The Basics of MRI

RF Coil Tuning


Variations in the size and tissue composition of the anatomy placed in an imaging coil affect the amount of RF energy getting into and the amount of signal detected from the imaged anatomy. For these reasons the RF coil should be tuned whenever it is known that the composition of the anatomy or material in the coil changes. Tuning the probe entails adjusting two types of capacitors on the RF probe. One capacitor is called the matching capacitor and the other the tuning capacitor. The matching capacitor matches the impedance of the coil with imaged object to that of the 50 Ohm cable coming from the spectrometer. The tuning capacitor changes the resonance frequency of the RF coil.

Very few imagers have an RF coil tuning mode of operation. Therefore this operation is performed external to the imager using either a vector-impedance meter or a return-loss bridge. A method using a return-loss bridge is presented here. The following instrumentation is required:

  1. return-loss bridge
  2. oscilloscope
  3. RF sweeper
  4. diode detector
  5. 5O W load
  6. assorted lengths of 50 W cable.

This instrumentation is connected as shown in the animation window. The oscilloscope is operated in the XY mode. The connection from the RF sweeper to the oscilloscope is provides a DC voltage proportional to the frequency to the x-axis input of the oscilloscope. This x-axis input must be calibrated. This can often be done on sweepers with an external frequency standard or from an internal frequency standard. The signal from the diode detector to the oscilloscope is a DC voltage from the half-wave rectified RF sent into the detector.

The oscilloscope displays the reflected power vs. frequency. The goal is to adjust the display so that the reflected power from the RF coil is zero at the resonance frequency of the nucleus you are examining. As the material within the coil changes, so does the bandwidth of the RF probe. This is significant because it affects the amount of RF power needed to produce a 90 degree pulse. The larger the bandwidth, the more power is needed to produce the 90 degree rotation.


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