PHY C22: NMRI

After exploring Ultrasound, let’s dive into another form of medical imaging!

  • Principles behind the use of NMRI
  • Functions of
    • uniform magnetic field
    • RF pulses
    • non-uniform magnetic field

What is Magnetic Resonance Imaging?
A method of medical imaging utilizing the principles of nuclear magnetic resonance.

So… what are the principles of nuclear magnetic resonance?

Nuclei of many elements can be said to SPIN*.

This property of spin makes them interact with magnetic fields.
While they spin, nuclei also precess:

The nucleus is represented by a blue sphere which spins (rotates) along an axis (the arrow), but also precesses (the arrow is rotating along another axis!)

An NMRI scanner utilizes this spin & precession using 3 main tools, each with different functions:

Uniform Magnetic Field

  • nuclei spin & precess naturally in random directions
  • When an external uniform magnetic field is applied, nuclei align such that their precession is about the direction of the magnetic field
  • the frequency of this precession (known as the Larmor frequency) depends on the nature of the nucleus & strength of external magnetic field
    • these precessions are in the radio-frequency range

Radio-frequency (RF) Pulses

  • Radio frequency pulses are sent at the same frequency as the nuclei’s Larmor frequency
  • nuclei absorb the energy, causing resonance
  • after a certain relaxation time, nuclei return to equilibrium state & release the energy as RF radiation
  • this RF pulse can be detected & processed

Non-uniform Magnetic Field

  • the non-uniform magnetic field is superposed on the uniform field
  • different locations now have different magnetic field strengths
  • thus, frequency of precession now depends on location
  • this enables us to change the region where nuclei are detected

External Resources:

  • A great simulation on how NMR works: https://www.drcmr.dk/CompassMR/
    • Select “coil” & “spin”
    • Slide B0 to change the strength of the constant magnetic field
    • Slide B1 to change the amplitude of the RF pulse
    • Slide B1 freq to change the frequency of the RF pulse
    • You will see how the nucleus gets excited when the RF pulse frequency is equal to its Larmor frequency

*DISCLAIMER:
In real life, nuclei exhibit a property called ‘Spin’, but don’t actually rotate like a top. However, the effects of their real spin properties are similar to what would happen if they really were spinning like tops. It’s a useful analogy for A-Levels,
but do read up on actual quantum spin if you’d like further understanding.

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