A coil consists of one or more loops of conductive wire, looped around the core of the coil. Coils are part of the hardware of MRI machines and are used to create a magnetic field or to detect a changing magnetic field by voltage induced in the wire. A coil is usually a physically small antenna.
The perfect coil produces a uniform magnetic field without significant radiation.
Different types of MRI coils are used in MR systems:
Gradient coils are used to produce controlled variations in the main magnetic field (B0) to provide spatial localization of the signals and to apply reversal pulses in some imaging techniques.
MR imaging radio frequency coils to receive and/or transmit the RF signal.
Shim coils provide auxiliary magnetic fields in order to compensate for inhomogeneities in the main magnetic field of the MRI machine.

See also Gradient Coil, Radio Frequency Coil, Hardware and Coil Loading.

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

Quick Overview

Eddy currents can cause artifacts in images and may seriously degrade overall magnet performance.

Common uses to reduce the influence of eddy currents on gradient fields are eddy current compensation and shielded gradient coils (active or passive). If persistent, it must be addressed by a service representative to verify the magnet performance.

See also Eddy Current Compensation.

Eliminate Gibb's artifact (Border ringing) for sharp images. Means of reducing the influence of eddy currents on pulsed gradient fields by employing an electrical preemphasis in the gradient amplifiers. Usually multiple time constants have to be used to correct for eddy current effects in various structures of the MR system such as the cryoshields and RF-shields.

See also Eddy Currents and Shielded Gradient Coils.

Electric currents induced in a conductor by a changing magnetic field or by motion of the conductor through a magnetic field.
One of the sources of concern about a potential hazard to subjects in very high magnetic fields or rapidly varying gradient or main magnetic fields. Can be a practical problem in the cryostat of superconducting magnets. Eddy currents can cause artifacts in images and may seriously degrade overall magnet performance. Common means to reduce the influence of eddy currents on gradient fields are eddy current compensation and shielded gradient coils (active or passive).

See also Eddy Current Compensation.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'

The word gradient (from grade) means the inclination of a surface along a given direction. In MRI, gradient stands for gradient field and/or gradient coil. Inside the main magnet are three gradient coils located, which produce the desired gradient (magnetic) fields. These fields are used to alter (collectively and sequentially) the influence of the static magnetic field B0 on the imaged object by inc- or decreasing the field strength and changing the direction. Through this influence selective spatial excitation and spatial encoding (each voxel resonate at a different frequency) is possible. Gradients are also utilized in another way for fast imaging sequences.

See also Slew Rate and Duty Cycle.