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Chapter 14: Artifacts Mark D. Herbst, MD, PhD
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Artifact Something on the image that does not represent something in the patient –Many causes Equipment malfunctions Environmental factors Patient motion Body composition
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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
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Wraparound
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Wrap-around artifact
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Aliasing: Example
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Wraparound Artfacts in 3D
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How to avoid Aliasing Use larger FOV http://www.e-mri.org/quality- artifacts/aliasing.htmlhttp://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.
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Increase FOV- Avoid Aliasing
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Chemical Shift - Example
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Chemical Shift Artifacts
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Chemical Shift
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Chemical Shift vs BW Per Pixel
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Chemical Shift Effect
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Chemical Shift - Second Kind
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In- phase Out-of- phase
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Motion Artifact Most common cause of bad images Periodic or random Produces ghost images in the phase encoding direction
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Motion Artifacts - Periodic
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Motion Artifacts - Random
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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
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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)
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Metal on CT
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Metal in bone
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Ferromagnetic Susceptibility Artifacts
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Susceptibility Artifacts
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How to reduce metal artifacts Use lower field strength Angle slices so screws are within one slice
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RF Leak Lines appear all over image in the phase encoding direction at different positions in the frequency encoding direction
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RF Noise
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Zero-phase Artifact=Zipper Artifact Occurs only with NEX=1 Occurs in the center of the image Remove it by going to NEX=2
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RF Artifact
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Zipper Artifact
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Magnetic Inhomogeneity Artifacts
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Diamagnetic Susceptibility Artifact
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