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).
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.