Normally all atomic nuclei in the body rotate around their own axis. This angular momentum is also called "nuclear spin". Due to their rotation, these nuclei generate a minimal magnetic field. The hydrogen nuclei, which occur most frequently in the body, are particularly important here. Under natural circumstances, the magnetic orientation of the hydrogen nuclei is purely random.
If, however, a strong magnetic field is applied from the outside, these atomic nuclei all arrange themselves in the same direction, namely in the longitudinal direction of the body. Magnetic resonance imaging uses this principle. There is a very strong magnetic field in the MRI machine. In addition to this magnetic field, the MRI device also emits radio waves with a high frequency onto the body during the MRI examination.
This changes the orientation of the hydrogen nuclei in the body. After each radio wave pulse, the hydrogen nuclei return in the longitudinal direction. The atomic nuclei emit special signals, which are measured and then combined into images by the computer. However, these signals are not sufficient to obtain usable images. For this reason, additional magnetic fields are applied to the body with the aid of so-called coils.