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Dephasing
 
The spins (protons) go out of phase in the transverse plane when the Rf pulse is switched of. This dephasing is due to static magnetic field inhomogeneities intrinsic (i.e. susceptibility changes at tissue interfaces) and extrinsic (i.e. field inhomogeneities of the main magnetic field) to the examined object.
See also T2*.
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• Related Searches:
    • Jet Effect
    • Dephasing Gradient
    • Spin Phase Effect
    • Intravoxel Incoherent Motion
    • Spin Echo Sequence
 
Further Reading:
  Basics:
Spin echoes, CPMG and T2 relaxation - Introductory NMR & MRI from Magritek
2013   by www.azom.com    
Comparison of Fast Spin-Echo Versus Conventional Spin-Echo MRI forEvaluating Meniscal Tears
June 2005   by www.ajronline.org    
Contrast mechanisms in magnetic resonance imaging
2004   by www.iop.org    
MRI Resources 
Contrast Agents - Artifacts - Guidance - Bioinformatics - Cochlear Implant - Absorption and Emission
 
Dephasing Gradient
 
Magnetic field gradient pulse used to create spatial variation of phase of transverse magnetization. For example, it may be applied prior to signal detection in the presence of a magnetic field gradient with opposite polarity (or of the same polarity if separated by a refocusing RF pulse) so that the resulting gradient echo signal will represent a more complete sampling of the Fourier transformation of the desired image.
See also Spoiler Gradient Pulse.
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Further Reading:
  Basics:
RARE
Monday, 3 December 2012   by www2.warwick.ac.uk    
MRI Resources 
Mobile MRI - Implant and Prosthesis pool - Open Directory Project - General - Mass Spectrometry - Pacemaker
 
Gradient Recalled Echo SequenceInfoSheet: - Sequences - 
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The gradient recalled echo MRI sequence generates gradient echoes as a consequence of echo refocusing. The initial slice selective RF pulse applied to the tissue is less than 90° (typically rotation angles are between 10° and 90°). Immediately after this RF pulse, the spins begin to dephase.
Instead of a refocusing 180° RF pulse, reversing the gradient polarity produces a gradient echo. A negative phase encoding gradient and a dephasing frequency encoding gradient are used simultaneous. The switch on of the frequency encoding gradient produces an echo caused by refocusing of the dephasing, which is caused by the dephasing gradient.
TR and flip angle together control the T1, and TE control T2* weighting.
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Further Reading:
  Basics:
Magnetic resonance imaging
   by www.scholarpedia.org    
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Diffusion Weighted SequenceInfoSheet: - Sequences - 
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 - Diffusion Weighted Imaging -
 
Diffusion weighted imaging can be performed similar to the phase contrast angiography sequence. The gradients must be increased in amplitude to depict the much slower motions of molecular diffusion in the body.
While a T1 weighted MRI pulse sequence is diffusion sensitive, a quantitative diffusion pulse sequence was introduced by Steijskal and Tanner. Its characteristic features are two strong symmetrical gradient lobes placed on either side of the 180° refocusing pulse in a spin echo sequence. These symmetrical gradient lobes have the sole purpose of enhancing dephasing of spins, thereby accelerating intravoxel incoherent motion (IVIM) signal loss.
Dephasing is proportional to the square of the time (diffusion time) during which the gradients are switched on and the strength of the applied gradient field. Therefore, the use of high field gradient systems with faster and more sensitive sequences, make diffusion weighting more feasible.
Areas in which the protons diffuse rapidly (swollen cells in early stroke, less restriction to diffusion) will show an increased signal when the echo is measured relative to areas in which diffusion is restricted. For increased accuracy of diffusion measurement and image enhancement, useful motion correction techniques such as navigator echo and other methods should be used. In addition to this, applying the b-value calculated by the strength and duration of motion probing gradients with a high rate of accuracy is very important.
See also Apparent Diffusion Coefficient, ADC Map, Lattice Index Map.

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Further Reading:
  Basics:
Diffusion-Weighted Imaging
   by spinwarp.ucsd.edu    
Diffusion Imaging: From Basic Physics to Practical Imaging
1999   by ej.rsna.org    
EVALUATION OF HUMAN STROKE BY MR IMAGING
2000
  News & More:
Functional imaging with diffusion-weighted MRI for lung biopsy planning: initial experience
Thursday, 10 July 2014   by 7thspace.com    
Diffusion-weighted MRI sensitive for metastasis in pelvic lymph nodes
Sunday, 15 June 2014   by www.2minutemedicine.com    
Hopkins researchers use diffusion MRI technique to monitor ultrasound uterine fibroid treatment
Monday, 8 August 2005   by www.eurekalert.org    
DWI Best in Subacute Stroke Imaging
Tuesday, 1 June 2004   by www.hospimedica.com    
MRI Resources 
Mass Spectrometry - Societies - Fluorescence - Safety Products - Abdominal Imaging - MRI Accidents
 
Flow ArtifactInfoSheet: - Artifacts - 
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Reduction Index, 
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 - Artifacts -
 
Quick Overview
Please note that there are different common names for this artifact.

Artifact Information
NAME Spin phase effect, flow
DESCRIPTION Vascular ghosts (ghosting artifact), anomalous intensities in images
REASON Movement of body fluids
HELP Flow compensation, presaturation, triggering

Flow effects in MRI produce a range of artifacts, e.g. intravascular signal void by time of flight effects; turbulent dephasing and first echo dephasing, caused by flowing blood.
Through movement of the hydrogen nuclei (e.g. blood flow), there is a location change between the time these nuclei experience a radio frequency pulse and the time the emitted signal is received (because the repetition time is asynchronous with the pulsatile flow).
The blood flow occasionally produces intravascular high signal intensities due to flow related enhancement, even echo rephasing and diastolic pseudogating. The pulsatile laminar flow within vessels often produces a complex multilayered band that usually propagates outside the head in the phase encoded direction. Blood flow artifacts should be considered as a special subgroup of motion artifacts.


Image Guidance
Artifacts can be reduced by reduction of phase shifts with flow compensation (gradient moment nulling), suppression of the blood signal with saturation pulses parallel to the slices, synchronization of the imaging sequence with the heart cycle (cardiac triggering) or can be flipped 90° by swapping the phase//frequency encoding directions.
See also Flow Related Enhancement and Flow Effects.
 
Images, Movies, Sliders:
 Knee MRI Sagittal T1 003  Open this link in a new window
 
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Further Reading:
  News & More:
MRI measure of blood flow over atherosclerotic plaque may detect dangerous plaque
Friday, 5 April 2013   by www.sciencecodex.com    
Advanced Visualization Techniques Could Change the Paradigm for Diagnosis and Treatment of Heart Disease
Thursday, 31 May 2012   by www.sciencedaily.com    
MRI Resources 
Process Analysis - Spine MRI - Stimulator pool - Spectroscopy - Implant and Prosthesis pool - Functional MRI
 
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