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'

Shimming by suitably adjusting the currents in shim coils for qualitiy assurance.

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.

From Hitachi Medical Systems America, Inc.;
the AIRIS made its debut in 1995. Hitachi followed up with the AIRIS II system, which has proven equally successfully. 'All told, Hitachi has installed more than 1,000 MRI systems in the U.S., holding more than 17 percent of the total U.S. MRI installed base, and more than half of the installed base of open MR systems,' says Antonio Garcia, Frost and Sullivan industry research analyst. Now Altaire employs a blend of innovative Hitachi features called VOSI™ technology, optimizing each sub-system's performance in concert with the other sub-systems, to give the seamless mix of high-field performance and the patient comfort, especially for claustrophobic patients, of open MR systems.

From Hitachi Medical Systems America, Inc.;
because of its dependability, the MRP-7000™ remains popular more than a decade after the first U.S. system was shipped. This system maintains a high resale value, what has made it one of the most sought-after scanners on the used MRI equipment market.