Shimming is the process in which the inhomogeneity of the static magnetic field of a MRI scanner is reduced. The are many different approaches to shimming and the best solution depends on many factors such as the cause of the inhomogeneity (is the magnet imperfect or is the susceptibility of the sample causing distortions of the field) , the field strength of the magnet, the order of magnitude of the inhomogeneity etc. One can typically differentiate shimming approaches in to two categories;
- Passive shimming which involves the addition of passive materials to the system and is typically used to compensate for magnet imperfections and
- Active shimming which uses (additional) coils to generate magnetic fields and is typically done to correct for inhomogeneities caused by the sample placed in the magnet.