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

Out-
      side
 



 
 'Transverse Magnetization' 
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 'Transverse Magnetization' found in 1 term [] and 42 definitions []
previous     21 - 25 (of 43)     next
Result Pages : [1]  [2 3 4 5 6 7 8 9]
MRI Resources 
Education - Guidance - Stimulator pool - Equipment - MR Guided Interventions - Colonography
 
Fast Low Angle ShotInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.MRI Resource Directory:
 - Sequences -
 
(FLASH) A fast sequence producing signals called gradient echo with low flip angles. FLASH sequences are modifications, which incorporate or remove the effects of transverse coherence respectively.
FLASH uses a semi-random spoiler gradient after each echo to spoil the steady state (to destroy any remaining transverse magnetization) by causing a spatially dependent phase shift. The transverse steady state is spoiled but the longitudinal steady state depends on the T1 values and the flip angle. Extremely short TR times are possible, as a result the sequence provides a mechanism for gaining extremely high T1 contrast by imaging with TR times as brief as 20 to 30 msec while retaining reasonable signal levels. It is important to keep the TE as short as possible to suppress susceptibility artifacts.
The T1 contrast depends on the TR as well as on flip angle, with short TE.
Small flip angles and short TR results in proton density, and long TR in T2* weighting.
With large flip angles and short TR result T1 weighted images.

TR and flip angle adjustment:

TR 3000 ms, Flip Angle 90°
TR 1500 ms, Flip Angle 45°
TR 700 ms, Flip Angle 25°
TR 125 ms, Flip Angle 10°

The apparent ability to trade TR against flip angle for purposes of contrast and the variation in SNR as the scan time (TR) is reduced.

See also Gradient Echo Sequence.
 
Images, Movies, Sliders:
 Fetus (Brain) and Dermoid in Mother  Open this link in a new window
      

Courtesy of  Robert R. Edelman

 
spacer
 
• Related Searches:
    • Breath Hold Imaging
    • Turbo Fast Low Angle Shot
    • Spoiled Gradient Echo Sequence
    • Gradient Echo Sequence
    • T2 Weighted Image
 
Further Reading:
  News & More:
Motion Compensation in MR Imaging
   by ccn.ucla.edu    
Turbo-FLASH Based Arterial Spin Labeled Perfusion MRI at 7 T
Thursday, 20 June 2013   by www.plosone.org    
Usefulness of MR Imaging for Diseases of the Small Intestine: Comparison with CT
2000   by www.ncbi.nlm.nih.gov    
MRI Resources 
Collections - Mobile MRI Rental - Open Directory Project - Societies - MRCP - Safety Products
 
Free Induction Decay
 
(FID) A free induction decay curve is generated as excited nuclei relax. The amplitude of the FID signal becomes smaller over time as net magnetization returns to equilibrium. If transverse magnetization of the spins is produced, e.g. by a 90° pulse, a transient MR signal will result that will decay toward zero with a characteristic time constant T2 (or T2*); this decaying signal is the free induction decay.
The signal peaks of the echoes fall onto this T2 decay curve, while at each echo the signals arise and decay with T2*. The typical T2 relaxation times being of the order of 5-200 ms in the human body. The first part of the FID is not observable (named the 'receiver dead time') caused by residual effects of the powerful exciting radio frequency pulse on the electronics of the receiver.
spacer

• View the DATABASE results for 'Free Induction Decay' (8).Open this link in a new window

 
Further Reading:
  Basics:
Free induction decay
   by en.wikipedia.org    
  News & More:
Magnetic resonance imaging
   by www.scholarpedia.org    
MRI Resources 
Anatomy - Equipment - MRI Technician and Technologist Jobs - Abdominal Imaging - Universities -
 
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 α. It tilts the magnetization by a flip angle α, 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 α = 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 α pulse.
As a result of the short repetition time, the z-magnetization cannot fully recover and after a few initial α pulses there is an equilibrium established between z-magnetization recovery and z-magnetization reduction due to the α 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:
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:
MRI evaluation of fatty liver in day to day practice: Quantitative and qualitative methods
Wednesday, 3 September 2014   by www.sciencedirect.com    
T1rho-prepared balanced gradient echo for rapid 3D T1rho MRI
Monday, 1 September 2008   by www.ncbi.nlm.nih.gov    
MRI Resources 
General - Pathology - Collections - Cochlear Implant - Resources - Functional MRI
 
Gradient Moment Nulling
 
Gradient moment nulling used as motion artifact suppression technique (MAST) reduces constant velocity motion distortion in standard spin echo or gradient echo pulse sequences. It is an adjustment to zero at the echo time (TE) of the net moments of the amplitude of the waveform of the magnetic field gradients with time. The zeroth moment is the area under the curve. The first moment is the 'center of gravity' etc.
The aim is to minimize the phase shifts acquired by the transverse magnetization of excited nuclei moving along the gradients (including the effect of refocusing radio frequency pulses), particularly for the reduction of image artifacts due to motion.
spacer

• View the DATABASE results for 'Gradient Moment Nulling' (7).Open this link in a new window

 
Further Reading:
  Basics:
Motion Compensation in MR Imaging
   by ccn.ucla.edu    
MRI Resources 
Mass Spectrometry - MRI Training Courses - Corporations - Education - Quality Advice - Claustrophobia
 
Gradient Motion Rephasing
 
(GMR) The application of strategic gradient pulses can compensate the objectionable spin phase effects of flow motion. That means the reducing of flow effects, e.g. gradient moment nulling of the first order of flow. The simplest velocity-compensated pulse sequence is the symmetrical second echo of a spin echo pulse sequence.
Gradient field changes can be configured in such a way that during an echo the magnetization signal vectors for all pixels have zero phase angle independent of velocities, accelerations etc. of the measured tissue. E.g. the adjustment to zero at the time TE of the net moments of the amplitude of the waveform of the magnetic field gradients with time. The zeroth moment is the area under the curve, the first moment is the 'center of gravity' etc. The aim is to minimize the phase shifts acquired by the transverse magnetization of excited nuclei moving along the gradients (including the effect of refocusing RF pulses), particularly for the reduction of image artifacts due to motion.
Also called Flow Compensation (FC), Motion Artifact Suppression Technique (MAST), Flow motion compression (STILL), Gradient Rephasing (GR), Shimadzu Motion Artifact Reduction Technique (SMART).
spacer

• View the DATABASE results for 'Gradient Motion Rephasing' (2).Open this link in a new window

 
Further Reading:
  Basics:
Motion Compensation in MR Imaging
   by ccn.ucla.edu    
MRI Resources 
Implant and Prosthesis - Lung Imaging - Contrast Enhanced MRI - Software - Examinations - MRI Accidents
 
previous      21 - 25 (of 43)     next
Result Pages : [1]  [2 3 4 5 6 7 8 9]
 Random Page
 
Share This Page
FacebookTwitterLinkedIn

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



Personalized protocols (age, gender, body habitus, etc.) lead to :
more automated planning 
improved patient comfort 
better diagnostics 
optimized image quality 
nothing 

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]