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Fieldmaps and distortion

Learn about fieldmaps, distortion prediction, and correction techniques in MRI imaging. Understand the impact of bandwidth on distortion and signal dropout. Discover how power calibration theory helps solve image mysteries. Explore when distortion matters and how to align distorted images effectively for better visualization. Dive into a preliminary motor mapping study using fieldmaps.

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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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