THE ADDRESS CHANGES MID-SCAN
MRI expects each k-space line to describe anatomy in the same position and state.
Movement between measurements changes signal magnitude, phase, or both. Reconstruction combines those inconsistent measurements as though nothing moved, so signal may be blurred, lost, or displayed in the wrong location.
Because phase encoding is built across many repetitions, motion artifacts usually extend in the phase-encoding direction. Motion can still affect other directions and dimensions depending on the acquisition.
THE PATTERN GIVES A CLUE
Periodic motion ghosts. Irregular motion smears.
Breathing, cardiac pulsation, or rhythmic flow can create regularly spaced replicas.
Random movement, swallowing, peristalsis, or changing breath depth can spread signal less predictably.
Motion during central k-space acquisition often produces prominent blur or contrast change because the center carries much of the image's overall signal and contrast information.
FIND THE SOURCE BEFORE THE FIX
Patient motion and flowing spins need different tools.
Explain the sequence, support the anatomy comfortably, use appropriate immobilization, communicate, and shorten the acquisition when possible.
Use clear breath-hold coaching, respiratory triggering or navigation, approved compensation methods, or faster/free-breathing techniques suited to the patient.
Check gating quality, lead placement, rhythm, trigger timing, and whether the selected acquisition handles irregular cycles appropriately.
Consider saturation bands, flow compensation or gradient-moment nulling, gating, phase-direction changes, and sequence-specific flow control.
Swapping phase and frequency can redirect ghosts away from critical anatomy, but it does not stop the motion. Always check whether the new direction creates wrap, flow, or other problems.
THE PANORAMA PHOTO
If someone moves while the camera builds the picture, parts repeat or smear.
A panorama is assembled from multiple views. A moving person may appear twice or stretch across the image. MRI likewise combines measurements collected at different times.
Repeated ghosts suggest periodic motion.
Follow the phase-encoding direction first.
Find what moved before choosing the correction.
Remember: Pattern, phase direction, moving source.
CHECK YOUR UNDERSTANDING
Read the artifact before changing the protocol.
Regular copies of the abdominal wall extend across the image. What should you check first?
Check the phase-encoding direction and the patient's breathing pattern. Periodic respiration commonly creates repeated ghosts along phase.
Why can flow create ghosting or signal loss?
Moving spins accumulate phase differently from stationary spins and may enter or leave the slice between excitation and readout. Pulsatile variation between phase steps can also create repeated signal.
Does changing phase direction remove motion?
No. It redirects where phase-encoded ghosting appears. That may protect the anatomy of interest, but the underlying motion still needs an appropriate prevention or compensation strategy.
LESSON 29 COMPLETE
You can connect a motion pattern with its likely source and correction family.Educational references
No single motion-correction method works for every patient, sequence, or anatomy. Use approved protocols, institutional policy, and scanner-specific guidance.