Gradients for open scanners
All the diagrams I have seen for gradients deal with cylindrical scanners. How are the gradients designed for open-bore scanners?
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Representative cross-section of an open-bore magnet showing location of planar gradient coils between main magnet coils and RF coils.
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The configuration of open-bore scanners places different constraints on the design of gradient, shim, and radio-frequency coils. As can be seen in the cross-section (left), a patient in an open scanner lies between the magnet poles rather than within a cylinder. The corresponding gradient coils must therefore be flat rather than cylindrical.
This is no particular challenge for for z-axis gradients in a paired Maxwell arrangement. In most open scanners the Bo field (and z-axis) are oriented vertically. The planar Maxwell loops simply lie flat against the upper and lower poles of the magnet and create their gradient in the vertical direction. |
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The design of x- and y-gradients is somewhat more challenging in that the 3D Golay curved saddle-coil configuration used in cylindrical scanners is not possible. A variety of flat gradient designs exist, with most sharing principles embodied in the biplanar configuration shown in the figure to the right. The inner arcs of the coils are primarily responsible for producing the desired gradient. By rotating these coils in plane by 90º either x- or y-gradient fields can be produced. In practice transverse gradients typically have a more sophisticated design in a fingerprint distributed winding arrangement. Mirror image coils are also typically placed along both the upper and lower poles of the magnet.
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Field lines associated with current flows in a simplified planar gradient coil showing how a transverse gradient is produced
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Nested stack of gradient coils along one pole of an open bore scanner.
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Mirror-image transverse gradient coils in a fingerprint pattern located along upper and lower poles of magnet.
Actual transverse gradient coil assembly from an open-bore permanent magnet scanner.
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References
Momo F, Adriani O, Gualtieri G, Sotgui A. Generalised Anderson coils for magnetic resonance imaging. J Phys E: Sci Instrum 1988; 21:565-568.
Petropoulos LS. Finite size disc gradient coil set for open vertical field magnets. Mag Res Imaging 2000:18:615-624.
Qing-ming H, Shan-shan C, Hong-zhi W et al. The optimized scheme of performance parameters of gradient coils for permanent magnetic open architecture nuclear magnetic resonance system. 5th International Conference on Bioinformatics and Biomedical Engineering, (iCBBE), Wuhan, China, 2011, pp 1-5.
Zhang R, Zhu J, Fu Y et al. An optimized target-field method for MRI transverse biplanar gradient coil design. Meas. Sci. Technol 2011; 22:1-7.
Momo F, Adriani O, Gualtieri G, Sotgui A. Generalised Anderson coils for magnetic resonance imaging. J Phys E: Sci Instrum 1988; 21:565-568.
Petropoulos LS. Finite size disc gradient coil set for open vertical field magnets. Mag Res Imaging 2000:18:615-624.
Qing-ming H, Shan-shan C, Hong-zhi W et al. The optimized scheme of performance parameters of gradient coils for permanent magnetic open architecture nuclear magnetic resonance system. 5th International Conference on Bioinformatics and Biomedical Engineering, (iCBBE), Wuhan, China, 2011, pp 1-5.
Zhang R, Zhu J, Fu Y et al. An optimized target-field method for MRI transverse biplanar gradient coil design. Meas. Sci. Technol 2011; 22:1-7.