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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:
    • Phase
    • Angiography
    • Blood Flow Imaging
    • Phase Contrast Angiography
    • Phase Shift
 
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MR–ANGIOGRAPHY(.pdf)
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Pregnancy - MRCP - Corporations - Used and Refurbished MRI Equipment - Colonography - MRI Technician and Technologist Schools
 
Phase Encoding
 
The process of locating a MR signal by altering the phase of spins in one dimension with a pulsed magnetic field gradient along that dimension prior to the acquisition of the signal.
If a gradient field is briefly switched on and then off again at the beginning of the pulse sequence right after the radio frequency pulse, the magnetization of the external voxels will either precess faster or slower relative to those of the central voxels.
During readout of the signal, the phase of the xy-magnetization vector in different columns will thus systematically differ. When the x- or y- component of the signal is plotted as a function of the phase encoding step number n and thus of time n TR, it varies sinusoidally, fast at the left and right edges and slow at the center of the image. Voxels at the image edges along the phase encoding direction are thus characterized by a higher 'frequency' of rotation of their magnetization vectors than those towards the center.
As each signal component has experienced a different phase encoding gradient pulse, its exact spatial reconstruction can be specifically and precisely located by the Fourier transformation analysis. Spatial resolution is directly related to the number of phase encoding levels (gradients) used. The phase encoding direction can be chosen, e.g. whenever oblique MR images are acquired or when exchanging frequency and phase encoding directions to control wrap around artifacts.
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• View the DATABASE results for 'Phase Encoding' (73).Open this link in a new window

 
Further Reading:
  Basics:
Magnetic resonance imaging
   by www.scholarpedia.org    
Aliasing or wrap around artifacts
Thursday, 31 March 2011   by de.slideshare.net    
MRI Resources 
MRI Training Courses - Mobile MRI - Equipment - Implant and Prosthesis pool - Shoulder MRI - Pregnancy
 
Postprocessing
 
MR images can be manipulated for evaluation in various ways. Postprocessing includes: Subtraction, addition, rotation, inversion, multiplanar reconstruction (MPR), maximum intensity projection (MIP), etc.
Subtraction is particularly useful in contrast enhanced MRI examinations (for example breast MRI, brain MRI). The pre contrast images are subtracted from the images after an injection of contrast agents (sometimes also called dye) for a better tumor detection.

See also Computer Aided Detection
 
Images, Movies, Sliders:
 PCA-MRA 3D Brain Venography Colored MIP  Open this link in a new window
    

 TOF-MRA Circle of Willis Inverted MIP  Open this link in a new window
    

 
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• View the DATABASE results for 'Postprocessing' (11).Open this link in a new window

 
Further Reading:
  Basics:
3D-DOCTOR Tutorial
   by www.ablesw.com    
  News & More:
Improvement of semantic segmentation through transfer learning of multi-class regions with convolutional neural networks on supine and prone breast MRI images
Thursday, 27 April 2023   by www.nature.com    
Cardiac MRI Becoming More Widely Available Thanks to AI and Reduced Exam Times
Wednesday, 19 February 2020   by www.dicardiology.com    
Searchterm 'Image' was also found in the following services: 
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Radio Frequency Noise ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview
Please note that there are different common names for this artifact.
Artifact Information
DESCRIPTION
Static on the image
REASON
Electromagnetic emissions
HELP
Shielding, eliminate the factor of disturbance
RF noise, which often appears as static on the image, can be caused by a medical device located anywhere in the MR procedure room. RF noise is a result of excessive electromagnetic emissions from the device that interference with the proper operation of the MR scanner. The interference is attenuated and aliased in the frequency direction.
mri safety guidance
Image Guidance
In the first case try to eliminate the factor of disturbance, if the problem persists screen the magnet- and room shielding.

See also Signal to Noise Ratio.
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• View the DATABASE results for 'Radio Frequency Noise Artifact' (3).Open this link in a new window

 
Further Reading:
  Basics:
MRI Artifact Gallery
   by chickscope.beckman.uiuc.edu    
MRI Resources 
Developers - Societies - MRI Physics - Libraries - Stimulator pool - Chemistry
 
Radio Frequency Overflow ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview
Please note that there are different common names for this artifact.
Artifact Information
NAME
Radio frequency overflow, data clipping
DESCRIPTION
Image non-uniform
REASON
Signal too intense
HELP
Manually decrease of the receiver gain
The received radio frequency signal is too strong, parts of the signal get lost by converting from analog to digital, resulting in a washed out image.
mri safety guidance
Image Guidance
Auto-prescanning usually adjusts the amplification at the receiver in a way, that the received signal could be processed without any loss. Else the receiver gain must be corrected manually.

See also Data Clipping Artifact, Artifact Overview and Artifacts Reduction Index.
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• View the DATABASE results for 'Radio Frequency Overflow Artifact' (2).Open this link in a new window

 
Further Reading:
  Basics:
RF Overflow Artifacts
   by www.mritutor.org    
  News & More:
Magnetic Resonance Imaging (MRI)
2003   by www.hull.ac.uk    
MRI Resources 
Chemistry - Blood Flow Imaging - Case Studies - Crystallography - Anatomy - Services and Supplies
 
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