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38 Cards in this Set
- Front
- Back
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What axis does flow motion appear in? |
Phase |
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What axis does chemical shift artefact appear in? |
Frequency |
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What axis does out of phase artefact appear in? |
frequency and phase |
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What axis does aliasing occur in? |
frequency and phase |
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What axis does zipper artefact occcur in? |
frequency |
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What axis does magnetic susceptibility occur in? |
frequency and phase |
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What axis does shading occur in? |
frequency and phase |
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What axis does patient motion occur in? |
phase |
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What axis does cross-talk occur in? |
slice select |
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What axis does cross-excitation occur in? |
slice select |
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What axis does morie occur in? |
phase and frequency |
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what axis does magic angle occur in? |
phase and frequency |
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Remedy for flow motion? |
swap phase and frequency (may need anti aliasing) gating (variable TR, variable image contrast, increased scan time) pre-sat (may lose a slice) gmn (increases min TE) |
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Remedy for chemical shift? |
increase BW (decrease min TE and SNR) reduce FOV (reduces SNR) use chemical saturation (reduces SNR) |
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Remedy for out of phase artefact |
select TE at periodicity of Fat and Water |
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Remedy for aliasing |
no frequency wrap no phase wrap |
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Remedy for magnetic susceptibility |
use SE remove metal |
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Remedy for shading |
check shim load coil correctly |
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Remedy for cross-excitation |
interleaving squaring off RF pulses |
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Remedy for Moire |
use SE patient not to touch bore |
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Remedy for magic angle |
change TE slightly alter position |
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How does magic angle appear? |
High signal intensity on tissue containing collagen |
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What angle does the magic angle appear? |
55 |
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How does Moire appear? |
Black and white banding artefact on edge of FOV. Always seen on gradient echo imaging |
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How does cross talk or excitation appear? |
adjacent slices in acquisition have different image contast |
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How does truncation apear |
Banding artefact at interfaces of high and low signal |
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Why does truncation appear |
under sampling of data(too few k space lines filled) so that interfaces of high and low signal are incorrectly represented in an image. |
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How does chemical shift appear? |
A dark edge at the interface between fat and water. |
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What axis does chemical shift appear? |
Frequency |
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What causes chemical shift? |
Chemical shift is caused by different chemical environments of fat and water. |
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What is the precessional different between fat and water at 1.5T? |
Fat precesses at 220Hz less than water |
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How do we reduce chemical shfit? |
Use large bandwidth smallest FOV possible Use chemical saturation Decrease field strength |
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How does chemical misregistration appear? |
Dark signal around certain organs where fat and water inferfaces occur within the same voxel |
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Why does chemical misregistration occur? |
Fat and water are out of phase with each other and cancel each other out |
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How do you get rid of chemical misregistration? |
Select TE that matches the periodicity of fat and water at your field strength Use a spin echo sequence (not GRE) |
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What is the periodicity of fat and water at 1.5T and 0.5 T |
1.5T = multiple of 4.2ms 0.5T = multiple of 7ms |
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Why is magnetic susceptibility artefact more prominent in GRE sequences? |
Gradient reversal cannot compensate for the phase difference at the itnerface |
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How do you decrease metal artefact? |
Remove object Spin echo sequence Decrease TE - more dephasing between tissues Broad Receive band width |
