Magnetic Resonance - Technology Information Portal Welcome to MRI Technology••


Your MRI scanner(s) in 5 years should be :
more automated 
much quicker 
voice controlled 
iPhone compatible 
as it is right now 

Dear Guest, Your Attention Please:  

In the next days, the daily page limit for non-members will decrease to 20 pages to split up resources in favour of the MR-TIP Community. Beyond this limitation, most resources will still be available for everyone. If you want to join the MR-TIP Community, please register here.

MRI Artifacts
Ultrasound Imaging Artifacts Open this link in a new window
Aliasing Artifact 
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
REASON Undersampling in k-space
HELP Larger FOV, oversampling, foldover suppression

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.

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.

• View the DATABASE results for 'Aliasing Artifact' (11).Open this link in a new window

Case Study 1 :
The bright tissue on the left side of the image (red arrow) is the aliasing artifact wrapped in from the outer right side of the image.
Case Study 2 :
This case study shows a 3D acquisition technique with aliasing (in the red circle) in the slice selection direction. The image of the upper leg wraps into the image of the lower leg. Using the phase encoding gradient for two directions causes this artifact.
Similar Artifacts:
 • Phase Encoded Motion Artifact
 • Sample Imperfection (Artifact)
  Artifact by Patient Movement top
Share This Page

Forgot your UserID/Password ?  

      Ups uses cookies! By browsing, you agree to our use of cookies.

Magnetic Resonance - Technology Information Portal
Member of SoftWays' Medical Imaging Group - MR-TIP • Radiology-TIP • US-TIP • The-Medical-Market
Copyright © 2003 - 2016 SoftWays. All rights reserved. [ 25 September 2017]
Terms of Use | Privacy Policy | Advertising
 [last update: 2017-09-05 04:51:00]