title: B(1) destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil.
abstract: RF behavior in the human head becomes complex at ultrahigh magnetic fields. A bright center and a weak periphery are observed in images obtained with volume coils, while surface coils provide strong signal in the periphery. Intensity patterns reported with volume coils are often loosely referred to as "dielectric resonances," while modeling studies ascribe them to superposition of traveling waves greatly dampened in lossy brain tissues, raising questions regarding the usage of this term. Here we address this question experimentally, taking full advantage of a transceiver coil array that was used in volume transmit mode, multiple receiver mode, or single transmit surface coil mode. We demonstrate with an appropriately conductive sphere phantom that destructive interferences are responsible for a weak B(1) in the periphery, without a significant standing wave pattern. The relative spatial phase of receive and transmit B(1) proved remarkably similar for the different coil elements, although with opposite rotational direction. Additional simulation data closely matched our phantom results. In the human brain the phase patterns were more complex but still exhibited similarities between coil elements. Our results suggest that measuring spatial B(1) phase could help, within an MR session, to perform RF shimming in order to obtain more homogeneous B(1) in user-defined areas of the brain.
authors.affiliation: Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, Minneapolis, Minnesota 55455, USA. pfvdm@cmrr.umn.edu
authors.forename: Pierre-François
authors.initials: PF
authors.lastname: Van de Moortele
authors.forename: Can
authors.initials: C
authors.lastname: Akgun
authors.forename: Gregor
authors.initials: G
authors.lastname: Adriany
authors.forename: Steen
authors.initials: S
authors.lastname: Moeller
authors.forename: Johannes
authors.initials: J
authors.lastname: Ritter
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Collins
authors.forename: Michael B
authors.initials: MB
authors.lastname: Smith
authors.forename: J Thomas
authors.initials: JT
authors.lastname: Vaughan
authors.forename: Kāmil
authors.initials: K
authors.lastname: Uğurbil
mesh_terms: D000465:Algorithms; D016477:Artifacts; D001921:Brain / Q000033:anatomy & histology*; D006257:Head / Q000033:anatomy & histology*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014159:Transducers