MRI involves the absorption and emission of energy by nuclei at a specific resonant (Larmor) frequency. The Larmor frequency scales directly with main magnetic field strength (Bo), and for clinical MRI lies in the range of tens to hundreds of MHz. These frequencies are part of the electromagnetic spectrum commonly used for radio transmission.
A sophisticated electronic "RF-front end" is responsible for generating, shaping, and amplifying the electrical currents required to produce the B1 field. The basic components in this RF-transmit chain are:
Pulse modulator. The B1 fields used in nearly all clinical MR imaging applications are not transmitted as continuous waves, but in short (1-5 ms) bursts, called RF-pulses. The continuous carrier wave from the frequency synthesizer must therefore be "chopped up" into small pieces and these pieces appropriately "shaped" into pulses as dictated by the particular imaging application. The contours of each RF-pulse are specified using 100-200 data points, and are therefore of low-frequency (measured in kHz). The pulse-shape data is used to modulate the carrier wave so that the resultant output is a mixture of frequencies centered around the carrier.
Quadrature Hybrid Coupler. The output of the power amplifier is typically split into two equal parts by means of a quadrature hybrid coupler device. The resultant outputs are 90° out of phase with one another and are used to feed the two ports of the quadrature transmit coil. The two outputs of the coupler are commonly known as I and Q, standing for "in phase" and "quadrature" respectively.
Transmit/Receive (T/R) Switch and Coil. The currents in the I and Q outputs of the coupler is now headed for the RF-transmitter coils. As scanners may have several possible transmitter coils, electronic switching circuitry is necessary to make sure that current is delivered to the proper coil at the proper time. Additionally, sometimes the same coils are used to both transmit B1 and receive the MR signal. For these coils a special T/R switch is required to isolate the two functions and make sure the powerful electric currents used for transmission do not go into and burn up the sensitive receiver circuitry.
Advanced Discussion (show/hide)»
Although RF-coils transmit electromagnetic radiation in the range of "radio waves", the emitted MR signal is not itself a conventional radio wave. (See the excellent but highly technical discussion in Hoult DI. The origins and present status of the radio wave controversy in NMR. Concepts Mag Reson Part A 2009;34A(4):193-216.) It is OK to talk about radio frequencies, but probably not radio waves.
Introduction to NMR/MRI amplifiers. CPC Amps, Inc. 2012.
Spectrum Wiki Website. Just type in the frequency of interest and this site will tell you how it is used world-wide.
What are the function(s) of radiofrequency (RF) coils?
What are gradient coils?
How do RF-transmit coils work?
Isn't the MR signal just re-emission of absorbed radio waves?