The power balance of multichannel transmit coils is a central consideration in assessing performance and safety issues. At ultrahigh fields, in addition to absorption and reflection, radiofrequency (RF) radiation into the far field becomes a concern.
Materials and methods
We engineered a system for in situ measurement of complex-valued scattering parameter (S-parameter) matrices of multichannel transmit coils that allows for the calculation of the reflected and accepted power for arbitrary steering conditions. The radiated power from an RF coil inside a large single-mode waveguide couples to that mode. Finite-difference time-domain simulations were used for the calculations, and E-field probes were used to measure the electric field distribution, and hence the radiated power, in the waveguide. To test this concept, an eight-channel shielded-loop array for 7T imaging was studied inside a 280-cm-long cylindrical waveguide with a 60-cm diameter.
For a 7T parallel-transmit coil, the S-parameters were measured and the reflected power calculated as a function of steering conditions. Maximum radiated power was observed for the circularly polarized mode.
A system was developed for in situ S-parameter measurements combined with a method for determining radiated power, allowing a complete assessment of the power balance of multichannel transmit coils at 7T.
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This work was supported by the European Metrology Research Programme (EMRP; Grant no. HLT06). The EMRP is jointly funded by the EMRP participating countries within the European Association of National Metrology Institutes (EURAMET) and the European Union.
Conflict of interest
The authors declare that they have no conflict of interest.
A statement on ethical compliance is not applicable as the described measurments were all done on phantoms. No human subjects and no animals were involved in this study.
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Weidemann, G., Seifert, F., Hoffmann, W. et al. Measurements of RF power reflected and radiated by multichannel transmit MR coils at 7T. Magn Reson Mater Phy 29, 371–378 (2016). https://doi.org/10.1007/s10334-016-0551-6
- Magnetic resonance imaging
- Parallel transmission
- Power balance
- Radiated power
- RF safety