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Radiofrequency coils (RF coils) are the receivers, and sometimes also the transmitters, of radiofrequency (RF) signals in the 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 coils generating and receiving electromagnetic fields. Atomic nuclei of interest in MRI studies have their own resonant frequencies, in the radiofrequency portion of the electromagnetic spectrum. 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 ''field'' transmitter at radio frequency, but a poor EM ''radiation'' transmitter at radio frequency. The receiver coil picks up the RF electromagnetic radiation produced by nuclear relaxation inside the subject. This signal consists of true EM radiation (radio waves), and these leave the subject as RF radiation, but they are of such low power as to also not cause appreciable RF interference that can be picked up by nearby radio tuners (in addition, MRI scanners are generally situated in metal mesh lined rooms which act as Faraday cages.) ==Types of Radiofrequency Coil== RF coils for MRI can be grouped into two different classes: volume coils and surface coils. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Radiofrequency coil」の詳細全文を読む スポンサード リンク
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