Wrap-Around Reduction
How do I get rid of phase wrap-around artifacts?
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The simplest way to eliminate the phase wrap-around artifact is to increase the phase field-of-view (FOV) to encompass the entire anatomic dimension of the subject in that direction. This solution is illustrated below showing how wrap-around is progressively eliminated as the phase FOV is increased from 15 cm to 20 cm to 30 cm.
FOV = 15 cm
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FOV = 20 cm
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FOV = 30 cm
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This method cannot be exercised with impunity, however, for if the FOV is increased, spatial resolution will suffer. To maintain spatial resolution, therefore, the number of phase-encoding steps must be increased. Phase wrap-around will be eliminated, but at the cost of increased imaging time.
As an alternative strategy, the frequency- and phase-encoding axes may be swapped so that the shorter dimension of the subject is oriented in the phase-encode direction. If the field-of-view is very small, however, even the shorter anatomic dimension of the subject may still exceed the field-of-view, and phase wrap-around will still occur. Additionally, switching phase and frequency may introduce different unwanted artifacts over the image (such as motion-induced phase ghosts or chemical-shift artifacts), thus limiting the usefulness of this simple strategy.
Another group of techniques eliminates phase wrap-around by minimizing the signal from the tissue outside the field-of-view. These techniques include using a surface coil (which doesn't detect signals far away from it) and applying saturation pulses outside the field-of-view (to eliminate the signal from tissues there).
Finally, "phase oversampling" or "no-phase-wrap" techniques are available that largely eliminate the phase wrap-around artifact in most cases. These techniques are the subject of the next Q&A.
Another group of techniques eliminates phase wrap-around by minimizing the signal from the tissue outside the field-of-view. These techniques include using a surface coil (which doesn't detect signals far away from it) and applying saturation pulses outside the field-of-view (to eliminate the signal from tissues there).
Finally, "phase oversampling" or "no-phase-wrap" techniques are available that largely eliminate the phase wrap-around artifact in most cases. These techniques are the subject of the next Q&A.
References
Axel L, Morton D. Correction of phase wrapping in magnetic resonance imaging. Med Phys 1989;16:284-287.
Heiland S. From A as in aliasing to Z as in zipper: artifacts in MRI. Clin Neuroradiol 2008; 1:25-36.
Pusey E, Yoon C, Anselmo ML, Lufkin RB. Aliasing artifacts in MR imaging. Comput Med Imag Graphics 1988;12:219-224.
Zhuo J, Gullapalli RP. AAPM/RSNA physics tutorial for residents. MR artifacts, safety, and quality control. Radiographics 2006;26:275-297.
Axel L, Morton D. Correction of phase wrapping in magnetic resonance imaging. Med Phys 1989;16:284-287.
Heiland S. From A as in aliasing to Z as in zipper: artifacts in MRI. Clin Neuroradiol 2008; 1:25-36.
Pusey E, Yoon C, Anselmo ML, Lufkin RB. Aliasing artifacts in MR imaging. Comput Med Imag Graphics 1988;12:219-224.
Zhuo J, Gullapalli RP. AAPM/RSNA physics tutorial for residents. MR artifacts, safety, and quality control. Radiographics 2006;26:275-297.
Related Questions
Why does the phase wrap-around artifact occur?
Why does the phase wrap-around artifact occur?