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/aliasi

ng.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-phase

Out-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