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Velocity Compensation
 
See Gradient Moment Nulling.
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Motion Compensation in MR Imaging
   by ccn.ucla.edu    
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Motion Compensation Pulse SequencesInfoSheet: - Sequences - Intro, Overview, Types of, etc.MRI Resource Directory: - Sequences -
 
Pulse sequences, designed to be insensitive to flow, e.g. at every even echo, a spin echo sequence is not flow sensitive. Velocity compensation is achieved by using gradients, which are either symmetrical around a 180° pulse and switched on twice as is the case for motion compensated spin echo pulse sequences, or two antisymmetrical gradient lobes without 180° pulse, which is the way to produce a velocity compensated gradient echo pulse sequence.
The signal of the second echo (and all other even echoes) is independent of the velocity of the object. Thus, velocity-based motion effects stemming from the entire voxel or from spins within a voxel (intravoxel incoherent motion) are suppressed with such pulse sequences.
If higher order motion is relevant, as it may be in turbulent jets across valves, acceleration and jerk effects can also be compensated for by the use of appropriate combinations of gradient- and radio frequency pulses.
With the increasingly stronger gradients, echo times in MR systems can be shortened to the point at which effects other than velocity effects hardly ever become relevant.
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Further Reading:
  Basics:
Motion Compensation in MR Imaging
   by ccn.ucla.edu    
  News & More:
Patient movement during MRI: Additional points to ponder
Tuesday, 5 January 2016   by www.healthimaging.com    
Motion-compensation of Cardiac Perfusion MRI using a Statistical Texture Ensemble(.pdf)
June 2003   by www.imm.dtu.dk    
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Flow Related Enhancement
 
(FRE) Flow related enhancement could be seen most for blood flow, but also for other liquids with some MR imaging techniques, as an increase in intensity due to the washout of saturated spins. FRE provides positive contrast ("bright blood") of vascular details in time of flight MRA as well as the physiologic characterization of blood flow.
If stationary spins within the scanned region experience only an incomplete T1 relaxation between the repeated radio frequency (RF) excitations, this results in fewer signal of the stationary tissue (compared to inflowing blood with completely relaxed spins). The degree of the flow related enhancement is proportional to the blood flow velocity and the used repetition time. The use of flow compensation (gradient moment nulling) improves the FRE especially in gradient echo sequences.
 
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Conventional MRI and MR Angiography of Stroke
2012   by www.mc.vanderbilt.edu    
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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).
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• 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    
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Flow CompensationInfoSheet: - Artifacts - Case Studies, Reduction Index, etc.
 
Flow compensation is based on the principle of even echo rephasing and a function of specific pulse sequences, wherein the application of strategic gradient pulses can compensate for the objectionable spin phase effects of flow motion. Gradient moment nulling of the first order of flow is another adjustment for the reduction of flow artifacts.
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. The simplest velocity-compensated pulse sequence is the symmetrical second echo of a spin echo pulse sequence.
Strategic gradient pulses are integrated in special sequences (e.g. CRISP, Complex Rephasing Integrated with Surface Probes) and for the most sequences flow compensation is an optional parameter.
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• View the DATABASE results for 'Flow Compensation' (14).Open this link in a new window

 
Further Reading:
  Basics:
Motion Compensation in MR Imaging
   by ccn.ucla.edu    
Flow comp off: An easy technique to confirm CSF flow within syrinx and aqueduct
Wednesday, 2 January 2013   by medind.nic.in    
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