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Fieldmaps and distortion

Fieldmaps and distortion. What is a fieldmap? How can we predict distortion? How can we correct distortion?. Field map comes from 2 GE FLASH images. Ratio of phase images gives  . Data from Siemens scanner is scaled weird. TE = 7.65ms. Scaled phase data: phase evolution during dTE.

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Fieldmaps and distortion

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  1. Fieldmaps and distortion • What is a fieldmap? • How can we predict distortion? • How can we correct distortion? Fieldmaps

  2. Field map comes from 2 GE FLASH images Fieldmaps

  3. Ratio of phase images gives  Fieldmaps

  4. Data from Siemens scanner is scaled weird Fieldmaps

  5. TE = 7.65ms Scaled phase data: phase evolution during dTE TE = 5.19ms Transverse plane Fieldmaps

  6. Distortion and signal drop-out vs. bandwidth: low resolution Fieldmaps

  7. Distortion and signal drop-out vs. bandwidth: low resolution Fieldmaps

  8. Distortion and signal drop-out vs. bandwidth: high resolution Fieldmaps

  9. Distortion and signal drop-out vs. bandwidth: high resolution Fieldmaps

  10. Mystery … solved Theory: power calibration applies saturation pulse before image acquisition. If operator delays before starting scan, magnetization returns to equilibrium and 1st image has T2* contrast. If operator does not delay, magnetization is not at equilibrium, and contrast is T2* with T1. Fieldmaps

  11. When distortion matters • Many data sets are best visualized in 3D • 3D anatomy is usually a FLASH-family sequence • 64 or 128 higher pixel bandwidth • often acquired on a different day (different day = different fieldmap) • Go ahead and try to align a distorted EPI to a FLASH volume. Fieldmaps

  12. Motor mapping prelim. study Fieldmaps

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