title: Signal-to-noise ratio and MR tissue parameters in human brain imaging at 3, 7, and 9.4 tesla using current receive coil arrays.
abstract: OBJECTIVE
  Relaxation times, transmit homogeneity, signal-to-noise ratio (SNR) and parallel imaging g-factor were determined in the human brain at 3T, 7T, and 9.4T, using standard, tight-fitting coil arrays.
  
  
  METHODS
  The same human subjects were scanned at all three field strengths, using identical sequence parameters and similar 31- or 32-channel receive coil arrays. The SNR of three-dimensional (3D) gradient echo images was determined using a multiple replica approach and corrected with measured flip angle and T2 (*) distributions and the T1 of white matter to obtain the intrinsic SNR. The g-factor maps were derived from 3D gradient echo images with several GRAPPA accelerations.
  
  
  RESULTS
  As expected, T1 values increased, T2 (*) decreased and the B1 -homogeneity deteriorated with increasing field. The SNR showed a distinctly supralinear increase with field strength by a factor of 3.10 ± 0.20 from 3T to 7T, and 1.76 ± 0.13 from 7T to 9.4T over the entire cerebrum. The g-factors did not show the expected decrease, indicating a dominating role of coil design.
  
  
  CONCLUSIONS
  In standard experimental conditions, SNR increased supralinearly with field strength (SNR ∼ B0 (1.65) ). To take full advantage of this gain, the deteriorating B1 -homogeneity and the decreasing T2 (*) have to be overcome.
authors.affiliation: Max Planck Institute for Biological Cybernetics, Magnetic Resonance Center, Tübingen, Germany.
authors.forename: Rolf
authors.initials: R
authors.lastname: Pohmann
authors.affiliation: Department of Biomedical Magnetic Resonance, Otto-von-Guericke University Magdeburg, Germany.|German Centre for Neurodegenerative Diseases (DZNE), Site Magdeburg, Germany.|Leibniz Institute for Neurobiology, Magdeburg, Germany.|Center for Behavioral Brain Sciences, Magdeburg, Germany.
authors.forename: Oliver
authors.initials: O
authors.lastname: Speck
authors.affiliation: Max Planck Institute for Biological Cybernetics, Magnetic Resonance Center, Tübingen, Germany.|Department for Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany.
authors.forename: Klaus
authors.initials: K
authors.lastname: Scheffler
mesh_terms: D001931:Brain Mapping / Q000379:methods*; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D008297:Male; D019047:Phantoms, Imaging; D059629:Signal-To-Noise Ratio
keywords: relaxation times; signal-to-noise ratio; ultra-high field