THE ENCODED MAP IS TOO SMALL
Wrap occurs when signal-producing anatomy extends beyond the encoded FOV.
MRI can assign spatial positions uniquely only within the sampled field of view. When the object is larger than that FOV, outside signal may be mapped to a location one FOV away, placing it on the opposite side of the image.
Wrap is most often noticed in the phase-encoding direction because frequency encoding is commonly protected by oversampling and filtering.
ONE FOV APART LOOKS LIKE THE SAME ADDRESS
Undersampling makes two real locations share one displayed location.
Signal remains within the uniquely encoded region.
The portion beyond one edge appears at the opposite edge.
This is a sampling problem, not patient motion. Unlike a series of fading ghosts, wrap often looks like a recognizable piece of anatomy folded across the image.
CORRECTION OPTIONS
Make the encoded region fit, or prevent outside signal from folding into it.
Cover the full signal-producing anatomy in the phase direction. Pixel size changes unless the matrix is adjusted too.
“No phase wrap” options enlarge the sampled phase region while keeping the displayed FOV. Scan time or other limits may change.
Place the vulnerable direction where anatomy fits better, while checking motion, flow, and artifact direction.
Center the patient and coil appropriately; approved saturation bands can reduce unwanted signal outside the region of interest.
Parallel imaging deliberately undersamples and uses coil sensitivity information to unfold the image. Incorrect calibration, excessive acceleration, coil limitations, or anatomy outside the supported volume can leave residual aliasing. Treat it as an acquisition-and-reconstruction issue, not automatically as a simple small-FOV wrap.
THE CLOCK FACE
Move past 12 and you return to 1.
A clock has repeating positions. MRI wrap behaves similarly: move one full FOV beyond the encoded boundary and signal returns at the corresponding position on the opposite side.
Does the phase FOV include the anatomy?
Which axis is phase encoding?
Can an approved no-wrap option expand sampling?
Remember: If the anatomy does not fit, the extra signal can fold.
CHECK YOUR UNDERSTANDING
Recognize wrap before choosing the fix.
An arm outside the axial abdominal FOV appears over the opposite flank. What is the likely artifact?
Aliasing, also called wraparound. The arm extends beyond the encoded FOV and is mapped onto the opposite side.
Why is phase wrap more commonly visible than frequency wrap?
Frequency encoding is commonly oversampled and filtered by the system, while phase oversampling can cost scan time and is more dependent on protocol selection.
Will increasing phase FOV always leave resolution unchanged?
No. With the acquisition matrix held constant, a larger FOV creates larger pixels and lowers spatial resolution. Matrix, time, SNR, and coverage must be considered together.
LESSON 30 COMPLETE
You can recognize wrap and choose a correction that respects the full protocol.Educational references
Aliasing behavior and no-wrap controls vary by dimensionality, sequence, acceleration method, coil, and vendor. Follow approved protocols and scanner-specific guidance.