Hydrogen Ions

The physics of MRI scanning is complex and relies on the behaviour of hydrogen ion nuclei in the presence of a high-intensity magnetic field and radiofrequency pulses (Fig 1).

Hydrogen ion nuclei are normally continually spinning around an axis that ‘wobbles’. This wobble is called precession and is a feature seen in many spinning objects, e.g. a gyroscope or the Earth. The spin and precession of nuclei in the presence of the magnetic field becomes aligned, but this alignment will alter when the nuclei are subjected to a pulse. As a consequence, their energy level rises. On withdrawal of the pulse, the nuclei return to their lower energy state and emit energy, which can be detected.

Different tissue types display varying emission patterns. In addition, the emission patterns can be influenced by differing patterns within the radiofrequency pulse.

These various patterns deliver ‘weightings’ that can highlight materials with differing characteristics, e.g. water content, fat content.

Fig 1: The behaviour of hydrogen ion nuclei in the presence of a high-intensity magnetic field and radiofrequency pulses