RF Receiver Coils
Surface coils are often used for better signal reception. This is because they are sensitive to signals close to the coil, which means that surface coils detect the signal in an organ of interest more efficiently. But they also amplify noise signals more efficiently too, so why are they better? Their sensitivity to noise signals from the rest of the patient—further away from the coil—is reduced.
Although they are placed on or around the surface of a patient, they may be optimised to image deep-body structures. The signal from the RF power amplifier drives the transmit coils such that the desired RF magnetic field is produced. Note that it is not the transmission and reception of radio waves which is used in MRI, but magnetic induction from the oscillating magnetic fields (Hoult. The magnetic resonance myth of radio waves. Concepts in Magnetic Resonance 1: 1-5, 1989).
Sometimes the transmit coil (or body coil) is also used as the receive coil. If the transmit and receive coils are different coils, the receive coil must be detuned and the preamplifier blocked so that the large transmit signal cannot burn them out. The transmit coil is also separated from the gradient coils by an RF shield (not shown).
The RF power received from the RF power amplifier comes in two signals, which have a 90° phase difference. The RF transmit coil converts the power into a circularly polarised RF magnetic field (this is quadrature coil design).
The answer is in the information above, though the word isn't used. It simply means that the transmit coil is switched off so that energy is not transferred between the two coils. The transmit (body) coil sees a much larger volume than a surface coil used to receive the signal, and so it picks up the noise associated with RF eddy currents in that larger patient volume. If energy were then transferred to to surface coil from the transmit coil, SNR would be significantly affected. Of course if the body coil is also being used as the receive coil, then decoupling is not an issue.
Transmit coils: homogeneity in the RF magnetic field they produce. Receive coils: high SNR.
A phased array coil is essentially a number of overlapping coils (or elements), each with their own receiver detection circuitry (= expensive!). The term "phased array" comes from radar and is a bit confusing (where phase shift techniques are used in beam steering…whatever that is). A more appropriate name that you should use is "multi coil array".
They are used to increase the surface area acquired whilst maintaining the SNR obtainable with a single surface coil. The acquisition in each coil / element takes place simultaneously too, and so no increase in acquisition time is necessary. The multiple coil nature of the phased array coil is used in parallel imaging strategies such as SMASH and SENSE. SENSE uses the additional information of the sensitivity of each individual coil / element to allow fewer lines of k-space to be acquired. This information makes up for the fewer data acquired (e.g. speeds up a scan), although a trade-off of SNR occurs.