Artifacts_ MR Flashcards
How can Gibbs artifact be minimized
Truncation errors (Gibbs artifact) can be minimized by increasing the number of phase-encode steps or by reducing the field of view.
What was the most likely step taken to eliminate the artifact seen in the first image?
Increasing the matrix size decreased truncation artifact (aka Gibbs, or ringing).
choosing a smaller FOV can also help eliminate this artifact
aliasing
also called Moire fringe artifact, commonly seen in GRE sequences and is a result of alternating constructive and destructive interference
Moire fringe artifact is commonly associated with what sequence?
GRE
aliasing (ear lobe into middle of image)
flow artifact
Bounce point artifact
- This artifact results from a mismatch of TR and TI, with either TR too long or TI too short
How do you decrease aliasing on MRI?
Why should we always get OOP before IP in the clinical setting?
water saturation
The accidental water saturation can be focal or affect an entire slice or even many slices. Watch for bright fat and very dark or black watery tissues.
Swap artifact with Dixon Fat suppression
- Dixon acquires in phase (IP) and out of phase (OOP) images
- Signal in IP: Water + fat
- Signal on OOP: Water - fat
- Water (fat suppressed) image: IP + OOP (Water + fat + water - fat = 2 x water)
- Fat (water suppressed) image: IP - OOP (Water + fat - (water - fat) = 2 x fat)
- Part of the process is for the scanner to identify what is fat and what is water and assign to the appropriate image
- If identification goes wrong, areas can be swapped between fat and water image (swap artifact)
- Important to always have the fat image reconstructed in case swap artifact occurs, since the tissue of interest will be visible in the fat image in the area of swap artifact
- If fat image is not reconstructed, information is lost.
- Repeating without fat saturation is an option, although more difficult to identify areas of enhancement and in this case fat saturation was not used, but Dixon which is a form of fat suppression
- Decreasing TR would not change the artifact, but would create more proton density weighting (depending on how much TR is changed)
- It is not an inversion recovery sequence, so not inversion time to change.
what is this artifact and what can be done to fix it?
k-space data error (herringbone)
A single erroneous data point (spike) in k-space creates a line pattern through the image
if data point is on the x-axis the lines will be verticle, if the data point error is on the y-axis the lines will be horizontal. If the lines are diagonal, the data error is not on an axis.
how to fix: Repeat the scan without parameter changes (usually due to random error). But, if it occurs frequently as faulty coil may be the problem
k-space data error (herringbone artifact)
Due to a data flitch/ reconstruction error
To fix this just reconstruct again and hopefully the error in k-space will resolve
chemical shift artifact, of the first kind
occurs in the frequency encoding direction.
black –> white is the direction of the frequency encoding direction.
dielectric (conductivity artifact)
most commonly in the setting of FLUID (ascites or amniotic fluid)
incident external RF field induces an electric current in the fluid and generates a magnetic field opposite to the main magnetic field
usually seen in 3T magnets
