title: 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.
  
  
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany.
authors.forename: Gerd
authors.initials: G
authors.lastname: Weidemann
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Seifert
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany.
authors.forename: Werner
authors.initials: W
authors.lastname: Hoffmann
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany.
authors.forename: Harald
authors.initials: H
authors.lastname: Pfeiffer
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany.
authors.forename: Reiner
authors.initials: R
authors.lastname: Seemann
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany. bernd.ittermann@ptb.de.
authors.forename: Bernd
authors.initials: B
authors.lastname: Ittermann
mesh_terms: D000465:Algorithms; D060733:Electromagnetic Radiation; D004867:Equipment Design; D004864:Equipment and Supplies; D008279:Magnetic Resonance Imaging / Q000379:methods*; D009682:Magnetic Resonance Spectroscopy; D061214:Patient Safety; D019047:Phantoms, Imaging; D020959:Polyethylene / Q000737:chemistry; D011846:Radio Waves; D012542:Scattering, Radiation
chemical_list: D020959:Polyethylene
keywords: Magnetic resonance imaging; Parallel transmission; Power balance; RF safety; Radiated power