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Image Data Filter
 
Various reconstruction filters can be applied to MR images to reduce noise.
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Filtering
 
Filtering deletes components of the signal, high or low frequencies, band-pass, analog or digital. Whatever pattern or algorithms can be defined for data decimation.
Low pass filtering attenuates high frequency data and passes low frequency data. The reconstructed image will look a little blurrier, but nearly similar to the original image. The blurring is caused by the fact that the high spatial frequencies are lost, which contain information about edges in the image.
High pass filtering attenuates low frequencies and passes high frequencies. Most of the objects and contrast of the original image are lost in the reconstructed image, but the edges are clearly visible because high frequency data has been preserved.
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Further Reading:
  Basics:
MR Image Reconstruction from Raw Data
   by dukemil.egr.duke.edu    
  News & More:
The Scientist and Engineer's Guide to Digital Signal Processing
   by www.dspguide.com    
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FilterForum -
related threads
 
Filtering is any process that alters the relative frequency content. This can be done with an analog (conventional electrical) filter, e.g. to remove higher frequency components so as to avoid aliasing in digitizing. Filtering can be carried out numerically on the digitized data. Raw data can be filtered prior to the image calculation. The Hanning filter is provided with various weightings, e.g. for the reduction of edge oscillation.
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Further Reading:
  News & More:
LCR Electronics Offers Quick Delivery on New EMI Filters That Eliminate RF Interference
Wednesday, 11 May 2011   by www.prnewswire.com    
Vascular Filters of Functional MRI: Spatial Localization Using BOLD and CBV Contrast
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Ringing Artifact Reduction
 
Multiplication of acquired MR data by a function smoothly tapering off at higher spatial frequencies so as to reduce ringing artifacts near edges in the corresponding image or spectrum due to truncation and Gibbs phenomenon. It is a form of filtering.
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Aliasing ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
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 - Artifacts -
 
Quick Overview
Please note that there are different common names for this MRI artifact.
Artifact Information
NAME
Aliasing, backfolding, foldover, phase wrapping, wrap around
DESCRIPTION
Image wrap around
Aliasing is an artifact that occurs in MR images when the scanned body part is larger than field of view (FOV). As a consequence of the acquired k-space frequencies not being sampled densely enough, whereby portions of the object outside of the desired FOV get mapped to an incorrect location inside the FOV. The cyclical property of the Fourier transform fills the missing data of the right side with data from behind the FOV of the left side and vice versa. This is caused by a too small number of samples acquired in, e.g. the frequency encoding direction, therefore the spectrums will overlap, resulting in a replication of the object in the x direction.
Aliasing in the frequency direction can be eliminated by twice as fast sampling of the signal or by applying frequency specific filters to the received signal.
A similar 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. Phase encoding gradients are scaled for the field of view only, therefore tissues outside the FOV do not get properly phase encoded relative to their actual position and 'wraps' into the opposite side of the image.
mri safety guidance
Image Guidance
Use a larger FOV, RFOV or 3D Volume, apply presaturation pulses to the undesired tissue, adjust the position of the FOV, or select a small coil which will only receive signal from objects inside or near the coil. The number of phase encoding steps must be increased in phase direction, unfortunately resulting in longer scan times.
When this is not possible it can be corrected by oversampling the data. Aliasing is eliminated by Oversampling in frequency direction. No Phase Wrap (Foldover Suppression) options typically correct the phase encoding by doubling the field of view, doubling the number of phase encodes (to keep resolution constant) and halving the number of averages (to keep scan time constant) then discarding the additional data and processing the image within the desired field of view (but this is more time consuming).
Tissue outside this doubled area can be folded nevertheless into the image as phase wrap. In this case combine more than 2 number of excitations / number of signal averages with foldover suppression.
See also Aliasing, Foldover Suppression, Oversampling, and Artifact Reduction - Aliasing.
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