03-03 Shimming of the Magnet

None of the above mentioned magnet systems will produce a perfect ho­mo­ge­neous field, but careful design may allow for fields where the in­ho­mo­ge­nei­ties are far bet­ter than 100 parts-per-million (ppm) within the region of interest. Field in­ho­mo­ge­nei­ties reduce the efficiency of imaging experiments and prohibit spectroscopic in­ves­ti­ga­tions. To improve the field characteristics, most magnet systems are de­li­ver­ed with shim coils. When currents are passed through these coils, correctional fields of known geometry are produced and can compensate for the inherent in­ho­mo­ge­nei­ty of the magnet.

Homogeneities better than 0.01 ppm can routinely be achieved with high-field analytical magnetic resonance magnets over small sample volumes (less than 1 cm³. Using in-vivo MR spectroscopy with localized shimming, homogeneities of less than 1 ppm can be achieved for small volumes. For MR imaging where lar­ger volumes are used, poorer homogeneity is acceptable.

The shim coils can be placed in liquid helium inside the superconducting main magnetic field and adjusted one-by-one to shape the field (active shimming).

A similar effect can be achieved by mounting small ferromagnetic metal pie­ces at the appropriate locations inside or outside the magnet bore. Each of these pieces will contribute to the magnetic field and, if the symmetry of the field is kept, a very homogeneous field can be obtained (passive shimming).

03-04 Magnetic Shielding

Shielding is applied to limit the fringe field of the magnet, to compensate for in­ho­mo­ge­ne­ities of the magnetic field, partly to increase the field strength, and to pro­tect the environment (Table 03-03).

Table 03-03:
Minimal field strengths at which certain de­vi­ces may start malfunctioning.
Shielding can be necessary to protect the hos­pi­tal environment from the magnetic field ema­nat­ing from the MR system. Cer­tain equipment must not be exposed to mag­ne­tic fields.

Passive shielding involves large quantities of iron, easily 30 tons, sym­me­tri­cal­ly placed around the magnet.

Active shielding is accomplished by additional superconductive coils. Whereas the inner set of coils produces the main magnetic field, the outer set contains and reduces the fringe field which surrounds the magnet. Commonly, both sets are electrically coupled for fail-safe operation.