Current carrying coils designed to produce a desired magnetic field gradient (so that the magnetic field will be stronger in some locations than others).
Proper design of the size and configuration of the coils is necessary to produce a controlled and uniform gradient. Three paired orthogonal current-carrying coils located within the magnet that are designed to produce desired gradient magnetic fields, which collectively and sequentially are superimposed on the main magnetic field (B0) so that selective spatial excitation of the imaging volume can occur.
Gradients are also used to apply reversal pulses in some fast imaging techniques. Gradient coils in general vary the main magnetic field, so that each signal can be related to an exact location. The gradient coil configuration for the z-axis consists of e.g., Helmholtz pair coils, and of paired saddle coils for the x- and y-axis.

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The figure 8 coil consists of two loops, connected in the middle like the number eight. This type of coil is used in magnetic resonance imaging and transcranial magnetic stimulation (TMS).

This term is commonly used for a particular kind of gradient coil, commonly used to create magnetic field gradients perpendicular to the main magnetic field. A golay coil (a special kind of saddle coils) produces a linear gradient in the x and y axes that requires wires running along the bore of the magnet. Such a coil produces a very linear field, but the linearity is lost rapidly away from the central plane. A number of pairs with different axial separations can be used to improve this.

Pair of current carrying coils used to create uniform magnetic field in the center of the space between them. For circular coils, their separation equals their radius.

A particular kind of gradient coil, commonly used to create magnetic field gradients along the direction of the main magnetic field. The maxwell coil consists of a pair of coils separated by 1.73 times their radius. Current flows in the opposite sense in the two coils, and produces a very linear gradient.

Pairs of saddle coils (such as the Golay coils) are used as the x- and y-axis of gradient coils and as extremity coils. This configuration produces a very linear, homogeneous magnetic field along its central axis.

Current-carrying gradient coils with reduced gradient fringe field inside of the magnet cryostat structures like cryoshields and He-vessel. The shielding can be accomplished by secondary actively driven coils or by passive screens, which are inductively coupled to the gradient coils. In both cases eddy currents outside of the gradient system will be reduced.

See also Eddy Currents.