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Related Experiment Videos

B(1) destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil.

Pierre-François Van de Moortele1, Can Akgun, Gregor Adriany

  • 1Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, Minneapolis, Minnesota 55455, USA. pfvdm@cmrr.umn.edu

Magnetic Resonance in Medicine
|November 5, 2005
PubMed
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Ultrahigh magnetic fields complicate radiofrequency (RF) behavior in the brain. Destructive interference, not dielectric resonance, causes signal loss in the brain periphery, enabling targeted RF shimming for improved MRI.

Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Biophysics
  • Electromagnetism

Background:

  • Radiofrequency (RF) behavior in the human head is complex at ultrahigh magnetic fields.
  • Observed intensity patterns in MRI, like bright centers and weak peripheries with volume coils, are often misattributed to "dielectric resonances."
  • Previous modeling studies suggested traveling waves dampened in brain tissues, questioning the "dielectric resonance" terminology.

Purpose of the Study:

  • To experimentally investigate the cause of peripheral signal loss in ultrahigh magnetic field MRI.
  • To clarify whether "dielectric resonances" or other phenomena are responsible for observed RF intensity patterns.
  • To evaluate the potential of B(1) phase measurements for RF shimming in MRI.

Main Methods:

Related Experiment Videos

  • Utilized a transceiver coil array in various modes (volume transmit, multiple receiver, single transmit surface coil).
  • Employed a conductive sphere phantom to demonstrate the underlying physical principles.
  • Conducted experiments and simulations to analyze B(1) field behavior and spatial phase.

    • Main Results:

    • Demonstrated that destructive interference, not standing waves, causes weak peripheral B(1) fields in phantoms.
    • Observed consistent relative spatial phase patterns between receive and transmit B(1) across different coil elements.
    • Found similarities in B(1) phase patterns between coil elements in human brain imaging, despite increased complexity.

    Conclusions:

    • The study refutes the "dielectric resonance" explanation for peripheral signal loss in ultrahigh field MRI.
    • Destructive interference is identified as the primary mechanism responsible for peripheral B(1) signal reduction.
    • Measuring spatial B(1) phase offers a potential method for in-session RF shimming to achieve more homogeneous B(1) fields in targeted brain regions.