LOCAL MAGNETIC FIELDS
Inside the main magnetic field, the hydrogen protons are subjected to the magnetic fields of the neighboring protons causing them to get out of synchronization. This effect diminishes when the protons belong to small molecules because faster Brownian motion allows the molecules to reorient. However, the frequency of the Brownian movements of larger molecules, such as lipids, is lower and in these cases the local magnetic field is not abolished.
In consequence, taking into account the magnetic field generated by the biochemical structure of the tissues, the effective magnetic field (Be) is: Be = Bo + Bgrad + Bmo, where Bo is the magnetic field of the magnet, Bgrad is the magnetic field of the gradient coils and Bmol is the molecular magnetic field.