title: Degenerate mode band-pass birdcage coil for accelerated parallel excitation.
abstract: 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.
authors.affiliation: AA Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129, USA. vijay243@nmr.mgh.harvard.edu
authors.forename: Vijayanand
authors.initials: V
authors.lastname: Alagappan
authors.forename: Juergen
authors.initials: J
authors.lastname: Nistler
authors.forename: Elfar
authors.initials: E
authors.lastname: Adalsteinsson
authors.forename: Kawin
authors.initials: K
authors.lastname: Setsompop
authors.forename: Ulrich
authors.initials: U
authors.lastname: Fontius
authors.forename: Adam
authors.initials: A
authors.lastname: Zelinski
authors.forename: Markus
authors.initials: M
authors.lastname: Vester
authors.forename: Graham C
authors.initials: GC
authors.lastname: Wiggins
authors.forename: Franz
authors.initials: F
authors.lastname: Hebrank
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Renz
authors.forename: Franz
authors.initials: F
authors.lastname: Schmitt
authors.forename: Lawrence L
authors.initials: LL
authors.lastname: Wald
mesh_terms: D016477:Artifacts; D004867:Equipment Design; D007089:Image Enhancement / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000295:instrumentation; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D019047:Phantoms, Imaging