B1 destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil

Pierre-Francois Van de Moortele, Can Akgun, Gregor Adriany, Steen Moeller, Johannes Ritter, Christopher M. Collins, Michael B. Smith, J. T Vaughan, Kamil Ugurbil

Research output: Contribution to journalArticlepeer-review

316 Scopus citations

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 B1 in the periphery, without a significant standing wave pattern. The relative spatial phase of receive and transmit B1 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 B1 phase could help, within an MR session, to perform RF shimming in order to obtain more homogeneous B1 in user-defined areas of the brain.

Original languageEnglish (US)
Pages (from-to)1503-1518
Number of pages16
JournalMagnetic resonance in medicine
Volume54
Issue number6
DOIs
StatePublished - Dec 2005

Keywords

  • Human brain
  • MRI
  • RF coil array, B field
  • Ultrahigh magnetic field

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