MRI interpretation
MRI signal production

Key points

  • MRI images are a map of proton energy within tissue of the body
  • The body contains protons which are orientated at random
  • Within the bore of an MRI scanner the protons become aligned to the magnetic field
  • The MRI machine produces repeating sequences of radiofrequency pulses to 'excite' protons in the body
  • As protons in the body 'relax' they give off radiofrequency 'signal' which is detected by the scanner and transformed into an image

To produce 'signal', the MRI scanner interacts with protons in the body. Randomly orientated protons become aligned with the powerful magnetic field in the bore of the scanner. A rapidly repeating sequence of radiofrequency pulses – produced by the scanner – then causes 'excitation' and 'resonance' of protons. As each radiofrequency pulse is removed, the protons ‘relax’ to realign with the magnetic field, and as they do so they give off radiofrequency 'signal' which is detected by the scanner and transformed into an image.

Proton excitation and relaxation

During scanning, signal is produced by the repeated process of alignment, excitation/resonance, and relaxation of protons in the body.

Free protons in body

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Free protons in body

  • Free protons within molecules of the body are orientated randomly, spinning on a North-South magnetic axis

Protons aligned in scanner

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Protons aligned in scanner

  • On entering the scanner, the protons align with the axis of the magnetic field (blue arrows) within the bore of the scanner

Radiofrequency pulse

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Radiofrequency pulse

  • A radiofrequency pulse is applied to ‘excite’ the protons
  • Protons are aligned at an angle to the magnetic field
  • The radiofrequency pulses also cause the protons to spin in phase with each other creating 'resonance'

Signal creation

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Signal creation

  • Milliseconds after removal of each radiofrequency pulse the excited protons 'relax', giving off radiofrequency signal which is detected by the scanner
  • Two types of relaxation occur - 1. Realignment of protons with the magnetic field and - 2. Dephasing of spinning protons (loss of resonance)
  • Two types of signal can be detected
  • T1 signal relates to the speed of realignment with the magnetic field – the more quickly the protons realign the greater the T1 signal
  • T2 signal relates to the speed of proton spin dephasing – the slower the dephasing the greater the T2 signal

Tissue differentiation - Fat v water

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Tissue differentiation - Fat v water

  • Protons in the body realign and dephase with varying rapidity depending on the tissue type
  • Detecting the signal after different time intervals allows different tissue types to be highlighted
  • Protons in fat realign quickly with high energy and produce high T1 signal – this phenomenon is exploited to produce 'T1-weighted' images which highlight fat in tissues of the body
  • Protons in water dephase slowly – this phenomenon is exploited to produce 'T2-weighted' images which highlight water in tissues of the body