Magnetic Resonance - Technology Information Portal Welcome to MRI Technology
Info
  Sheets

Out-
      side
 



 
 'phase encoding direction' 
SEARCH FOR    
 
  2 3 5 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Result : Searchterm 'phase encoding direction' found in 0 term [] and 27 definitions []
1 - 5 (of 27)     next
Result Pages : [1 2 3 4 5 6]
Searchterm 'phase encoding direction' was also found in the following service: 
spacer
Forum  (1)  
 
Phase Encoding
 
The process of locating a MR signal by altering the phase of spins in one dimension with a pulsed magnetic field gradient along that dimension prior to the acquisition of the signal.
If a gradient field is briefly switched on and then off again at the beginning of the pulse sequence right after the radio frequency pulse, the magnetization of the external voxels will either precess faster or slower relative to those of the central voxels.
During readout of the signal, the phase of the xy-magnetization vector in different columns will thus systematically differ. When the x- or y- component of the signal is plotted as a function of the phase encoding step number n and thus of time n TR, it varies sinusoidally, fast at the left and right edges and slow at the center of the image. Voxels at the image edges along the phase encoding direction are thus characterized by a higher 'frequency' of rotation of their magnetization vectors than those towards the center.
As each signal component has experienced a different phase encoding gradient pulse, its exact spatial reconstruction can be specifically and precisely located by the Fourier transformation analysis. Spatial resolution is directly related to the number of phase encoding levels (gradients) used. The phase encoding direction can be chosen, e.g. whenever oblique MR images are acquired or when exchanging frequency and phase encoding directions to control wrap around artifacts.
spacer
 
• Share the entry 'Phase Encoding':  Facebook  Twitter  LinkedIn  
 
• Related Searches:
    • Gradient
    • Precession
    • Pulse Sequence Timing Diagram
    • Magnetic Resonance
    • Frequency Encoding
 
Further Reading:
  Basics:
Magnetic resonance imaging
   by www.scholarpedia.org    
Aliasing or wrap around artifacts
Thursday, 31 March 2011   by de.slideshare.net    
MRI Resources 
Knee MRI - Fluorescence - MRI Physics - Non-English - Breast Implant - Contrast Agents
 
Sample Imperfection (Artifact)InfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview
Artifact Information
NAME
Sample imperfection
DESCRIPTION
Shifts of the signal in the phase encoding direction
REASON
Distorting the k-space trajectory, reduced bandwidth
HELP
Fat suppression, more excitations
Artifacts either by distorting the k-space trajectory (i.e. due to imperfect shimming) or as a consequence of the reduced bandwidth in the phase encode direction, commonly with EPI sequences.
While a standard spin warp-based sequence has an infinitely large bandwidth in the phase encode direction (about 1 or 2 kH), the bandwidth in EPI is related to the time between the gradient echoes (about a millisecond).
Hence even small frequency offsets can result in significant shifts of the signal in the phase encoding direction. Segmentation can introduce ghosting if there are significant difference in the amplitude and phase of the signal. This can be a particular problem when trying to acquire the segments in rapid succession.
mri safety guidance
Image Guidance
Suitable choices of excitation schemes and/or subsequent correction can help to reduce this artifact. The signal from fat can easily be offset by a large fraction of the FOV, and must be suppressed. The effect of frequency offsets can be reduced by collecting data with more than one excitation, which effectively increases the bandwidth in the phase encoding direction.
spacer
MRI Resources 
Pathology - Bioinformatics - Raman Spectroscopy - Homepages - Artifacts - NMR
 
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.
spacer

• 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    
Searchterm 'phase encoding direction' was also found in the following service: 
spacer
Forum  (1)  
 
Ghosting ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview
Artifact Information
NAME
Ghosting, ghost
DESCRIPTION
Displaced reduplications of image in phase encoding direction
REASON
Motion, heartbeat, respiration
HELP
Triggering, breath hold, pharmaceuticals to reduce bowel motion
Ghosting artifacts are in the most cases caused by movements (e.g., respiratory motion, bowel motion, arterial pulsations, swallowing, and heartbeat) and appear in the phase encoding direction.
mri safety guidance
Image Guidance
Ghosting artifacts can be reduced by respiratory and cardiac triggering, the use of breath holding pulse sequences, flow compensation or presaturation pulses, depending on their origin. To reduce bowel motion also pharmaceuticals, such as glucagon or scopolamine are useful. This will decrease artifacts from both peristalsis and breathing.

See also Motion Artifact, Phase Encoded Motion Artifact, Cardiac Motion Artifact, and Artifact Reduction - Motion.
spacer

• View the DATABASE results for 'Ghosting Artifact' (5).Open this link in a new window

 
Further Reading:
  Basics:
MRI Artifact Gallery
   by chickscope.beckman.uiuc.edu    
MRI Resources 
Corporations - Online Books - Fluorescence - Blood Flow Imaging - Mobile MRI Rental - Software
 
Rectangular Field of View
 
(RFOV) A different field of view (the scanned region) in the frequency and phase encoding directions that means the data acquisition with fewer measurement lines. Because there are fewer rows than columns, a rectangular image is obtained. To reduce the FOV in phase encoding direction (foldover direction) saves scan time by decreasing signal but invariable spatial resolution.
Also called HFI or undersampling.
mri safety guidance
Image Guidance
If the scanned object is oval, e.g. head or abdomen, a rectangular FOV is an easy to use scan parameter to reduce the scan time without loss of resolution.
spacer

• View the DATABASE results for 'Rectangular Field of View' (2).Open this link in a new window

MRI Resources 
Software - Safety Training - Pacemaker - Bioinformatics - MRI Accidents - MRI Physics
 
     1 - 5 (of 27)     next
Result Pages : [1 2 3 4 5 6]
 Random Page
 
Share This Page
FacebookTwitterLinkedIn

MR-TIP    
Community   
User
Pass
Forgot your UserID/Password ?    



New acceleration techniques will :
reduce scan times 
cause artifacts 
increase expenses 
be useful if you have a lot of experience 
doesn't do much 
never heard of 

Look
      Ups





MR-TIP.com uses cookies! By browsing MR-TIP.com, you agree to our use of cookies.

Magnetic Resonance - Technology Information Portal
Member of SoftWays' Medical Imaging Group - MR-TIP • Radiology-TIP • Medical-Ultrasound-Imaging • 
Copyright © 2003 - 2024 SoftWays. All rights reserved. [ 29 March 2024]
Terms of Use | Privacy Policy | Advertising
 [last update: 2024-02-26 03:41:00]