Manipulation of image intensity distribution at 7.0 T: Passive RF shimming and focusing with dielectric materials

Qing X. Yang, Weihua Mao, Jinghua Wang, Michael B. Smith, Hao Lei, Xiaoliang Zhang, Kamil Ugurbil, Wei Chen

Research output: Contribution to journalArticle

73 Scopus citations

Abstract

Purpose: 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. Materials and 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 B1 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. Conclusion: 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 B1 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).

Original languageEnglish (US)
Pages (from-to)197-202
Number of pages6
JournalJournal of Magnetic Resonance Imaging
Volume24
Issue number1
DOIs
StatePublished - Jul 1 2006

Keywords

  • FDTD
  • High-field MRI
  • RF field focusing
  • RF field inhomogeneity
  • RF field passive shim

ASJC Scopus subject areas

  • Radiology Nuclear Medicine and imaging

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