Magnetic Resonance Imaging (MRI) combines nuclear physics at the quantum level, strong external magnetic fields and radio wave resonance.
In essence, MRI uses the hydrogen atoms that are naturally abundant in the body (mainly in water and fat). A radio frequency pulse of a specific frequency makes these hydrogen nuclei respond. The strength and timing of their response depend on the hydrogen concentration and the surrounding cellular environment, which lets us tell different tissue types apart with excellent contrast.
How MRI works: the learning sequence
Video 1
Nuclear spin: Understanding nuclear magnetic moments and why hydrogen protons behave like tiny magnetic dipoles.
Video 2
External fields and precession: What happens when hydrogen nuclei are placed inside a strong external magnetic field (B0).
Video 3
Resonance: How hydrogen nuclei absorb energy when hit by a targeted radio frequency (RF) pulse at the Larmor frequency.
Video 4
Relaxation: How protons return to equilibrium and emit detectable radio frequency signals back to the MRI receivers.
Video 5
Spatial localisation: How magnetic field gradients determine the exact 3D location of the emitted signals to produce an image.
Check Your Understanding: MRI Basics Practice
1. Why are hydrogen nuclei the ones mainly used in human magnetic resonance imaging?
Hydrogen consists of a single proton, which has a net spin (magnetic moment), and it is extremely abundant in human tissue because of water (H2O) and fats.
Hydrogen is used because of its single-proton nuclear spin and its overwhelming abundance in water and fat throughout biological tissue.
2. What happens to hydrogen protons when a patient is placed inside the strong main magnetic field (B0)?
The strong external magnetic field (B0) makes the magnetic moments of the protons align parallel or anti-parallel to the field, and they precess around the field axis at the Larmor frequency.
Placing hydrogen nuclei in a strong magnetic field makes them align with or against the field direction and precess at a specific frequency.
3. During the relaxation phase after the radio frequency pulse is switched off, what do the hydrogen nuclei do?
As the protons relax back to equilibrium, they release the energy they absorbed as a faint radio frequency signal. The receiver coils record this signal to build the image.
During relaxation the protons give back the energy they absorbed as radio frequency signals, which the receiver coils collect and the MRI computer decodes.
video 1 - Nuclear Spin
video 2 - Precession
video 3 - Resonance
video 4 - Relaxation
video 5 - Location
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Test your predictions in a simulation.
The following is a simulation of how an MRI works
Click on "Simplified MRI"
Start by applying the main magnetic field. What do you see occurring with the hydrogen nuclei?
Increase the power of the radiowave source. What do you see occurring?
Now we alter the field by applying the gradient fields
Adjust the frequency until you get a response. What does this demonstrate?
Add a "tumor". What frequency is required to get a maximum signal from the tumor?