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.

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'

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'