title: Parallel imaging performance as a function of field strength--an experimental investigation using electrodynamic scaling.
abstract: In this work, the dependence of parallel MRI performance on main magnetic field strength is experimentally investigated. Using the general framework of electrodynamic scaling, the B0-dependent behavior of the relevant radiofrequency fields at a single physical field strength of 7 T is studied. In the chosen implementation this is accomplished by adjusting the permittivity and conductivity of a homogeneous spherical phantom. With different mixing ratios of decane, ethanol, purified water, N-methylformamide, and sodium chloride, field strengths in the range of 1.5 to 11.5 T are mimicked. Based on sensitivity maps of an eight-coil receiver array, the field-dependent performance of parallel imaging is assessed in terms of the geometry factor g, which reflects noise enhancement in parallel imaging reconstruction. At low field strengths the SNR penalty was nearly independent of B0 and favorably low for 1D reduction factors up to between 3 and 4. At higher field strengths the transition between favorable and prohibitive parallel imaging conditions was found to shift toward higher feasible reduction factors. These findings are in good agreement with previous theoretical predictions. From this agreement it is concluded that parallel MRI at high B0 benefits specifically from onsetting far-field behavior of the involved radiofrequency fields.
authors.affiliation: Institute for Biomedical Engineering, University of Zurich, Switzerland.
authors.forename: Florian
authors.initials: F
authors.lastname: Wiesinger
authors.forename: Pierre-Francois
authors.initials: PF
authors.lastname: Van de Moortele
authors.forename: Gregor
authors.initials: G
authors.lastname: Adriany
authors.forename: Nicola
authors.initials: N
authors.lastname: De Zanche
authors.forename: Kamil
authors.initials: K
authors.lastname: Ugurbil
authors.forename: Klaas P
authors.initials: KP
authors.lastname: Pruessmann
mesh_terms: D004574:Electromagnetic Fields; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019047:Phantoms, Imaging; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted