title: Manipulation of image intensity distribution at 7.0 T: passive RF shimming and focusing with dielectric materials.
abstract: OBJECTIVE
  To investigate the effects of high dielectric material padding on RF field distribution in the human head at 7.0 T, and demonstrate the feasibility and effectiveness of RF passive shimming and focusing with such an approach.
  
  
  METHODS
  The intensity distribution changes of gradient-recalled-echo (GRE) and spin-echo (SE) images of a human head acquired with water pads (dielectric constant = 78) placed in specified configurations around the head at 7.0 T were evaluated and compared with computer simulation results using the finite difference time domain (FDTD) method. The contributions to the B(1) field distribution change from the displacement current and conductive current of a given configuration of dielectric padding were determined with computer simulations.
  
  
  RESULTS
  MR image intensity distribution in the human head with an RF coil at 7.0 T can be changed drastically by placing water pads around the head. Computer simulations reveal that the high permittivity of water pads results in a strong displacement current that enhances image intensity in the nearby region and alters the intensity distribution of the entire brain.
  
  
  CONCLUSIONS
  The image intensity distribution in the human head at ultra-high field strengths can be effectively manipulated with high permittivity padding. Utilizing this effect, the B(1) field inside the human head of a given RF coil can be adjusted to reduce the B(1) field inhomogeneity artifact associated with the wave behavior (RF passive shimming) or to locally enhance the signal-to-noise ratio (SNR) in targeted regions of interest (ROIs; RF field focusing).
authors.affiliation: Center for NMR Research, Department of Radiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania, USA. qyang@psu.edu
authors.forename: Qing X
authors.initials: QX
authors.lastname: Yang
authors.forename: Weihua
authors.initials: W
authors.lastname: Mao
authors.forename: Jinghua
authors.initials: J
authors.lastname: Wang
authors.forename: Michael B
authors.initials: MB
authors.lastname: Smith
authors.forename: Hao
authors.initials: H
authors.lastname: Lei
authors.forename: Xiaoliang
authors.initials: X
authors.lastname: Zhang
authors.forename: Kamil
authors.initials: K
authors.lastname: Ugurbil
authors.forename: Wei
authors.initials: W
authors.lastname: Chen
mesh_terms: D001921:Brain / Q000473:pathology*; D055590:Electromagnetic Phenomena; D006257:Head / Q000473:pathology*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*; D011846:Radio Waves