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:
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
- 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
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.