Phase-encoded motion artifact
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Phase-encoded motion artifact is one of many MRI artifacts occurring as a result of tissue/fluid moving during the scan. It manifests as ghosting in the direction of phase-encoding, usually in the direction of the 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 repetition time (TR) and the frequency of the motion.
Motion artifacts can be distinguished from Gibbs or truncation artifacts because they extend across the entire field of view (FOV), unlike truncation artifacts that diminish quickly away from the boundary causing them.
Ways of identifying phase artifact include:
- identifying known moving/flowing structures and noting that the artifact is in line with them (horizontal or vertical depending on phase-encoding orientation)
- matching shape of ghost to that of flowing vessel (e.g. round pseudolesion due to aorta ghost)
- wide windowing to see repetitive ghost beyond confines of anatomy
- they can be distinguished from Gibbs or truncation artifacts because they extend across the entire field of view, unlike truncation artifacts that diminish quickly away from the boundary causing them
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, especially 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 movement
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