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Purpose of the non-uniform field in MRI?

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    large / strong (constant) magnetic field B1
    nuclei rotate about direction of field / precess (1)
    radio frequency / r.f. pulse B1
    causes resonance in nuclei , nuclei absorb energy (1)
    (pulse) is at the Larmor frequency (1)
    on relaxation / nuclei de-excite emit (pulse of) r.f. B1
    detected and processed B1
    non-uniform field (superimposed) B1
    allows for position of nuclei to be determined B1
    and for location of detection to be changed (1)

    (B6 plus any two extra details, 1 each, max 2) B2

    This is the mark scheme to a question that asks to explain the principles behind MRI. My question is, what is the non-uniform field superimposed to? Is it the same field as the first magnetic field or the RF pulse that puts the atoms in resonance? Where does the relaxation time of the atom after the RF pulse is stopped come in to all of this?
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    If a uniform field was used across the whole of the patient, all protons would have the same Larmour frequency, so it would be impossible to tell where the received radio wave came from. By slighly varying the field across the patent, this is made possible.
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    ^^^^^ what they said
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    (Original post by Pangol)
    If a uniform field was used across the whole of the patient, all protons would have the same Larmour frequency, so it would be impossible to tell where the received radio wave came from. By slighly varying the field across the patent, this is made possible.
    OK, then it's a strong non-uniform magnetic field that is applied to align the hydrogen atoms in the first place. The relaxation time gives an idea of the environment of the atom, but the frequency of the pulse emitted gives an idea of the POSITION of the atom, is that correct?

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