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Artifacts in Cardiac MRI

Artifacts in Cardiac MRI. Andreas Sigfridsson, IMV/IMT/CMIV <sigge@imv.liu.se> 2007-04-27. Dynamic imaging. Images are acquired line-by-line in k-space Motion during acquisition causes artifacts , not blurring Special care must be taken Triggering Acquisition ordering. Example.

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Artifacts in Cardiac MRI

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  1. Artifacts in Cardiac MRI Andreas Sigfridsson, IMV/IMT/CMIV <sigge@imv.liu.se> 2007-04-27

  2. Dynamic imaging • Images are acquired line-by-line in k-space • Motion during acquisition causes artifacts, not blurring • Special care must be taken • Triggering • Acquisition ordering

  3. Example • Simulation example • 256x256, TR=20 ms, total scan time ~5 s Image domain k-space

  4. Signal saturation • Signal recovery - relaxation

  5. Signal saturation • Progress to steady-state

  6. Varying signal during inflow • The MR signal is increased during inflow • Signal increase related to flow velocity • More magnetization because of T1 decay/recovery Flowing spins, fresh signal Static spins, saturated signal

  7. Varying signal intensity • Simulated six sinusoidal heart beats Intensity Image (animation)

  8. Varying signal intensity, results • Reconstructed image Normal contrast High contrast

  9. Varying signal intensity, k-space • Image artifacts because of k-space sampling • Different lines are acquired with different signal intensity

  10. Varying signal intensity, analysis • Analyze signal

  11. Intensity FT(Intensity) Slow sine variation

  12. Varying signal intensity, results • Reconstructed image Normal contrast High contrast

  13. Fast sine variation Intensity FT(Intensity)

  14. Fast sine variation Intensity FT(Intensity)

  15. “Physiological” variation • Aortic flow (handcrafted, not actual data) Intensity (whole scan) Intensity (one beat)

  16. “Physiological” variation Intensity variation (animation)

  17. “Physiological” variation FT(Intensity) Intensity

  18. “Physiological” variation Reconstructed images High contrast Normal contrast

  19. Remedies • Triggering • Wait for ECG R-tag • Wait additional trigger delay time • Acquire one (or several) k-space lines • Acquisition reordering • Measure current cardiac (or respiratory phase) • Choose k-space line to acquire • Creates a slow varying intensity curve along k-space

  20. Triggering - random phase • If triggering is not accurate, random variations will still be left Intensity FT(Intensity)

  21. Random phase, reconstructed Normal contrast High contrast

  22. Acquisition reordering Intensity FT(Intensity)

  23. Acquisition reordering Random Ordered

  24. Acquisition reordering • Reconstructed images Normal contrast High contrast

  25. Actual examples • ECG triggering failed

  26. Dynamic imaging • Images are acquired line-by-line in k-space • Motion during acquisition causes artifacts, not blurring • Special care must be taken • Triggering • Acquisition ordering • Artifacts might be predictable through use of Fourier transform

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