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Result : Searchterm 'Time Of Flight' found in 2 terms [] and 16 definitions []
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Phase Contrast SequenceMRI Resource Directory:
 - Sequences -
 
(PC) Phase contrast sequences are the basis of MRA techniques utilizing the change in the phase shifts of the flowing protons in the region of interest to create an image. Spins that are moving along the direction of a magnetic field gradient receive a phase shift proportional to their velocity.
In a phase contrast sequence two data sets with a different amount of flow sensitivity are acquired. This is usually accomplished by applying gradient pairs, which sequentially dephase and then rephase spins during the sequence. Both 2D and 3D acquisition techniques can be applied with phase contrast MRA.
The first data set is acquired with a flow compensated sequence, i. e. without flow sensitivity. The second data set is acquired with a flow sensitive sequence. The amount of flow sensitivity is controlled by the strength of the bipolar gradient pulse pair, which is incorporated into the sequence. Stationary tissue undergoes no effective phase change after the application of the two gradients. Caused by the different spatial localization of flowing blood to stationary tissue, it experiences a different size of the second bipolar gradient compared to the first. The result is a phase shift.
The raw data from the two data sets are subtracted. By comparing the phase of signals from each location in the two sequences the exact amount of motion induced phase change can be determined to have a map where pixel brightness is proportional to spatial velocity.
Phase contrast images represent the signal intensity of the velocity of spins at each point within the field of view. Regions that are stationary remain black while moving regions are represented as grey to white.
The phase shift is proportional to the spin's velocity, and this allows the quantitative assessment of flow velocities. The difference MRI signal has a maximum value for opposite directions. This velocity is typically referred to as venc, and depends on the pulse amplitude and distance between the gradient pulse pair. For velocities larger than venc the difference signal is decreased constantly until it gets zero. Therefore, in a phase contrast angiography it is important to correctly set the venc of the sequence to the maximum flow velocity which is expected during the measurement. High venc factors of the PC angiogram (more than 40 cm/sec) will selectively image the arteries (PCA - arteriography), whereas a venc factor of 20 cm/sec will perform the veins and sinuses (PCV or MRV - venography).

See also Flow Quantification, Contrast Enhanced MR Venography, Time of Flight Angiography, Time Resolved Imaging of Contrast Kinetics.
 
Images, Movies, Sliders:
 PCA-MRA 3D Brain Venography Colored MIP  Open this link in a new window
    

 
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• Related Searches:
    • Blood Flow Imaging
    • Magnetic Resonance Angiography MRA
    • Phase Shift
    • Blood Flow-Velocity
    • Phase
 
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MR–ANGIOGRAPHY(.pdf)
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Spin Phase EffectInfoSheet: - Artifacts - 
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The spins flow with the blood through a slice and experience a RF pulse. If they flow out of the slice by the time the signal is recorded (because the repetition time (TR) is asynchronous with the pulsatile flow), the flowing blood produces intravascular signal void by 'time of flight' effects, turbulent dephasing and first echo dephasing. The liquid flow occasionally produces an intravascular high signal intensity due to flow related enhancement, even echo rephasing and diastolic pseudogating.

See also Flow Artifact and Flow Effects.
 
Images, Movies, Sliders:
 TOF-MRA Circle of Willis Inverted MIP  Open this link in a new window
    

 
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• View the DATABASE results for 'Spin Phase Effect' (3).Open this link in a new window

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Vessel Separation
 
Postprocessing techniques for artery vein separation by segmentation of the entire vessel structures from the background with different approaches.
Phase contrast imaging can also be used to differentiate arteries and veins based on flow-velocity or flow direction. The fact that arterial and venous flows are in opposite directions also has the advantage to separate the vessels in time of flight angiography.
 
Images, Movies, Sliders:
 CE-MRA of the Carotid Arteries Colored MIP  Open this link in a new window
    
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 TOF-MRA Circle of Willis Inverted MIP  Open this link in a new window
    

 
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Further Reading:
  News & More:
Usefulness of non-contrast-enhanced magnetic resonance imaging prior to venous interventions
Tuesday, 18 September 2018   by www.ncbi.nlm.nih.gov    
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