"Active" shield confusion
I'm confused. I thought active shielding had to do with controlling the main magnetic field.
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There are three MR system components containing the word "active" that are commonly confused. Each of these is a coil system with some sort of counter current that corrects or modifies another magnetic field. They are:
Active Shielding. A set of coils used to reduce the static fringe field generated by the main magnet. For superconducting magnets these are usually located within the helium-filled cryostat just outside the main field windings. For open bore permanent scanners they are resistive coils located at the edge of the magnet pole pieces.
Active Shimming. A set of coils, typically resistive, used to increase magnetic field homogeneity. This contrasts with passive shimming, a technique to improve homogeneity by installing metallic shim plates inside the bore of the scanner.
Actively Shielded Gradients. This is the technique described in the previous question, wherein coils just superficial to the imaging gradients are used to dynamically cancel changing magnetic fields and reduce eddy currents generated in the main magnet structure.
Active Shielding. A set of coils used to reduce the static fringe field generated by the main magnet. For superconducting magnets these are usually located within the helium-filled cryostat just outside the main field windings. For open bore permanent scanners they are resistive coils located at the edge of the magnet pole pieces.
Active Shimming. A set of coils, typically resistive, used to increase magnetic field homogeneity. This contrasts with passive shimming, a technique to improve homogeneity by installing metallic shim plates inside the bore of the scanner.
Actively Shielded Gradients. This is the technique described in the previous question, wherein coils just superficial to the imaging gradients are used to dynamically cancel changing magnetic fields and reduce eddy currents generated in the main magnet structure.
References
Turner BD. Shielding issues for medical products. Conformity, May 2007, pp 48-53.
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.
Jezzard P. Shim coil design, limitations and implications. Abstracts from the International Society of Magnetic Resonance in Medicine (ISMRM) Annual Meeting, May, 2006.
Turner BD. Shielding issues for medical products. Conformity, May 2007, pp 48-53.
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.
Jezzard P. Shim coil design, limitations and implications. Abstracts from the International Society of Magnetic Resonance in Medicine (ISMRM) Annual Meeting, May, 2006.
Related Questions
What are actively shielded gradients?
How does active shimming work?
How do you reduce the size of the fringe field so it doesn't cause problems?
What are actively shielded gradients?
How does active shimming work?
How do you reduce the size of the fringe field so it doesn't cause problems?