A RF
coil, often a
transmit receive coil with a number of wires running along the z-direction, arranged to give a cosine current variation around the circumference of the
coil, which looks like a bird cage.
The bird cage
coil works on a different principle to conventionally tuned local and surround coils in that it behaves like a tuned transmission line with one complete cycle of standing wave around the circumference. The
frequency supply is generated by an oscillator, which is modulated to form a shaped pulse by a
product detector controlled by the waveform generator. The signal must be amplified to 1000's of
watts. This can be done using either solid state electronics, valves or a combination of both.
The bird cage
coil design provides the best
field homogeneity of all RF imaging coils.
One advantage is that it is simple to produce an exceedingly uniform
B1 radio frequency field over most of the coil's volume, with the result of images with a high degree of uniformity.
A
second advantage is that nodes with zero voltage occur 90° away from the driven part of the
coil, thus facilitating the introduction of a
second signal in quadrature, which produces a circularly polarized
radio frequency field.
This type of
volume coil is used for
brain (head) MRI, or
MR imaging of joints, such as the
wrist or
knees.
See also the related poll result: '
3rd party coils are better than the original manufacturer coils'
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