title: Radio frequency magnetic field mapping of a 3 Tesla birdcage coil: experimental and theoretical dependence on sample properties.
abstract: The RF B(1) distribution was studied, theoretically and experimentally, in phantoms and in the head of volunteers using a 3 T MRI system equipped with a birdcage coil. Agreement between numerical simulation and experiment demonstrates that B(1) distortion at high field can be explained with 3D full-Maxwell calculations. It was found that the B(1) distribution in the transverse plane is strongly dependent on the dielectric properties of the sample. We show that this is a consequence of RF penetration effects combined with RF standing wave effects. In contrast, along the birdcage coil z-axis the B(1) distribution is determined mainly by the coil geometry. In the transverse plane, the region of B(1) uniformity (within 10% of the maximum) was 15 cm with oil, 6 cm with distilled water, 11 cm with saline, and 10 cm in the head. Along z the B(1) uniformity was 9 cm with phantoms and 7 cm in the head.
authors.affiliation: Centre for Functional Magnetic Resonance Imaging of the Brain, Department of Clinical Neurology, John Radcliffe Hospital, University of Oxford, Headington, Oxford, UK. alecci@fmrib.ox.ac.uk
authors.forename: M
authors.initials: M
authors.lastname: Alecci
authors.forename: C M
authors.initials: CM
authors.lastname: Collins
authors.forename: M B
authors.initials: MB
authors.lastname: Smith
authors.forename: P
authors.initials: P
authors.lastname: Jezzard
mesh_terms: D001931:Brain Mapping / Q000379:methods*; D004574:Electromagnetic Fields; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D008962:Models, Theoretical; D019047:Phantoms, Imaging