These are coils which can send RF pulses and receive signals. There are two types of receiver coil: volume coils and body coils.
The volume coils can send RF pulses and receive the MR signal. They can obtain a signal of the whole body and ensure a homogeneous excitation but, if used as receivers, they reduce the signal to noise ratio (S/N ratio) as they receive the noise of the whole excited volume.
The body coils are just receivers. They are placed as close as possible to the region to be scanned. Their advantage is that they diminish the S/N ratio because the signal is not degraded by the noise of a large excited volume. This allows the scanning of small areas with thinner slices.
In the magnetic resonance system, there are three sets of gradient coils, oriented in the sagittal, transverse and dorsal planes to produce a linear variation of the magnetic field. This is necessary for slice selection and to encode phase and frequency to produce cross-sectional images. In each plane, two gradient coils are placed on each end of the main magnet. The first generates a magnetic field (B1) in the same direction as the main magnet (Bo). The second generates a magnetic field (B2) in the opposite direction. The aim is to create a smooth linear decreasing magnetic field (Br). Thus: Br = Bo + B1 - B2
 The signal comes from the excited protons and the noise comes from active nuclei moving from high and low states of energy due to the thermal activity caused by the MRI system.