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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References
Yee KS (1996) Numerical solution of initial boundary value problems involving Maxwell’s equations in isotropic media. IEEE Trans Antennas Propag 14:302–307
Collins CM, Wang Z (2011) Calculation of radiofrequency electromagnetic fields and their effects in MRI of human subjects. Magn Reson Med 65:1470–1482
Lemdiasov RA, Obi AA, Ludwig R (2011) A numerical postprocessing procedure for analyzing radio frequency MRI coils. Concepts Magn Reson Part A 38A:133–147
Kuehne A, Goluch S, Waxmann P, Seifert F, Ittermann B, Moser E, Laistler E (2015) Power balance and loss mechanism analysis in RF transmit coil arrays. Magn Reson Med 74:1165–1176
Guérin B, Gebhardt M, Serano P, Adalsteinsson E, Hamm M, Pfeuffer J, Nistler J, Wald LL (2015) Comparison of simulated parallel transmit body arrays at 3 T using excitation uniformity, global SAR, local SAR, and power efficiency metrics. Magn Reson Med 73:1137–1150
Wu X, Zhang X, Tian J, Schmitter S, Hanna B, Strupp J, Pfeuffer J, Hamm M, Wang D, Nistler J, He B, Vaughan TJ, Ugurbil K, de Moortele P-FV (2015) Comparison of RF body coils for MRI at 3 T: a simulation study using parallel transmission on various anatomical targets. NMR Biomed 28:1332–1344
El-Sharkawy AM, Qian D, Bottomley PA, Edelstein WA (2012) A multichannel, real-time MRI RF power monitor for independent SAR determination. Med Phys 39:2334–2341
Liu W, Kao CP, Collins CM, Smith MB, Yang QX (2013) On consideration of radiated power in RF field simulations for MRI. Magn Reson Med 69:290–294
Deniz CM, Vaidya MV, Sodickson DK, Lattanzi R (2015) Radiofrequency energy deposition and radiofrequency power requirements in parallel transmission with increasing distance from the coil to the sample. Reson Med, Magn. doi:10.1002/mrm.25646
Brunner DO, De Zanche N, Fröhlich J, Paska J, Pruessmann KP (2009) Travelling-wave nuclear magnetic resonance. Nature 457:994–998
Andreychenko A, Kroeze H, Klomp DW, Lagendijk JJ, Luijten PR, van den Berg CA (2013) Coaxial waveguide for travelling wave MRI at ultrahigh fields. Magn Reson Med 70:875–884
Alt S, Müller M, Umathum R, Bolz A, Bachert P, Semmler W, Bock M (2012) Coaxial waveguide MRI. Magn Reson Med 67:1173–1182
Weidemann G, Seifert F, Hoffmann W, Seemann R, Waxmann P, Ittermann B (2015) A system for in situ S-parameter measurements of MR transmit arrays. In: Proceedings of the 23rd scientific meeting, international society for magnetic resonance in medicine, Toronto, p 1775
Athey TW, Stuchly MA, Stuchly SS (1982) Measurement of radio frequency permittivity of biological tissues with an open-ended coaxial line: part I—experimental results. IEEE Trans Microw Theory Tech 30:82–86
Stuchly MA, Athey TW, Samaras GM, Taylor GE (1982) Measurement of radio frequency permittivity of biological tissues with an open-ended coaxial line: part II—experimental results. IEEE Trans Microw Theory Tech 30:87–92
Weidemann G, Seifert F, Hoffmann W, Pfeiffer H, Ittermann B (2015) A method for the measurement of the RF power radiated by 7T transmit coils. In: Proceedings of the 23rd scientific meeting, international society for magnetic resonance in medicine, Toronto, p 1865
Klepsch T, Lindel TD, Hoffmann W, Botterweck H, Ittermann B, Seifert F (2012) Calibration of fibre optic RF E/H-field probes using a magnetic resonance (MR) compatible TEM cell and dedicated MR measurement techniques. Biomed Tech 57(Suppl 1):119–122
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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The authors declare that they have no conflict of interest.
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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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DOI: https://doi.org/10.1007/s10334-016-0551-6