The RF (radio frequency) coils in a magnetic resonance imager are used for excitation of the nuclei and for signal detection, just like antennas. The purpose of MRI is to make slice images of the body, so only a small slice will be excited. When the coil receives the signal from that slice it also receives noise from the rest of the body which gives a noisy picture:

For a better noise performance a separate coil for detection is used, which can not "see" the whole body because it is too small and it will thus not receive all the noise:

The disadvantage is that the size of the slice that can be made will of course also decrease, but more than one coil can be used to cover the whole slice:

This approach leads to a variety of specialized coils: for instance head coils, knee coils and also flexible coils:

The more specialised a coil is, the better its image quality is.

With this basic knowledge you can understand why someone might need the
COil CONstruction UTility »