Radiofrequency coil

Radiofrequency coils (RF coils) are the receivers, and sometimes also the transmitters, of radiofrequency (RF) signals in equipment used in magnetic resonance imaging (MRI).

The MR signal in MRI is produced by the process of resonance, which is the result of radiofrequency coils. They consist of two electromagnetic coils, the transmitter and receiver, which generate the field and receive the resulting signal. Atomic nuclei of interest in MRI studies have their own resonant frequencies, in the radiofrequency portion of the electromagnetic spectrum.[1]

Although the electromagnetic fields produced by the transmitting coil are in the RF range of tens of megahertz (often in the shortwave radio portion of the electromagnetic spectrum) at powers usually exceeding the highest powers used by amateur radio, there is very little RF interference produced by the MRI machine. The reason for this is that the MRI is a very poor radio transmitter, and is without an antenna. The RF frequency electromagnetic field produced in the "transmitting coil" is a magnetic near-field with very little associated changing electric field component (such as all conventional radio wave transmissions have). Thus, the high-powered electromagnetic field produced in the MRI transmitter coil does not produce much electromagnetic radiation at its RF frequency, and the RF power is confined to the coil space and not radiated as "radio waves." Thus, the transmitting coil is a good EM near-field generator at radio frequency, but a poor EM radiation transmitter at radio frequency.

The receiver coil picks up the oscillations at RF frequencies produced by precession of the magnetic moment of nuclei inside the subject. The signal acquired by the coil is thus an induced emf, and is not the result of picking up radio waves. This is a common misconception, and unfortunately, has propagated through the literature. MRI scanners are generally situated in metal mesh lined rooms which act as Faraday cages.)