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Volume coils are the transmit and receive radiofrequency coils which are used to both transmit and receive the radiofrequency signal in MRI. Most MRI scanners have what is called a body coil – which is a volume coil built into the bore of the magnet which transmits the radiofrequency for most examinations. Certain types of imaging require additional smaller volume coils – such as in head and occasionally in extremity imaging (e.g. the knees).
Typical head imaging makes use of head coils, or birdcage coils (because of their appearance). Birdcage coils are an example of circularly polarized coils or quadrature excitation. Circularly polarized coils (in contrast to linearly polarized coils) add a second set of coils perpendicular to the first, driven with sinusoidal currents shifted by 90 degrees. This results in a rotational B1 field (hence circularly polarized). This system is more efficient, as noncontributory counter-rotating fields are 180 degrees out of phase and cancel each other out, whereas the in-sync subfields accumulate to produce the electromagnetic field.
Volume coils generally have a homogeneous RF excitation across a large volume. This is important for transmission, but has the downside of receiving a lot of noise when the target region of interest is small.
- 1. Jerrold T. Bushberg, John M. Boone. The Essential Physics of Medical Imaging. (2011) ISBN: 9780781780575