Gradients: Active shielding
What are actively shielded gradients?
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Design using actively shielded gradients. The gradient shields are slightly larger in diameter than the primary gradient coils and generate a counter magnetic field to reduce eddy currents induced in the magnet structure.
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The most effective way to control eddy currents is to prevent them from being generated in the first place. This was the motivation for developing actively-shielded (AKA self-shielded) gradients. In this configuration, shield coils are placed around the gradient coils used for imaging. These shield coils cancel/minimize the distant magnetic effects of the imaging gradients, reducing eddy currents induced shims, cryostat, main coils, and magnet housing. Today, nearly all MR scanners employ this technique which reduces eddy currents by a order of magnitude.
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As shown in the above diagram, the x-, y- and z- imaging gradient coils are mounted concentrically in the standard fashion. Just beyond the imaging gradients are three corresponding active shield coils. These have a topology in parallel with their associated imaging coils but are located at larger radii. The currents in these shields run in a direction opposite to those in the imaging gradient coils and are configured to minimize the magnetic flux that extends outside of the shields. Both the coils and shields are resistive and are driven by high voltage currents. Water cooling jackets, tubes, or channels typically are placed between the imaging and shielding coils to remove the appreciable heat generated from both.
The term “active shielding” should not be confused with “active shimming.” Shielding refers to containing or controlling the fringe fields external to the magnet; shimming refers to controlling fields within the magnet bore.
The term “active shielding” should not be confused with “active shimming.” Shielding refers to containing or controlling the fringe fields external to the magnet; shimming refers to controlling fields within the magnet bore.
References
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.
Mansfield P, Chapman BLW, Turner R, Bowley RM. Magnetic field screens (pdf). US Patent #4,978,920 (Dec. 18, 1990).
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.
Mansfield P, Chapman BLW, Turner R, Bowley RM. Magnetic field screens (pdf). US Patent #4,978,920 (Dec. 18, 1990).
Related Questions
I've heard that "eddy currents" cause problems for imaging. What are they?
I'm confused. I thought active shielding had to do with controlling the main magnetic field.
I've heard that "eddy currents" cause problems for imaging. What are they?
I'm confused. I thought active shielding had to do with controlling the main magnetic field.