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.
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.
Keywords:
Magnetic resonance imaging; Parallel transmission; Power balance; RF safety; Radiated power
Authors: Stefan Alt; Marco Müller; Reiner Umathum; Armin Bolz; Peter Bachert; Wolfhard Semmler; Michael Bock Journal: Magn Reson Med Date: 2011-10-21 Impact factor: 4.668
Authors: Bastien Guérin; Matthias Gebhardt; Peter Serano; Elfar Adalsteinsson; Michael Hamm; Josef Pfeuffer; Juergen Nistler; Lawrence L Wald Journal: Magn Reson Med Date: 2014-04-18 Impact factor: 4.668
Authors: Andre Kuehne; Sigrun Goluch; Patrick Waxmann; Frank Seifert; Bernd Ittermann; Ewald Moser; Elmar Laistler Journal: Magn Reson Med Date: 2014-10-16 Impact factor: 4.668
Authors: Anna Andreychenko; Hugo Kroeze; Dennis W J Klomp; Jan J W Lagendijk; Peter R Luijten; Cornelis A T van den Berg Journal: Magn Reson Med Date: 2012-09-28 Impact factor: 4.668
Authors: Wanzhan Liu; Chien-ping Kao; Christopher M Collins; Michael B Smith; Qing X Yang Journal: Magn Reson Med Date: 2012-04-03 Impact factor: 4.668
Authors: Xiaoping Wu; Xiaotong Zhang; Jinfeng Tian; Sebastian Schmitter; Brian Hanna; John Strupp; Josef Pfeuffer; Michael Hamm; Dingxin Wang; Juergen Nistler; Bin He; Thomas J Vaughan; Kamil Ugurbil; Pierre-Francois Van de Moortele Journal: NMR Biomed Date: 2015-08-30 Impact factor: 4.044