Measurements of RF power reflected and radiated by multichannel transmit MR coils at 7T

Abstract

Objective

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.

Results

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.

Conclusion

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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Acknowledgments

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.

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Correspondence to Bernd Ittermann.

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The authors declare that they have no conflict of interest.

Ethical approval

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

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Keywords

  • Magnetic resonance imaging
  • Parallel transmission
  • Power balance
  • Radiated power
  • RF safety