title: Encoding methods for B1(+) mapping in parallel transmit systems at ultra high field.
abstract: Parallel radiofrequency (RF) transmission, either in the form of RF shimming or pulse design, has been proposed as a solution to the B1(+) inhomogeneity problem in ultra high field magnetic resonance imaging. As a prerequisite, accurate B1(+) maps from each of the available transmit channels are required. In this work, four different encoding methods for B1(+) mapping, namely 1-channel-on, all-channels-on-except-1, all-channels-on-1-inverted and Fourier phase encoding, were evaluated using dual refocusing acquisition mode (DREAM) at 9.4 T. Fourier phase encoding was demonstrated in both phantom and in vivo to be the least susceptible to artefacts caused by destructive RF interference at 9.4 T. Unlike the other two interferometric encoding schemes, Fourier phase encoding showed negligible dependency on the initial RF phase setting and therefore no prior B1(+) knowledge is required. Fourier phase encoding also provides a flexible way to increase the number of measurements to increase SNR, and to allow further reduction of artefacts by weighted decoding. These advantages of Fourier phase encoding suggest that it is a good choice for B1(+) mapping in parallel transmit systems at ultra high field.
authors.affiliation: Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany.
authors.forename: Desmond H Y
authors.initials: DH
authors.lastname: Tse
authors.affiliation: Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany.
authors.forename: Michael S
authors.initials: MS
authors.lastname: Poole
authors.affiliation: Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany.
authors.forename: Arthur W
authors.initials: AW
authors.lastname: Magill
authors.affiliation: Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany.
authors.forename: Jörg
authors.initials: J
authors.lastname: Felder
authors.affiliation: Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany; German Center for Neurodegenerative Diseases (DZNE), Ernst-Robert-Curtius-Straße 12, 53117 Bonn, Germany.
authors.forename: Daniel
authors.initials: D
authors.lastname: Brenner
authors.affiliation: Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Straße, 52425 Jülich, Germany; Department of Neurology, Faculty of Medicine, RWTH Aachen University, JARA, Aachen, Germany. Electronic address: n.j.shah@fz-juelich.de.
authors.forename: N
authors.initials: N
authors.lastname: Jon Shah
keywords: High-field MRI; Parallel transmit; Phase-rotation; mapping