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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.

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Presentation on theme: "Chapter 14: Artifacts Mark D. Herbst, MD, PhD. Artifact Something on the image that does not represent something in the patient –Many causes Equipment."— Presentation transcript:

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 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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10 Increase FOV- Avoid Aliasing

11 Chemical Shift - Example

12 Chemical Shift Artifacts

13 Chemical Shift

14 Chemical Shift vs BW Per Pixel

15 Chemical Shift Effect

16 Chemical Shift - Second Kind

17 In- phase Out-of- phase

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

19 Motion Artifacts - Periodic

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21 Motion Artifacts - Random

22 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

23 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)

24 Metal on CT

25 Metal in bone

26 Ferromagnetic Susceptibility Artifacts

27 Susceptibility Artifacts

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

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

30 RF Noise

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

32 RF Artifact

33 Zipper Artifact

34 Magnetic Inhomogeneity Artifacts

35 Diamagnetic Susceptibility Artifact


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