X- and Y-Gradients
How do you make x- and y-direction gradients?
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Double-saddle (Golay) coil configuration for producing a y-gradient. To produce an x-gradient the coils are rotated by 90º along the surface of the cylinder.
Vintage (ca.1986) MR scanner showing gradients (y-gradient Golay coil marked by red arrow, z-gradient Maxwell coil marked by yellow arrow).
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The design for transverse gradients used in cylindrical MR magnets is based on a "double-saddle" coil configuration originally described in 1958 by Marcel Golay. The simplest form of this coil set consists of 4 inner and 4 outer arcs on the surface of a cylinder connected by 8 straight wires running parallel to the z-axis. The current along the inner arcs are primarily responsible for creating the required gradient, while the straight wires parallel to the z-axis serve as return pathways for current and do not contribute to the gradient field.
The homogeneity of the simple Golay configuration can be improved significantly by adding more arcs and curves. Nearly all modern MR scanners use some variation of a "fingerprint" coil design with distributed windings. Many advanced configurations have been developed, including bizarre- appearing 3D computer assisted designs. |
Advanced Golay design in fingerprint pattern, very typical for modern MR scanners in 2014.
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Distributed winding Golay coils in fingerprint pattern.
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Advanced 3D design.
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References
Carlson JW. Compact transverse magnetic gradient coils and dimensioning method therefor. United States Patent #4,755,755, July 5, 1988. (Contains a good discussion of Golay coil geometry and construction.)
Schenck JF, Hussain MA, Edelstein WA. Transverse gradient magnetic field coils for nuclear magnetic resonance imaging. United States Patent #4,646,024, Feb. 24, 1987. (Explanation of the fingerprint coil design and physics).
Golay MJE. Field homogenizing coils for nuclear spin resonance instrumentation. Rev Sci Instrum 1958; 29:313-316.
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.
Schmitt F. The gradient system. Understanding gradients from an EM perspective. Proc Intl Soc Mag Reson Med 2013; 21:1-13.
Carlson JW. Compact transverse magnetic gradient coils and dimensioning method therefor. United States Patent #4,755,755, July 5, 1988. (Contains a good discussion of Golay coil geometry and construction.)
Schenck JF, Hussain MA, Edelstein WA. Transverse gradient magnetic field coils for nuclear magnetic resonance imaging. United States Patent #4,646,024, Feb. 24, 1987. (Explanation of the fingerprint coil design and physics).
Golay MJE. Field homogenizing coils for nuclear spin resonance instrumentation. Rev Sci Instrum 1958; 29:313-316.
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.
Schmitt F. The gradient system. Understanding gradients from an EM perspective. Proc Intl Soc Mag Reson Med 2013; 21:1-13.