A signal to noise improvement method that is accomplished by taking the average of several FID`s made under similar conditions to suppress the effects of random variations or random artifacts. It is a common method to increase the SNR by averaging several measurements of the signal.
The number of averages is also referred to as the number of excitations (NEX) or the number of acquisitions (NSA). Doubling the number of acquisitions will increase the SNR by √2. The approximate amount of improvement in signal to noise (SNR) ratio is calculated as the square root of the number of excitations.
By using multiple averages, respiratory motion can be reduced in the same way that multiple averages increase the signal to noise ratio. NEX/NSA will increase SNR but will not affect contrast unless the tissues are being lost in noise (low CNR). Scan time scales directly with NEX/NSA and SNR as the square root of NEX/NSA.
The use of phase array coils allows the number of signal averages to be decreased with their superior SNR and resolution, thereby decreasing scan time.

An RF array coil consisting of several separately resonant elements, any one of which can be selected as the receiver coil at a particular time. Coils not in use are decoupled. Applications of switchable coils include imaging the whole spine without patient repositioning (where the coil elements may collectively be known as a ladder coil), imaging of bilateral structures such as TMJ or orbit using separate coils, or imaging using a coil with selectable field of view.

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