Reducing Fringe FieldsHow do you reduce the size of the fringe field so it doesn't cause problems?
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The fringe field is reduced in size by a process called magnetic shielding. Magnetic shielding may be either passive or active.
In the early 1980's MR scanners were generally unshielded and magnet rooms was made extremely large (2-4 times the size of modern rooms) to accommodate the large fringe fields. Where required external magnetic fields were constrained using passive shielding methods. In passive shielding iron beams or steel plates are incorporated into the walls, ceiling, and/or floor of the magnet room. These materials concentrate magnetic flux lines close to the scanner. In some cases passive shielding material is applied directly to the outer surfaces of the cryostat or magnet shell.
Passive shielding adds considerable weight and expense to MR siting and creates difficulties in achieving high homogeneity within the magnet bore itself. To overcome these limitations, actively shielded magnets were developed in the early 1990's and have now become standard on all superconducting clinical scanners. Occasionally passive shielding is still required, but the amount of steel is significantly less than it would be otherwise. Although a number of 7.0 T magnets also have active shielding, additional passive shielding is usually required due to the very high fields involved.
In the early 1980's MR scanners were generally unshielded and magnet rooms was made extremely large (2-4 times the size of modern rooms) to accommodate the large fringe fields. Where required external magnetic fields were constrained using passive shielding methods. In passive shielding iron beams or steel plates are incorporated into the walls, ceiling, and/or floor of the magnet room. These materials concentrate magnetic flux lines close to the scanner. In some cases passive shielding material is applied directly to the outer surfaces of the cryostat or magnet shell.
Passive shielding adds considerable weight and expense to MR siting and creates difficulties in achieving high homogeneity within the magnet bore itself. To overcome these limitations, actively shielded magnets were developed in the early 1990's and have now become standard on all superconducting clinical scanners. Occasionally passive shielding is still required, but the amount of steel is significantly less than it would be otherwise. Although a number of 7.0 T magnets also have active shielding, additional passive shielding is usually required due to the very high fields involved.
Main and shielding coils of a superconducting magnet before being enclosed in a cryostat.
The two coil sets are electrically connected oppositely in series and thus have fields pointing in opposite directions. For example, the main windings might generate a magnetic field of 2.0 T while the shielding coils generate a field of 0.5 T in the opposite direction. The net effect is a magnetic field of 1.5 T in the center of the magnet. Although the usable field strength in the operating volume is reduced by the active shielding, the reduction effect on the fringe field is substantially greater.
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In active shielding, the fringe fields are suppressed within the cryostat itself rather than contained by ferromagnetic materials outside the scanner. An actively shielded magnet design incorporates at least one additional set of superconductive windings (shield coils) that oppose a portion of the external field generated by the main windings. These secondary shield coil windings are co-axial with the main windings and external to them, as shown in the image left and diagram below.
Active shield coils (blue) are in series with the main magnet windings (orange) but carry current in the opposite direction.
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References
Practical EM Shielding. On-line tutorial available from LearnEMC.com available at this link.
Turner BD. Shielding issues for medical products. Conformity, May 2007, pp 48-53.
Practical EM Shielding. On-line tutorial available from LearnEMC.com available at this link.
Turner BD. Shielding issues for medical products. Conformity, May 2007, pp 48-53.
Related Questions
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