Magnetic shimming refers to the process of reducing magnetic field inhomogeneities. It is an important aspect of optimizing image quality. Field inhomogeneities can be the result by both intrinsic magnet properties and the surrounding environment of the MR magnet – such as the presence of nearby metallic and electrical structures and the fields of nearby scanners. Magnetic field homogeneity needs to be considered on a patient-by-patient basis in order to fine tune the resulting image resolution and quality.
There are two main types of shimming: passive and active (sometimes both may be used). Active shimming involves the use of shim coils. These coils have electric currents directed through them, which generates their own magnetic field, which cancel the remaining field gradients in the main field. Passive shimming involves the use of ferromagnetic pellets or pieces of sheet metal attached within the scanner bore.
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