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Degenerate mode band‐pass birdcage coil for accelerated parallel excitation
Author(s) -
Alagappan Vijayanand,
Nistler Juergen,
Adalsteinsson Elfar,
Setsompop Kawin,
Fontius Ulrich,
Zelinski Adam,
Vester Markus,
Wiggins Graham C.,
Hebrank Franz,
Renz Wolfgang,
Schmitt Franz,
Wald Lawrence L.
Publication year - 2007
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.21247
Subject(s) - excitation , electromagnetic coil , physics , nuclear magnetic resonance , phase (matter) , azimuth , basis (linear algebra) , degenerate energy levels , optics , computational physics , acoustics , geometry , mathematics , quantum mechanics
An eight‐rung, 3T degenerate birdcage coil (DBC) was constructed and evaluated for accelerated parallel excitation of the head with eight independent excitation channels. Two mode configurations were tested. In the first, each of the eight loops formed by the birdcage was individually excited, producing an excitation pattern similar to a loop coil array. In the second configuration a Butler matrix transformed this “loop coil” basis set into a basis set representing the orthogonal modes of the birdcage coil. In this case the rung currents vary sinusoidally around the coil and only four of the eight modes have significant excitation capability (the other four produce anticircularly polarized (ACP) fields). The lowest useful mode produces the familiar uniform B 1 field pattern, and the higher‐order modes produce center magnitude nulls and azimuthal phase variations. The measured magnitude and phase excitation profiles of the individual modes were used to generate one‐, four‐, six‐, and eightfold‐accelerated spatially tailored RF excitations with 2D and 3D k ‐space excitation trajectories. Transmit accelerations of up to six‐fold were possible with acceptable levels of spatial artifact. The orthogonal basis set provided by the Butler matrix was found to be advantageous when an orthogonal subset of these modes was used to mitigate B 1 transmit inhomogeneities using parallel excitation. Magn Reson Med 57:1148–1158, 2007. © 2007 Wiley‐Liss, Inc.