B0 field mapping

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B0 field mapping

• Geoff Todd

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What is a B0 field map?
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First, a few definitions

• B0 The strong static magnetic field generated
  by an MRI scanner.
• Field map An image of the intensity of the
  magnetic field across space.

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So how does this relate to our work?

• In an ideal situation, the magnetic field in an
  MRI scanner would be homogenous.
• Unfortunately, biological tissue develops small
  magnetization that counteracts the applied field.
• Bone and air have little effect on the magnetic
  field.

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So how does this relate to our work?

• When a head is put into a uniform field, the
  field becomes distorted.
• Static field inhomogeneities usually cause
  geometric distortions, although they can also
  lead to signal losses under severe conditions.

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Solutions?

• Shimming

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Shimming

• Active metallic coils that produce an adjustable
  current
• Passive pieces of steel placed within the area
  to produce a magnetic field
• However, even after shimming, there will often
  remain substantial variation involving distortion
  or dropout.

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Solutions?

• Shimming
• Use magnet with low field strength
• Use a magnetic field map

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Jezzard and Balaban (1995)

• In head imaging, such internal field
  inhomogeneities often occur around the petrous
  bone, leading to distortions in the temporal
  lobes, and also surrounding the sinuses, often
  leading to distortion around the anterior frontal
  regions. The magnetic field inhomogeneities
  cause pixels in the echo planar image to be
  shifted from where they should appear, and if the
  inhomogeneities are sufficiently bad, gross image
  distortion results.

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Jezzard and Balaban (1995)

• If the magnetic field inhomogeneities can be
  characterized by means of a residual field map
  throughout the volume of interest, then the
  distorted pixels can be relocated and intensity
  corrected to give a geometric distortion-free
  image.

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A simple calculation
Jezzard and Balaban (1995)
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Cusack, Brett, and Osswald (2003)
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Phantom distortion correction
Huettel et al. (2004)
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Structural Overlay effect?
Jezzard and Balaban (1995)
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Preprocessing Data Framework
Recording of MR signal
Image reconstruction
Online Quality of Assurance
Distortion correction
Head motion correction/ realignment
Coregistration and/ or normalization
Spatial and temporal smoothing
Final quality assurance
Adapted from Huettel et al. (2004)
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Sagittal view on distortion
Jezzard and Balaban (1995)
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Field map effects on brain imaging
http//imaging.mrc-cbu.cam.ac.uk/imaging/FmBackgro
und
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Close up
Raw EPI
Undistorted EPI
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How does this affect us?

• Application of undistortion increases the
  statistical power of group studies.
• Increase in power due to an increase in the
  overlap of activation of different subjects.

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Power improvement
http//imaging.mrc-cbu.cam.ac.uk/imaging/FmPower
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Personal Plan

• Use vudistortioncorrection program to analyze
  previous data in the lab and determine if
  statistical power matches and/ or rises above
  previous results.

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vuiiswiki
http//wiki.vuiis.vanderbilt.edu/index.php/vuDisto
rtionCorrection
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Future directions of field mapping

• Generating multiple field maps
• May create extra variance in the voxel
  time-series and might not want to use for
  time-varying distortions (Hutton et al. 2002).
• Vanderbilt group, however, has found a better
  way to create multiple field maps.

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Dynamic field mapping and distortion correction
for fMRINing Xu and J. Michael Fitzpatrick

• Propose dynamic field mapping to use a single
  reference image and a single corresponding
  acquired field map and the phase information
  extracted from the complex image data of each EPI
  image in the time series.
• A separate field map is derived for each EPI
  image. The derived field maps are then used for
  distortion correction.
• Can correct for image distortion that varies
  dynamically without sacrificing temporal
  resolution.
• Validated using simulated data with experimental
  results showing improved performance in
  comparison to correction using a single field
  map.

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References

• Jezzard, P., and Balaban, R. S. 1995. Correction
  for geometric distortion in echo planar images
  from B0 field variations. Magnetic Resonance in
  Medicine. 34 65-73.
• Cusack, R., Brett, M., and Osswald, K. 2003. An
  Evaluation of the Use of Magnetic Field Maps to
  Undistort Echo-Planar Images. NeuroImage. 18
  127-142
• Hutton, C. et al. 2002. Image Distortion
  Correction in fMRI A Quantitative Evaluation.
  NeuroImage. 16 217-240.
• Huettel, S., Song, A., McCarthy, G. (2004).
  Functional Magnetic Resonance Imaging.
  Sunderland, MA Sinauer Associates, Inc.

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Thank You!