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Phase direction in mri

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MRI phase encoding

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MRI phase encoding direction artifacts wrap around radiology reference

Phase-encoding direction in MRI is one of the in-plane spatial directions used to locate signal. A short gradient is applied before signal readout, causing spins at different positions to accumulate different phase shifts. Repeating the sequence with different phase-gradient strengths fills the phase-encoding dimension of k-space.
It is perpendicular to the frequency-encoding (readout) direction.

Why it matters clinically

Most motion-related artifacts occur along the phase-encoding direction:
  • Respiratory, cardiac, swallowing, and vascular pulsation produce repeated ghost images in this direction.
  • Wrap-around (aliasing) usually occurs in the phase direction if anatomy outside the field of view is included.
  • MRI technologists select this direction to move ghosts and wrap-around away from the structure being assessed.
For example, in a brain MRI, phase direction may be set anterior-posterior or right-left so pulsation ghosts from vessels or CSF do not obscure the brainstem or pituitary. In thoracic imaging, it may be chosen to prevent cardiac or respiratory ghosts from crossing a lesion.

Practical rule

Choose the phase-encoding direction:
  1. Along the shorter body dimension when possible, to reduce wrap-around.
  2. So expected motion/flow ghosts project away from the region of interest.
  3. With awareness that more phase-encoding steps improve resolution but increase scan time.
Phase encoding is typically slower than frequency encoding because separate repetitions are required for each phase-encoding step. The textbook notes that increasing resolution in the phase-encoding axis increases acquisition time. Bradley and Daroff's Neurology in Clinical Practice, Magnetic Resonance Image Reconstruction, lines 43-46.
For a clear artifact-focused explanation, MRIQuestions on PE direction describes how phase direction is selected to control motion and wrap-around artifacts.

Phase direction inmri

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MRI phase encoding

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MRI phase encoding direction explanation motion ghost artifacts

Phase direction in MRI means the direction in which spatial position is encoded by changing the phase of proton signals.
  • It is one of the two in-plane directions of an MR image.
  • The other is the frequency-encoding (readout) direction.
  • The phase direction is perpendicular to the frequency direction.

Main importance

Motion artifacts and ghosting appear mainly in the phase-encoding direction.
Examples:
  • In brain MRI, eye motion can cause ghosts. Phase direction is often selected left-to-right so the ghosts do not cross the brain.
  • In chest or abdomen MRI, cardiac and respiratory motion creates ghosts along the selected phase direction. The direction is chosen so they move away from the organ or lesion of interest.
  • Wrap-around or aliasing also commonly occurs in the phase direction if body anatomy extends beyond the field of view.
Key exam point:
Changing phase direction does not remove motion artifact. It changes the direction in which the ghost artifact is displayed.
Higher resolution in the phase-encoding direction requires more phase-encoding steps and therefore a longer scan time. Bradley and Daroff's Neurology in Clinical Practice, Magnetic Resonance Image Reconstruction, lines 43-46.
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