.
     
  4. Aug 11, 2014 #3

    Dale

    Staff: Mentor

    You are correct, no need for self-doubt here!

    Both are valid interpretations, but I prefer to use the former interpretation. The reason is that MRI, at the common clinical frequencies, is generally done in the near field, which is why the coils are designed as loops rather than dipoles.

    What most people think of as EM waves are in the far field. Those are the ones that radiate off to infinity and where the E and B are orthogonal to each other and to the direction of propagation. While some energy is radiated off to the far field in MRI, that is not what is generally detected. However, there has been some recent work on far field MRI (or at least farther-than-usual-field): http://www.nature.com/nature/journal/v457/n7232/full/nature07752.html

    Yes, you cannot avoid the EM wave production. But again, you have to be careful not to overuse far-field intuition when you are dealing with the near-field.
     
    Last edited by a moderator: May 6, 2017
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