title: Improved homogeneity of the transmit field by simultaneous transmission with phased array and volume coil.
abstract: OBJECTIVE
  To improve the homogeneity of transmit volume coils at high magnetic fields (> or =4 T). Due to radiofrequency (RF) field/tissue interactions at high fields, 4 T to 8 T, the transmit profile from head-sized volume coils shows a distinctive pattern with relatively strong RF magnetic field B(1) in the center of the brain.
  
  
  METHODS
  In contrast to conventional volume coils at high field strengths, surface coil phased arrays can provide increased RF field strength peripherally. In theory, simultaneous transmission from these two devices could produce a more homogeneous transmission field. To minimize interactions between the phased array and the volume coil, counter rotating current (CRC) surface coils consisting of two parallel rings carrying opposite currents were used for the phased array.
  
  
  RESULTS
  Numerical simulations and experimental data demonstrate that substantial improvements in transmit field homogeneity can be obtained.
  
  
  CONCLUSIONS
  We have demonstrated the feasibility of using simultaneous transmission with human head-sized volume coils and CRC phased arrays to improve homogeneity of the transmit RF B(1) field for high-field MRI systems.
authors.affiliation: Department of Neurosurgery, Yale University, New Haven, CT 06519, USA. nikolai.avdievich@yale.edu
authors.forename: Nikolai I
authors.initials: NI
authors.lastname: Avdievich
authors.forename: Sukhoon
authors.initials: S
authors.lastname: Oh
authors.forename: Hoby P
authors.initials: HP
authors.lastname: Hetherington
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Collins
mesh_terms: D001921:Brain / Q000473:pathology*; D001931:Brain Mapping; D003198:Computer Simulation; D004574:Electromagnetic Fields; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods*; D008962:Models, Theoretical; D019047:Phantoms, Imaging; D011846:Radio Waves; D012984:Software