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Chapter 14: Artifacts

Chapter 14: Artifacts. Mark D. Herbst, MD, PhD. Artifact. Something on the image that does not represent something in the patient Many causes Equipment malfunctions Environmental factors Patient motion Body composition. Aliasing.

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Chapter 14: Artifacts

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  1. Chapter 14: Artifacts Mark D. Herbst, MD, PhD

  2. Artifact • Something on the image that does not represent something in the patient • Many causes • Equipment malfunctions • Environmental factors • Patient motion • Body composition

  3. Aliasing • When signal form a voxel is represented in the wrong voxel. • Occurs when the FOV is smaller than the body part • Also known as ‘wrap-around” or “fold-over” artifact

  4. Wraparound

  5. Wrap-around artifact

  6. Aliasing: Example

  7. Wraparound Artfacts in 3D

  8. How to avoid Aliasing • Use larger FOV • http://www.e-mri.org/quality-artifacts/aliasing.html • Apply “no phase wrap” – this acquires data for a larger FOV and discards the data outside your original FOV, but it takes 2x longer to do since it is done in the phase encoding direction, unless NEX is cut in half.

  9. Increase FOV- Avoid Aliasing

  10. Chemical Shift - Example

  11. Chemical Shift Artifacts

  12. Chemical Shift

  13. Chemical Shift vs BW Per Pixel

  14. Chemical Shift Effect

  15. Chemical Shift - Second Kind

  16. In-phaseOut-of-phase

  17. Motion Artifact • Most common cause of bad images • Periodic or random • Produces ghost images in the phase encoding direction

  18. Motion Artifacts - Periodic

  19. Motion Artifacts - Periodic

  20. Motion Artifacts - Random

  21. How to reduce motion artifacts • Consider swapping phase and frequency directions to move artifact away from area of interest (SPF) • Apply sat pulses to reduce signal from pulsating vessels • Apply flow compensation • Apply EKG triggering or gating –requires constant heart rate • Consider “Stark technique”—high NEX T1WI

  22. Metal Artifacts • Can warp image with black area with white border • Can appear as just black dots if microscopic bits of metal (post-op shoulder)

  23. Metal on CT

  24. Metal in bone

  25. Ferromagnetic Susceptibility Artifacts

  26. Susceptibility Artifacts

  27. How to reduce metal artifacts • Use lower field strength • Angle slices so screws are within one slice

  28. RF Leak • Lines appear all over image in the phase encoding direction at different positions in the frequency encoding direction

  29. RF Noise

  30. Zero-phase Artifact=Zipper Artifact • Occurs only with NEX=1 • Occurs in the center of the image • Remove it by going to NEX=2

  31. RF Artifact

  32. Zipper Artifact

  33. Magnetic Inhomogeneity Artifacts

  34. Diamagnetic Susceptibility Artifact

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