title: Improved steering of the RF field of traveling wave MR with a multimode, coaxial waveguide.
abstract: OBJECTIVE
  Magnetic resonance imaging of humans at high magnetic field strengths is strongly influenced by the interference of the radiofrequency (RF) electromagnetic field and the body. To minimize this effect, multiple RF sources could be used. A novel setup (called multimode, coaxial waveguide) is proposed that facilitates RF shimming based on the traveling waves.
  
  
  METHODS
  The multimode, coaxial waveguide combines the coaxial waveguide, cylindrical waveguide, high dielectric permittivity lining, and eight radial stub antennas. Each antenna excites multiple waveguide modes. Based on modes orthogonality, a method was devised to decompose an excitation pattern of single stub antenna into waveguide modes.
  
  
  RESULTS
  The number of modes present in the excitation pattern of a single stub antenna increased with the higher effective permittivity of the dielectric lining. Thus, RF shimming performance of the setup was improved. An average homogeneity of 10% was demonstrated for a single slice of each principle plane in the human head at 7 T.
  
  
  CONCLUSIONS
  Traveling wave RF shimming is feasible both in axial and longitudinal directions and is improved with an increased amount of orthogonal waveguide modes. Nevertheless, with the currently available RF amplifiers at 7 T, the performance of the setup is limited to low flip angles.
authors.affiliation: Imaging Division, University Medical Center Utrecht, The Netherlands.
authors.forename: A
authors.initials: A
authors.lastname: Andreychenko
authors.forename: H
authors.initials: H
authors.lastname: Kroeze
authors.forename: V O
authors.initials: VO
authors.lastname: Boer
authors.forename: J J W
authors.initials: JJ
authors.lastname: Lagendijk
authors.forename: P R
authors.initials: PR
authors.lastname: Luijten
authors.forename: C A T
authors.initials: CA
authors.lastname: van den Berg
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D007089:Image Enhancement / Q000379:methods*; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D019047:Phantoms, Imaging; D011846:Radio Waves; D015203:Reproducibility of Results; D012542:Scattering, Radiation; D012680:Sensitivity and Specificity; D014159:Transducers; D051598:Whole Body Imaging / Q000295:instrumentation*
keywords: 7 T; RF shimming; multimode coaxial waveguide; traveling wave