The elimination of main field inhomogeneities caused by ferro-magnetic material around the magnet and by manufacturing tolerances is referred to as shimming. These inhomogeneities have a strong influence on image quality and therefore must be minimized as much as possible, within the limits of specification.
In passive shimming small pieces of sheet metal or ferromagnetic pellets are affixed at various locations within the scanner bore to improve homogeneity. Conversely, active shimming uses currents directed through specialized coils to generate a “corrective” magnetic field.
Active shim coils can be:
1) Superconducting, located within the liquid helium-containing cryostat; or 2) resistive, mounted on the same support structure as the gradient coils within the room-temperature inner walls of the scanner. Both types of active shims require their own power supplies and are controlled by special circuitry. Some scanners use both types.
2) MEDICARE has all the tools to perform Shimming of Siemens super conductive magnets and all other permanent magnets (MRI machines) of any company by which the image quality can be improved and gives positive value in diagnosis and filming.