TwinTree Insert

03-03 Shimming

one 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 Shielding

spaceholder redMagnetic Shielding. This kind of shielding is applied to limit the fringe field of the magnet (see Figure 18-04), to com­pen­sate for inhomoge­neities of the magnetic field, partly to in­crease the field strength, and to pro­tect the environment.

Shielding can be necessary to protect the hos­pi­tal environment from the magnetic field ema­nat­ing from the MR system. Cer­tain equipment and devices must not be ex­posed to mag­ne­tic fields, for instance nu­clear cameras, CT scanners, neuro- and biostimulation devices (e.g., pacemakers), magnetic cards (e.g., credit cards), com­pu­ters, disks, tapes, mechanical watches, and cameras (see In­ci­den­tal Ha­zards).

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.

At 3 Tesla, for instance, active shielding can can bring down the 5-Gauss line to less than five meters from the magnetic isocenter.

spaceholder redRadiofrequency (Faraday) Shielding. Table 02-01 showed that the resonance frequencies of all MR scanners overlap with commercial, military, and amateur radio and television frequencies. Elec­tric machines can also create electromagnetic waves.

It can easily happen that the receiver of the MR imaging equipment picks up such radio signals from the outside world, which then interfere with the signals from the examined sample or patient. This leads to noisy images or, in the worst case, the complete loss of ima­ges.

Faraday shielding is used as a protection against electromagnetic interference. High-field systems require a complete Faraday cage (usually a copper cage with windows, including an electrically conducting screen), which has to be grounded. Connections from the inside of the cage to the outside have to be very carefully made and shielded (Figure 03-09).

Figure 03-09:
Simple Faraday cage. Copper covers floor, walls and ceiling of the entire room. Windows, the door and all cables connecting the room with the outside are also shielded to maintain elec­tri­cal isolation. Usually — but not on this example installation — the shielding is co­ve­red with plaster and wallpapered, hiding the copper cage.