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'

A RF coil, often a transmit receive coil with a number of wires running along the z-direction, arranged to give a cosine current variation around the circumference of the coil, which looks like a bird cage. The bird cage coil works on a different principle to conventionally tuned local and surround coils in that it behaves like a tuned transmission line with one complete cycle of standing wave around the circumference. The frequency supply is generated by an oscillator, which is modulated to form a shaped pulse by a product detector controlled by the waveform generator. The signal must be amplified to 1000's of watts. This can be done using either solid state electronics, valves or a combination of both.
The bird cage coil design provides the best field homogeneity of all RF imaging coils.
One advantage is that it is simple to produce an exceedingly uniform B1 radio frequency field over most of the coil's volume, with the result of images with a high degree of uniformity.
A second advantage is that nodes with zero voltage occur 90° away from the driven part of the coil, thus facilitating the introduction of a second signal in quadrature, which produces a circularly polarized radio frequency field.
This type of volume coil is used for brain (head) MRI, or MR imaging of joints, such as the wrist or knees.

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

A coil designed to excite or detect spins using two orthogonal transmit and/or receive channels. As a transmitter coil, there is a factor of 2 reductions in power required. The receiver coil has a better SNR than a linearly polarized 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.

A Helmholtz pair consists of two identical circular magnetic coils that are placed symmetrically one on each side of the experimental area along a common axis, and separated by a distance equal to the radius of the coil. Actually, a slightly larger separation improves the field uniformity. Each coil carries an equal electrical current flowing in the same direction. A cylindrical region extending between the centers of the two coils and approximately 1/5 of their diameter will have a nearly spatially uniform magnetic field.
In MRI, the Helmholtz pair coils are used as z-gradient coils to produce linear variations in the main magnetic field along the z-axis. Also in use as z-gradient coils are the Maxwell coils (three coils in a slightly more complicated geometry than the Helmholtz configuration). These coils are only occasionally used as RF coils for imaging.

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.

A coil that produces an RF field with circular polarization. The RF power received from the RF power amplifier comes in two signals (quadrature detection), which have a phase difference of 90°. The RF transmit coil converts the power into a circularly polarized RF magnetic field.
Quadrature coils can be used as both, transmit and/or receive coil. When used as a transmitter coil a factor of two power reduction over a linear coil results; as a receiver an increase in SNR of up to a factor of √2, can be achieved.

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

Single turn solenoid is a transmit and receive RF imaging coil that, in general, has a cylindrical shape. The solenoidal configuration of this coil is a further developed planar surface coil. While surface coils have a strong coupling and high signal to noise ratio to nearby signals they have a marked loss of signal beyond one radius from the center of the coil. With a solenoidal coil design the imaging object lies within one radius from the coil center, which improved imaging (of extremities, such as wrist or knee).

See also Radio Frequency Coil and Imaging of the Extremities.