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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 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
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.
Motion Artifact • Most common cause of bad images • Periodic or random • Produces ghost images in the phase encoding direction
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)
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
Zero-phase Artifact=Zipper Artifact • Occurs only with NEX=1 • Occurs in the center of the image • Remove it by going to NEX=2
