Phase-encoded motion artifact

Phase-encoded motion artifact artefact is one of many MRI artifactsartefact, and occurs 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 artifactsartefacts 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 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 artifactartefact include:

1. identifying known moving/flowing structures and noting that the artifactartefact 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 psuedolesion 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 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