Hydrogen nuclei (protons) have magnetic properties, called nuclear spin. They behave like tiny rotating magnets, represented by vectors.

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The sum of all the tiny magnetic fields of each spin is called net magnetization or macroscopic magnetization. Normally, the direction of these vectors is randomly distributed. Thus, the sum of all the spins gives a null net magnetization.

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Within a large external magnetic field (called B0), nuclear spins align with the external field. Some of the spins align with the field (parallel) and some align against the field (anti-parallel).

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Main nuclei imaged in human MRI

  • In clinical MRI, Hydrogen is the most frequently imaged nucleus due to its great abundance in biological tissues.
  • Other nuclei such as 13C, 19F, 31P, 23Na have a net nuclear spin and can be imaged in MRI. However, they are much less abundant than hydrogen in biological tissues and require a dedicated RF chain, tuned to their resonance frequency.