theory

Last reviewed 01/2018

The body should be regarded as a mass of small randomly arranged magnets. These magnets represent the nuclei of hydrogen atoms - protons - which have polarity and thus are able to alter their orientation if they are subjected to a strong magnetic field. MRI utilizes these properties in a 3-stage process.

• Precession: When a patient is placed in a scanner then some of the protons within the patient become aligned along the axis of the magnetic field. When these protons become magnetized they then rotate and wobble - precess - around the magnetized field axis.
• Resonance: If there is then application of changes in radiofrequency then the orientation of rotation and wobbling can be altered. Different types of radiofrequency pulse can be used to produce different types of emitted signal with different structures.
• Emission: Once the radiofrequency pulse is turned off then the protons begin to lose their phase cohesion and this results in the emission of very small radiofrequency signals. The magnitude, phase, amplitude and frequency of these signals are detected by a magnetic resonance imager and used to generate an image.