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'

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'