Gradient Coils
What are gradient coils?
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Complete gradient system showing coils mounted along the inner bore of the scanner driven by a powerful current amplifiers and cooled by water chillers in the adjacent MR equipment room. (Radiofrequency (RF) coils, located just inside the gradient coils, are also pictured. See analogic.com for more information.)
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Gradients are loops of wire or thin conductive sheets on a cylindrical shell lying just inside the bore of an MR scanner. When current is passed through these coils a secondary magnetic field is created. This gradient field slightly distorts the main magnetic field in a predictable pattern, causing the resonance frequency of protons to vary in as a function of position. The primary function of gradients, therefore, is to allow spatial encoding of the MR signal. Gradients also are critical for a wide range of "physiologic" techniques, such as MR angiography, diffusion, and perfusion imaging.
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Historically, gradients coils were were composed of individual wires wrapped onto cylindrical formers made of fiberglass and coated with epoxy resin. Many laboratory instruments and very-high-field human scanners still use this method. Today, however, most widely manufactured superconducting scanners utilize distributed windings in a "fingerprint" pattern consisting of multiple thin metallic strips or large copper sheets etched into complex patterns and applied to the cylinder.
Gradient coils with discrete wire windings in a 7T scanner (from Human Connectisome Project)
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Gradient coil with thin metallic strips being applied in a fingerprint pattern to a former (courtesy Siemens)
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Gradient coil with distributed windings etched into copper conducting sheets (courtesy GEMS)
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Gradient cabinet containing 3 independent power amplifiers for x-, y-, and z-gradients.
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Three sets of gradient coils are used in nearly all MR systems: the x-, y-, and z-gradients. Each coil set is driven by an independent power amplifier and creates a gradient field whose z-component varies linearly along the x-, y-, and z-directions, respectively. The design of the z-gradients is usually based on circular (Maxwell) coils, while the transverse (x- and y-) gradients typically have a saddle (Golay) coil configuration. The physical principles underlying these designs will be considered in subsequent questions.
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Gradient coils for the 3 cardinal directions: x, y, and z
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Applying a gradient causes a frequency variation of protons as a function of position along the direction of the gradient. This change in frequency can be used for spatial encoding. If the gradient is played out during slice selection and again during signal readout, a slice can be selected perpendicular to the gradient direction. For example, if the z-gradient is turned on in this fashion, a transverse slice is created in a supine patient. Oblique slices can be obtained by turning on two or more gradients simultaneously.
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A frequent misconception about gradient fields is that the x- and y-gradients somehow skew or shear the main (Bo) field transversely. That is not the case as is shown in the diagram to the right. The x- and y-gradients provide augmentation in the z-direction to the Bo field as a function of left-right or anterior-posterior location in the gantry. The x- and y-gradients (ideally, at least) do not produce substantial components within the bore perpendicular to Bo.
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Action of gradient fields. Note that the x- and y-gradients do NOT generate transverse components that rotate/tip Bo from side-to-side or up-and down. The x- and y-gradients act only to create in-plane "skewing" of the z-components of Bo.
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References
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.
Lauterbur PC. Image formation by induced local interactions: examples employing nuclear magnetic resonance (pdf). Nature 1973; 242:190-191. (The classic paper demonstrating how gradients can be used for imaging that won Lauterbur the Nobel Prize in 2003).
Hidalgo-Tabon SS. Theory of gradient coil design methods for magnetic resonance imaging. Concepts Mag Res Part A 2001; 36A:223-242.
Lauterbur PC. Image formation by induced local interactions: examples employing nuclear magnetic resonance (pdf). Nature 1973; 242:190-191. (The classic paper demonstrating how gradients can be used for imaging that won Lauterbur the Nobel Prize in 2003).
Related Questions
What is a gradient?
I'm confused. Isn't a gradient some type of coil?
How do you make a z-direction gradient?
How do you make x- and y-direction gradients?
All the diagrams I have seen for gradients deal with cylindrical scanners. How are the gradients designed for open-bore scanners?
What is a gradient?
I'm confused. Isn't a gradient some type of coil?
How do you make a z-direction gradient?
How do you make x- and y-direction gradients?
All the diagrams I have seen for gradients deal with cylindrical scanners. How are the gradients designed for open-bore scanners?