CIE A Level Physics (9702) 2019-2021

Revision Notes

25.5.1 Principles of Nuclear Magnetic Resonance Imaging (NMRI)

Uses of Nuclear Magnetic Resonance Imaging (NMRI)

  • Nuclear Magnetic Resonance Imaging (NMRI) is defined as:

A procedure in which radio waves and a powerful magnet are used to create detailed images of areas inside the bodies

  • The purpose of this is to obtain diagnostic information about internal structures for medical purposes
  • NMRI is used for imaging the brain, the spine, the soft tissues of the joints, and inside the bones
  • The main principles behind NMRI are:
    • A strong uniform magnetic field is applied (0.5 – 3.0 T)
    • Hydrogen nuclei, or protons, precess, meaning they rotate about the field direction
    • Radio frequency pulse is applied
    • This is at the Larmor frequency (the frequency of precession/rotation)
    • This causes resonance (nuclei absorb this energy)
    • On relaxation, the nuclei emit the radio frequency pulse in the form of photons
    • The emitted pulse is detected and processed by the detector
    • A non-uniform magnetic field is superimposed on the uniform field
    • This allows the position of the resonating nuclei to be determined
    • This also allows the position of the detection to be changed, meaning different slices can be studied

Exam Tip

This is commonly a 6 mark exam question. Practice writing all the points that make up the main principles together in clear and concise sentences, making sure you mention all the bullet points mentioned above.

Uses of a Non-Uniform Magnetic Field in NMRI

  • Magnetic resonance imaging (MRI) requires the use of a non-uniform magnetic field superimposed on a large uniform magnetic field
    • The Larmor frequency depends on the applied magnetic field strength
    • So, a non-uniform field means the Larmor frequency will be different in different regions of the person being studied
    • Therefore, the frequency of precession (rotation) of hydrogen nuclei is different at different points


The purpose of the two fields are outlined below:

  • Large constant magnetic field
    • To align the nuclei
    • To cause the Larmor frequency (frequency of precession/rotation) to be set in the radio frequency range
  • Non-uniform magnetic field
    • To allow the nuclei, ie. the region of precessing, to be located
    • To enable the thickness of the slice being studied to be varied

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