title: Numerical and experimental evaluation of RF shimming in the human brain at 9.4 T using a dual-row transmit array.
abstract: OBJECTIVE
  To provide a numerical and experimental investigation of the static RF shimming capabilities in the human brain at 9.4 T using a dual-row transmit array.
  
  
  METHODS
  A detailed numerical model of an existing 16-channel, inductively decoupled dual-row array was constructed using time-domain software together with circuit co-simulation. Experiments were conducted on a 9.4 T scanner. Investigation of RF shimming focused on B1(+) homogeneity, efficiency and local specific absorption rate (SAR) when applied to large brain volumes and on a slice-by-slice basis.
  
  
  RESULTS
  Numerical results were consistent with experiments regarding component values, S-parameters and B1(+) pattern, though the B1(+) field was about 25% weaker in measurements than simulations. Global shim settings were able to prevent B1(+) field voids across the entire brain but the capability to simultaneously reduce inhomogeneities was limited. On a slice-by-slice basis, B1(+) standard deviations of below 10% without field dropouts could be achieved in axial, sagittal and coronal orientations across the brain, even with phase-only shimming, but decreased B1(+) efficiency and SAR limitations must be considered.
  
  
  CONCLUSIONS
  Dual-row transmit arrays facilitate flexible 3D RF management across the entire brain at 9.4 T in order to trade off B1(+) homogeneity against power-efficiency and local SAR.
authors.affiliation: High-Field Magnetic Resonance Center, Max Planck Institute for Biological Cybernetics, Spemannstr. 41, 72076, Tübingen, Germany, jens.hoffmann@tuebingen.mpg.de.
authors.forename: Jens
authors.initials: J
authors.lastname: Hoffmann
authors.forename: Gunamony
authors.initials: G
authors.lastname: Shajan
authors.forename: Klaus
authors.initials: K
authors.lastname: Scheffler
authors.forename: Rolf
authors.initials: R
authors.lastname: Pohmann
mesh_terms: D001921:Brain / Q000033:anatomy & histology*; D003198:Computer Simulation; D064368:Healthy Volunteers; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D011846:Radio Waves; D059629:Signal-To-Noise Ratio