Shimming by adjusting the position of suitable pieces of ferromagnetic metal within or around the main magnet of a MRI system. Typically, the iron is in the form of small plates which are held in removable trays. The magnetic field is plotted, the position and quality of the iron required is then calculated and the appropriate 'shims' put in place. Several repetitions of this process will normally be required in order to achieve the optimum arrangement.
Passive shimming does not replace adjustable shimming, but is installed during the commissioning of a unit to improve the homogeneity of the bare magnet.

Correction of inhomogeneity of the magnetic field produced by the main magnet of a MRI system due to imperfections in the magnet or to the presence of external ferromagnetic objects. May involve changing the configuration of the magnet or the using of shim coils (active shimming) or adding or removing steel from the magnets poles (passive shimming) to fine-tune the magnetic field.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'

From GE Healthcare;
a friendly and less confining appearance targets the 7% of individuals who refuse to have an MRI because of claustrophobia. This open MRI system is also up to three times faster than other scanners, therefore the Signa OpenSpeed™ reducing exam time and scheduling issues. In addition, a swing table provides better access and supports up to 500 pounds.

From GE Healthcare;
the Signa Ovation™ is a patient-friendly open MRI scanner designed not only to handle a typical patient mix, but to accommodate larger patients, patients who are claustrophobic, and others who have difficulty tolerating the close quarters of conventional MR machines.

From FONAR Corporation;
in October of 2004, the company changed the product name of the Stand-Up™ MRI to the Upright™ MRI. The Indomitable™, Upright™ MRI is the only open MRI in the world that can perform positional MRI (pMRI), i.e. the Upright MRI™ scans patients in upright, weight-bearing positions, in addition to the conventional lie-down positions. The Upright™ MRI is the only device that can scan patients in the position their symptoms occur, in their position of pain. In early clinical reports independently confirm the effectiveness and potential of positional MRI. In October 2000, Fonar received permission to market the Indomitable™ from the FDA.