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

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

Content Provider	World Health Organization (WHO)-Global Index Medicus
Author	Weidemann, Gerd Seifert, Frank Hoffmann, Werner Pfeiffer, Harald Seemann, Reiner Ittermann, Bernd
Description	Country affiliation: Germany Author Affiliation: Weidemann G ( Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany.); Seifert F ( Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany.); Hoffmann W ( Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany.); Pfeiffer H ( Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany.); Seemann R ( Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany.); Ittermann B ( Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany. bernd.ittermann@ptb.de.)
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.
File Format	HTM / HTML
ISSN	09685243
Issue Number	3
Journal	Magnetic Resonance Materials in Physics, Biology and Medicine
Volume Number	29
e-ISSN	13528661
Language	English
Publisher	Springer
Publisher Date	2016-06-01
Publisher Place	Germany
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Discipline Diagnostic Imaging
Content Type	Text
Resource Type	Article
Subject	Radiology, Nuclear Medicine and Imaging Biophysics Radiological and Ultrasound Technology

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