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The radiofrequency (RF) transmitter is the generator of the radiofrequency current which is delivered to the transmitting coil. This creates a signal which is used to excite protons in the imaging field. Radiofrequency coils can be both transmitters and receivers of the radiofrequency signal or receivers alone. A radiofrequency coil that performs both of these actions is called a transmit receiver coil or a transceiver coil.
A simplified version of a transmit coil is a loop of wire with a current passed through it at Larmor frequency, situated at right angles to the main magnetic field. This creates an oscillating magnetic field at 90 degrees to B0. This rotates the magnetization in a pulse sequence. This energy is transmitted at the resonant frequency of hydrogen in the form of a short, intense burst of RF. More complicated versions of this single loop transmit coil can be created by having multiloop systems (for example, two-loop Helmholtz coils).
Typically, the B1 field is approximately 25 μT. This means that the radiofrequency pulse must be applied for approximately 0.23 ms to achieve a selective 90 degree rotation of the magnetization vector. Transmit coils are also capable of receiving radiofrequency and thus also act as receiver coils.
Transmit coils that produced an oscillating B1 field in only a single dimension (such as single-loop and Helmholtz coils) are referred to as linearly polarized. Linearly polarized RF coils were classically used as transmit-receive coils, but have now been replaced by the more efficient quadrature or circularly polarized coils. Transmit-receive coils were once the norm in medical imaging, but are now used less commonly. However they remain the standard in head and knee imaging. The transmit/receive coils used in MRI are called volume coils.
- 1. Jerrold T. Bushberg, John M. Boone. The Essential Physics of Medical Imaging. (2011) ISBN: 9780781780575