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.
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
