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Pseudo frequency? Related to phase shift

  1. Aug 20, 2013 #1
    I am reading a text about how MRI works and I came across a thing that I am totally confused.

    The book mentions about that few particles side by side with their nuclei spinning at the same frequency but they all have a phase difference. Since MRI cannot measure the phase differences directly, it will try to encode the phase differences in term of so called pseudo-frequency. With each unique pseudo frequency the MRI will know where that particle coming from.

    The thing I am confused is how can each nuclei share a common frequency yet they have a different frequency due to their phase differences?

    Am I misunderstanding something????
  2. jcsd
  3. Aug 21, 2013 #2


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    I think the phase difference you are referring to is determined by the differences in distance from the nuclei to the antenna.
  4. Aug 21, 2013 #3


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    They precess at the same frequency in a uniform magnetic field. Once a field gradient is applied, their frequencies differ slightly depending on position. The gradient means that the applied field changes a little bit, linearly with position. This results in the basic frequency being shifted, typically, by parts per million.
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