Phase-encoded motion artifact

Phase-encoded motion artifact is one of many MRI artifact, and occurs as a result of tissue/fluid moving during the scan and manifests as ghosting in the direction of phase encoding, usually in the direction of short axis of the image (i.e left to right on axial or coronal brains, and anterior to posterior on axial abdomen). 

These artifacts may be seen from arterial pulsations, swallowing, breathing, peristalsis, and physical movement of a patient. When projected over anatomy it can mimic pathology, and needs to be recognized. Motion that is random such as the patient moving produces a smear in the phase direction. Periodic motion, such as respiratory or cardiac/vascular pulsation, produces discrete, well-defined ghosts. The spacing between these ghosts is related to the TR and frequency of the motion.

Motion artifacts can be distinguished from Gibbs or truncation artifacts because they extend across the entire FOV, unlike truncation artifacts that diminish quickly away from the boundary causing them.

Ways of identifying phase artifact include:

  1. identifying known moving/flowing structures and noting that the artifact is in line with them (horizontal or vertical depending on phase encoding orientation)
  2. matching shape of ghost to that of flowing vessel (e.g. round pseudolesion due to aorta ghost)
  3. wide windowing to see repetitive ghost beyond confines of anatomy
  4. they can be distinguished from Gibbs or truncation artifacts because they extend across the entire FOV, unlike truncation artifacts that diminish quickly away from the boundary causing them
Remedy

Solutions to phase mismapping include:

  • cardiac/respiratory gating
  • spatial presaturation bands placed over moving tissues (e.g. over the anterior neck in sagittal cervical spines) 
  • spatial presaturation bands placed outside the FOV, especial before the entry or after the exit slice for reducing ghosting from vascular flow: arterial and venous
  • scanning prone to reduce abdominal excursion
  • switching phase and frequency directions
  • increasing the number of signal averages
  • shorten the scan time when motion is from patient moving
MRI physics
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Article Information

rID: 1868
Section: Physics
Tag: mri
Synonyms or Alternate Spellings:
  • Phase mismapping in MRI
  • Phase-encoded motion artefacts
  • Phase missmapping

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Cases and Figures

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    Case 1: phase artifact from sigmoid sinus
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    Case 2: phase-encoded motion artifact
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