Gradient coils

Last revised by Andrew Murphy on 2 Apr 2020

Gradient coils are used to produce deliberate variations in the main magnetic field (B0). There are three sets of gradient coils, one for each direction. The variation in the magnetic field permits localization of image slices as well as phase encoding and frequency encoding. The set of gradient coils for the z axis are Helmholtz pairs, and for the x and y axes, paired saddle coils.

Gradient coils need to provide linear gradations of the magnetic field. When turned on, one end or side the bore of the magnet has a lesser strength and the other a greater strength than the static main magnetic field. Drop off of the z gradient near the bore of the magnet can cause misplacement of objects in the image if signal is detected beyond the gradients. This should not happen with appropriate engineering.

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