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


Out-
      side
 



 
 'Gradient Echo' 
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 'Gradient Echo' found in 20 terms [] and 114 definitions []
1 - 5 (of 134)     next
Result Pages : [1 2 3 4]  [5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ... ]
Searchterm 'Gradient Echo' was also found in the following services: 
spacer
News  (1)  Resources  (2)  Forum  (6)  
 
Gradient EchoForum -
related threads
 
(GE) An echo signal generated from a free induction decay by means of a bipolar switched magnetic gradient. The echo is produced by reversing the direction of a magnetic field gradient or by applying balanced pulses of magnetic field gradient before and after a refocusing RF pulse so as to cancel out the position dependent phase shifts that have accumulated due to the gradient.
In the latter case, the gradient echo is generally adjusted to be coincident with the RF spin echo. When the RF and gradient echoes are not coincident, the time of the gradient echo is denoted echo time (TE) and the difference in time between the echoes is denoted time difference (TD).
Gradient echo does not refocus the effects of main field inhomogeneity and therefore is generally used with a short echo time. Disadvantages of gradient echo imaging are compromised anatomic details and artifacts in regions with varying susceptibility e.g. between the air-containing sinuses and brain and especially between haemorrhages and normal tissue.
See also Susceptibility Artifact.
spacer
 
• Share the entry 'Gradient Echo':  Facebook  Twitter  LinkedIn  
 
• Related Searches:
    • Cine Sequence
    • Fast Low Angle Shot
    • Dual Echo Sequence
    • Gradient Echo Sequence
    • Sequences
 
Further Reading:
  Basics:
Mapping of low flip angles in magnetic resonance(.pdf)
Saturday, 1 January 2011   by www.hal.inserm.fr    
A LANTHANIDE LANTHOLOGY(.pdf)
   by www.phy.davidson.edu    
Enhanced Fast GRadient Echo 3-Dimensional (efgre3D) or THRIVE
   by www.mri.tju.edu    
MRI Resources 
Supplies - Blood Flow Imaging - Societies - - Fluorescence - Directories
 
Coherent Gradient EchoInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.MRI Resource Directory:
 - Sequences -
 
Coherent gradient echo sequences can measure the free induction decay (FID), generated just after each excitation pulse or the echo formed prior to the next pulse. Coherent gradient echo sequences are very sensitive to magnetic field inhomogeneity. An alternative to spoiling is to incorporate residual transverse magnetization directly into the longitudinal steady state. These GRE sequences use a refocusing gradient in the phase encoding direction during the end module to maximize remaining transverse (xy) magnetization at the time when the next excitation is due, while the other two gradients are, in any case, balanced.
When the next excitation pulse is sent into the system with an opposed phase, it tilts the magnetization in the -a direction. As a result the z-magnetization is again partly tilted into the xy-plane, while the remaining xy-magnetization is tilted partly into the z-direction.
A fully refocused sequence with a properly selected and uniform f would yield higher signal, especially for tissues with long T2 relaxation times (high water content) so it is used in angiographic, myelographic or arthrographic examinations and is used for T2* weighting. The repetition time for this sequence has to be short. With short TR, coherent GE is also useable for breath hold and 3D technique. If the repetition time is about 200 msec there's no difference between spoiled or unspoiled GE. T1 weighting is better with spoiled techniques.
The common types include GRASS, FISP, FAST, and FFE.
The T2* component decreases with long TR and short TE. The T1 time is controlled by flip angle. The common TR is less than 50 ms and the common TE less than 15 ms
Other types have stronger T2 dependence but lower SNR. They include SSFP, CE-FAST, PSIF, and CE-FFE-T2.
Examples of fully refocused FID sequences are TrueFISP, bFFE and bTFE.

spacer

• View the DATABASE results for 'Coherent Gradient Echo' (6).Open this link in a new window

MRI Resources 
Spectroscopy pool - Corporations - Education - Functional MRI - RIS - Education pool
 
Gradient Echo SequenceForum -
related threadsInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.
 
Gradient Echo Sequence Timing Diagram (GRE - sequence) A gradient echo is generated by using a pair of bipolar gradient pulses. In the pulse sequence timing diagram, the basic gradient echo sequence is illustrated. There is no refocusing 180° pulse and the data are sampled during a gradient echo, which is achieved by dephasing the spins with a negatively pulsed gradient before they are rephased by an opposite gradient with opposite polarity to generate the echo.
See also the Pulse Sequence Timing Diagram. There you will find a description of the components.
The excitation pulse is termed the alpha pulse a. It tilts the magnetization by a flip angle a, which is typically between 0° and 90°. With a small flip angle there is a reduction in the value of transverse magnetization that will affect subsequent RF pulses. The flip angle can also be slowly increased during data acquisition (variable flip angle: tilt optimized nonsaturation excitation). The data are not acquired in a steady state, where z-magnetization recovery and destruction by ad-pulses are balanced. However, the z-magnetization is used up by tilting a little more of the remaining z-magnetization into the xy-plane for each acquired imaging line.
Gradient echo imaging is typically accomplished by examining the FID, whereas the read gradient is turned on for localization of the signal in the readout direction. T2* is the characteristic decay time constant associated with the FID. The contrast and signal generated by a gradient echo depend on the size of the longitudinal magnetization and the flip angle. When a = 90° the sequence is identical to the so-called partial saturation or saturation recovery pulse sequence. In standard GRE imaging, this basic pulse sequence is repeated as many times as image lines have to be acquired. Additional gradients or radio frequency pulses are introduced with the aim to spoil to refocus the xy-magnetization at the moment when the spin system is subject to the next a pulse.
As a result of the short repetition time, the z-magnetization cannot fully recover and after a few initial a pulses there is an equilibrium established between z-magnetization recovery and z-magnetization reduction due to the a pulses.
Gradient echoes have a lower SAR, are more sensitive to field inhomogeneities and have a reduced crosstalk, so that a small or no slice gap can be used. In or out of phase imaging depending on the selected TE (and field strength of the magnet) is possible. As the flip angle is decreased, T1 weighting can be maintained by reducing the TR. T2* weighting can be minimized by keeping the TE as short as possible, but pure T2 weighting is not possible. By using a reduced flip angle, some of the magnetization value remains longitudinal (less time needed to achieve full recovery) and for a certain T1 and TR, there exist one flip angle that will give the most signal, known as the "Ernst angle".
Contrast values:
PD weighted: Small flip angle (no T1), long TR (no T1) and short TE (no T2*)
T1 weighted: Large flip angle (70°), short TR (less than 50ms) and short TE
T2* weighted: Small flip angle, some longer TR (100 ms) and long TE (20 ms)

Classification of GRE sequences can be made into four categories:
T1 weighted or incoherent/(RF or gradient) spoiled GRE sequences
T1/T2* weighted or coherent//refocused GRE sequences
T2 weighted contrast enhanced GRE sequences
ultrafast GRE sequences
See also Gradient Recalled Echo Sequence, Spoiled Gradient Echo Sequence, Refocused Gradient Echo Sequence, Ultrafast Gradient Echo Sequence.
 
Images, Movies, Sliders:
 MRI Liver In Phase  Open this link in a new window
    
 MRI Liver Out Of Phase  Open this link in a new window
    
 MVP Parasternal  Open this link in a new window
 Breast MRI Images T1 Pre - Post Contrast  Open this link in a new window
 Circle of Willis, Time of Flight, MIP  Open this link in a new window
    
SlidersSliders Overview

 
spacer

• View the DATABASE results for 'Gradient Echo Sequence' (70).Open this link in a new window

 
Further Reading:
  Basics:
Enhanced Fast GRadient Echo 3-Dimensional (efgre3D) or THRIVE
   by www.mri.tju.edu    
  News & More:
T1rho-prepared balanced gradient echo for rapid 3D T1rho MRI
Monday, 1 September 2008   by www.ncbi.nlm.nih.gov    
Searchterm 'Gradient Echo' was also found in the following services: 
spacer
News  (1)  Resources  (2)  Forum  (6)  
 
Gradient Echo Multi SliceInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.
 
(GEMS) This pulse sequence uses a changeable flip angle instead of a 90° pulse and a gradient instead of a RF pulse to rephase the FID.
T2*, T1 weighted and proton density images can be acquired. The flip angle in combination with the TR determines the T1 weighting and the TE controls the amount of dephasing. To minimize T2* the echo time should be short.
See also Gradient Echo Sequence.
spacer
MRI Resources 
Abdominal Imaging - DICOM - Bioinformatics - Devices - Mobile MRI - Movies
 
Multiplanar Gradient EchoInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.
 
(MPGE) GE sequence, with a multislice acquisition in one TR.
spacer
 
Further Reading:
  News & More:
Mapping of low flip angles in magnetic resonance(.pdf)
Saturday, 1 January 2011   by www.hal.inserm.fr    
MRI Resources 
Directories - RIS - Nerve Stimulator - Education pool - Spectroscopy - Shoulder MRI
 
     1 - 5 (of 134)     next
Result Pages : [1 2 3 4]  [5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ... ]
 Random Page
 
Share This Page
FacebookTwitterLinkedIn

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



Acoustic Noise Reduction (Silent, Quiet, etc.) :
cannot get better 
is in its first steps 
is done by earplugs, headphones 
must get better 
is bad, I miss something 
is unnecessary 

Look
      Ups





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. [ 22 January 2017]
Terms of Use | Privacy Policy | Advertising
 [last update: 2017-01-13 03:47:00]