Chapter 14 artifacts
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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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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

  • 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


Wraparound


Wrap-around artifact


Aliasing: Example


Wraparound Artfacts in 3D


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.


Increase FOV- Avoid Aliasing


Chemical Shift - Example


Chemical Shift Artifacts


Chemical Shift


Chemical Shift vs BW Per Pixel


Chemical Shift Effect


Chemical Shift - Second Kind


In-phaseOut-of-phase


Motion Artifact

  • Most common cause of bad images

  • Periodic or random

  • Produces ghost images in the phase encoding direction


Motion Artifacts - Periodic


Motion Artifacts - Periodic


Motion Artifacts - Random


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


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)


Metal on CT


Metal in bone


Ferromagnetic Susceptibility Artifacts


Susceptibility Artifacts


How to reduce metal artifacts

  • Use lower field strength

  • Angle slices so screws are within one slice


RF Leak

  • Lines appear all over image in the phase encoding direction at different positions in the frequency encoding direction


RF Noise


Zero-phase Artifact=Zipper Artifact

  • Occurs only with NEX=1

  • Occurs in the center of the image

  • Remove it by going to NEX=2


RF Artifact


Zipper Artifact


Magnetic Inhomogeneity Artifacts


Diamagnetic Susceptibility Artifact


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