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 'Oversampling' 
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Result : Searchterm 'Oversampling' found in 3 terms [] and 7 definitions []
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Anti Aliasing
 
Anti aliasing is a method to prevent aliasing artifacts by properly choosing the sampling rate. The signal is sampled at a higher bandwidth than required.

See Oversampling.
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Backfolding ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - 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    
MRI Resources 
Movies - Blood Flow Imaging - DICOM - Education pool - Spectroscopy pool - Health
 
Field Inhomogeneity ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview
Artifact Information
NAME
Field inhomogeneity
DESCRIPTION
Image distortion signal loss
REASON
HELP
Larger FOV, oversampling
A disturbance of the field homogeneity, because of magnetic material (inside or outside the patient), technical problems or scanning at the edge of the field.
When images were obtained in a progression from the center to the edge of the coil, the homogeneity of the field observed by the imaged volume, changes when the distance from the center of the volume increase. The same problem appears by scanning at a distance from the isocenter in left-right direction or too large field of view.
There are different types of bad image quality, the images are noisy, distorted or the fat suppression doesn't work because of badly set shim currents.
E.g. by using an IR sequence, changes in the T1 recovery rates of the tissues are involved. The inversion time at the center of the imaged volume is appropriate to suppress fat, but at the edge of the coil the same inversion time is sufficient to suppress water. Since the inversion time is not changed, the T1 recovery rates will increase.
mri safety guidance
Image Guidance
Take a smaller imaging volume (and for fat suppression a volume shimming), take care that the imaged region is at the center of the coil and that no magnetic material is inside the imaging volume.
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• View the DATABASE results for 'Field Inhomogeneity Artifact' (3).Open this link in a new window

 
Further Reading:
  Basics:
MRI Artifact Gallery
   by chickscope.beckman.uiuc.edu    
MRI Resources 
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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

MRI Resources 
Functional MRI - Coils - Societies - Claustrophobia - Pediatric and Fetal MRI - Equipment
 
Zipper ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview
Artifact Information
NAME
Zipper, star
DESCRIPTION
Bands through image center
REASON
Hardware or software problems
HELP
Larger FOV, oversampling
Zipper artifacts appear as dashed lines. There are various causes for this MRI artifact.
Most of zipper artifacts result from inhomogeneities of the magnetic field caused by interferences with radio frequency from various sources ('your radio is working in the scanner room means your shielding is not working'). Software and equipment problems can also cause zipper lines in both directions.
mri safety guidance
Image Guidance
Scanning room door during the acquisition of images is closed, shielding is working, and all disturbing devices are removed?

See also Room Shielding, Active Shielding, Radio Frequency Shielding.
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MRI Resources 
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