Why MRI ghosts follow the phase-encoding direction

Why ghosts always follow the phase direction

One line of frequency encoding is read in a few milliseconds, too fast for the body to move. Phase encoding is built up line by line over the whole scan, seconds to minutes. If the anatomy moves between lines, the lines no longer agree, and the disagreement appears as copies spread along the phase direction, whatever the direction of the real movement.

You cannot remove the ghost by changing direction, but you can choose where it lands. Aim it away from the anatomy the radiologist needs.

Motion ghosting

Repeated faint copies of moving anatomy, or general blurring, spread along the phase direction.

The anatomy is in a different position at each phase-encoding step (breathing, swallowing, eye movement, tremor, peristalsis). Phase encoding takes seconds to minutes, frequency encoding only milliseconds, so the error shows in phase.

• Explain the scan, make the patient comfortable, immobilize with pads and straps
• Shorten the scan: parallel imaging, fewer averages, single-shot sequences
• PROPELLER / BLADE / MultiVane
• Swap phase direction or add a saturation band over the moving structure
• Breath-hold, respiratory triggering or navigator; antiperistaltic agent for bowel if ordered

Faster scans lose SNR. Radial methods take longer per slice and can add streaks. Swapping phase may need oversampling.