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 'Phase Oversampling' 
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Phase Oversampling
 
The data acquisition beyond the FOV in phase encoding direction, with doubling the number of acquisitions and the scan time.

See also Oversampling and Aliasing Artifact.
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Foldover SuppressionInfoSheet: - Sequences - 
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A problem occurs in the phase encoding direction, where the phases of signal-bearing tissues outside of the FOV in the y-direction are a replication of the phases that are encoded within the FOV. This signal will be mapped (wrapped, backfolded) back into the image at incorrect locations.
Foldover suppression (phase oversampling, no phase wrap) is a user-selectable parameter that maps this signal to its correct location outside the FOV, then discards any signal from outside the FOV before displaying the image. In order to be able to choose this parameter, in most cases more than an average is necessary.

See also Phase Wrapping Artifact and Oversampling.
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No Phase Wrap
 
(NPW / PNW - Phase No Wrap) If the receiving RF coil is sensitive to tissue signal arising from outside the desired FOV, this undesired signal may be incorrectly mapped, or wrapped back to a location within the image and is seen as artifact. This problem occurs in the phase encoding direction, where the phases of signal-bearing tissues outside of the FOV in the y-direction are a replication of the phases that are encoded within the FOV.
A user-selectable parameter maps this signal to its correct location outside the FOV, then discards any signal from outside the FOV before displaying the image. No phase wrap works by filling k-space to the same extent, using twice as many phase encoding steps. In order to be able to choose this parameter, in most cases more than an average is necessary.

See Foldover Suppression and Oversampling.
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• View the DATABASE results for 'No Phase Wrap' (5).Open this link in a new window

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Oversampling
 
Oversampling is the increase in data to avoid aliasing and wrap around artifacts. Aliasing is the incorrectly mapping of tissue signal from outside the FOV to a location inside the FOV. This is caused by the fact, that the acquired k-space frequency data is not sampled density enough.
Oversampling in frequency direction, done by increasing the sampling frequency, prevents this aliasing artifact. The proper frequency based on the sampling theorem (Shannon sampling theorem/Nyquist sampling theorem) must be at least twice the frequency of each frequency component in the incoming signal. All frequency components above this limit will be aliased to frequencies between zero and half of the sampling frequency and combined with the proper signal information, which creates the artifact. Oversampling creates a larger field of view, more data needs to be stored and processed, but this is for modern MRI systems not a real problem. Oversampling in phase direction (no phase wrap), to eliminate wrap around artifacts, by increasing the number of phase encoding steps, results in longer scan/processing times.
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Further Reading:
  Basics:
The Basics of MRI
   by www.cis.rit.edu    
The Scientist and Engineer's Guide to Digital Signal Processing
   by www.dspguide.com    
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Backfolding ArtifactInfoSheet: - Artifacts - 
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 - Artifacts -
 
Quick Overview
Please note that there are different common names for this artifact.
Artifact Information
NAME
Backfolding, foldover, phase wrapping, wrap around
DESCRIPTION
Image wrap around
Backfolding always occurs due to wrong phase encoding caused by objects outside the planned FOV. Phase encoding gradients are scaled for the field of view only. Tissues outside the FOV do not get properly phase encoded relative to their actual position and 'wraps' into the opposite side of the image. The Backfolding artifact projects image contents which fall outside the imaging FOV back into the image; the back folded information thus reappearing on the other side of the image. In fact, information along the phase encoding direction can be viewed as projected onto a cylindrical screen with a circumference corresponding to the linear field of view dimension in the phase encoding direction.

See also Aliasing Artifact.
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• View the DATABASE results for 'Backfolding Artifact' (2).Open this link in a new window

 
Further Reading:
  Basics:
Aliasing or wrap around artifacts
Thursday, 31 March 2011   by de.slideshare.net    
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