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

Out-
      side
 



 
 'Nyquist Limit' 
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 'Nyquist Limit' found in 1 term [] and 2 definitions [], (+ 1 Boolean[] results
1 - 4 (of 4)     
Result Pages : [1]
MRI Resources 
Corporations - Contrast Agents - Liver Imaging - Implant and Prosthesis pool - Directories - Examinations
 
Nyquist Limit
 
According to Shannon's sampling theorem, the sampling frequency should be twice the frequency being sampled. The maximum measurable frequency is therefore equal to one half the sampling rate. This is the so-called Nyquist limit. When the frequency is higher than the Nyquist limit, aliasing occurs.
spacer
 
• Share the entry 'Nyquist Limit':  Facebook  Twitter  LinkedIn  
Searchterm 'Nyquist Limit' was also found in the following service: 
spacer
Ultrasound  (4) Open this link in a new window
Aliasing
 
If the receiving RF coil is sensitive to tissue signal arising from outside the desired FOV, this undesired signal may be incorrectly mapped to a location within the image, a phenomenon known as aliasing. This is 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 sampling frequency should be at least twice the frequency being sampled. The maximum measurable frequency is therefore equal to half the sampling frequency. This is the so-called Nyquist limit. When the frequency is higher than the Nyquist limit, aliasing occurs.
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. This signal will be mapped, or wrapped back into the image at incorrect locations, and is seen as artifact.

See also Aliasing Artifact.
spacer

• View the DATABASE results for 'Aliasing' (19).Open this link in a new window

 
Further Reading:
  News & More:
The Effects of Breathing Motion on DCE-MRI Images: Phantom Studies Simulating Respiratory Motion to Compare CAIPIRINHA-VIBE, Radial-VIBE, and Conventional VIBE
Tuesday, 7 February 2017   by www.kjronline.org    
MRI Resources 
Colonography - Directories - Most Wanted - Quality Advice - Cardiovascular Imaging - Breast Implant
 
Spectral Width
 
The overall width in hertz needed to observe a particular NMR spectrum. This width is generally set using the Nyquist limit; namely, that the temporal sampling rate must be equal to twice the maximum spread in frequencies.
spacer
MRI Resources 
Movies - DICOM - Distributors - Mass Spectrometry - Cardiovascular Imaging - Nerve Stimulator
 
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.
spacer

• View the DATABASE results for 'Oversampling' (10).Open this link in a new window

 
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    
Searchterm 'Nyquist Limit' was also found in the following service: 
spacer
Ultrasound  (4) Open this link in a new window
     1 - 4 (of 4)     
Result Pages : [1]
 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]