Coils carrying a relatively small current that are used to provide auxiliary magnetic fields in order to compensate for inhomogeneities in the main magnetic field of a MR system.

See also Shimming.

A saddle coil has a cylindrical body, with one or more turns of wire (older versions) or foil (newer versions) on each side. It generates a very homogenous field in the direction of its long axis. To achieve a higher B₁ homogeneity there are different developments like Folded Litz-foil Saddle Coils or Etched Litz-foil Saddle Coils.

See also the related poll result: '3rd party coils are better than the original manufacturer coils'

Radio frequency coil that surrounds either the whole body, or one specific region, such as the head or a knee. Volume coils have a better RF homogeneity than surface coils, which extends over a large area. The most commonly used design is a (birdcage) bird cage coil. This consists of a number of wires running along the z-direction, arranged to give a cosine current variation around the circumference of the coil. It is possible to use the same coil to transmit and receive, or to use two separate coils. The largest volume coil, the body coil is typically a transmit and receive coil. If two separate coils are used, works the body coil as the transmitter and a smaller coil as the receiver.

See also the related poll result: '3rd party coils are better than the original manufacturer coils'

The interaction of the patient with the RF coil, which causes shifts of the resonance frequency and damping of the coil's resonance and hence reduction of the quality factor because of magnetic induction and dielectric losses in the patient.
The design and construction of a MRI coil is determined by the load on the coil. The load is either a phantom or the actual sample being imaged.

See also Radio Frequency Coil.

RF coil pair arranged with their magnetic fields at right angles to each other in such a way as to minimize their mutual magnetic interaction.