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Magnetic Resonance Materials in Physics, Biology and Medicine

Published in:

01-06-2016 | Research Article

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

Authors: Gerd Weidemann, Frank Seifert, Werner Hoffmann, Harald Pfeiffer, Reiner Seemann, Bernd Ittermann

Published in: Magnetic Resonance Materials in Physics, Biology and Medicine | Issue 3/2016

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.

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 65:1470–1482CrossRefPubMedPubMedCentral

Lemdiasov RA, Obi AA, Ludwig R (2011) A numerical postprocessing procedure for analyzing radio frequency MRI coils. Concepts Magn Reson Part A 38A:133–147CrossRef

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 74:1165–1176CrossRefPubMed

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 73:1137–1150CrossRefPubMedPubMedCentral

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 28:1332–1344CrossRefPubMed

El-Sharkawy AM, Qian D, Bottomley PA, Edelstein WA (2012) A multichannel, real-time MRI RF power monitor for independent SAR determination. Med Phys 39:2334–2341CrossRefPubMedPubMedCentral

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 69:290–294CrossRefPubMedPubMedCentral

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

10.

Brunner DO, De Zanche N, Fröhlich J, Paska J, Pruessmann KP (2009) Travelling-wave nuclear magnetic resonance. Nature 457:994–998CrossRefPubMed

11.

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 70:875–884CrossRefPubMed

12.

Alt S, Müller M, Umathum R, Bolz A, Bachert P, Semmler W, Bock M (2012) Coaxial waveguide MRI. Magn Reson Med 67:1173–1182CrossRefPubMed

13.

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

14.

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 30:82–86CrossRef

15.

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 30:87–92CrossRef

16.

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

17.

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

Title: Measurements of RF power reflected and radiated by multichannel transmit MR coils at 7T
Authors: Gerd Weidemann
Frank Seifert
Werner Hoffmann
Harald Pfeiffer
Reiner Seemann
Bernd Ittermann
Publication date: 01-06-2016
Publisher: Springer Berlin Heidelberg
Published in: Magnetic Resonance Materials in Physics, Biology and Medicine / Issue 3/2016
Print ISSN: 0968-5243
Electronic ISSN: 1352-8661
DOI: https://doi.org/10.1007/s10334-016-0551-6

At a glance: The ONWARDS insulin icodec trials

Springer Medicine

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

Abstract

Objective

Materials and methods

Results

Conclusion

At a glance: The ONWARDS insulin icodec trials

Springer Medicine

Abstract

Objective

Materials and methods

Results

Conclusion

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