Artifacts Flashcards
Which coil/FOV pairing is more sensative to aliasing artifacts?
Large coil with a small FOV
Aliasing in the FREQUENCY direction is compensated for automatically by frequency filtering —- True/False
TRUE
What are common causes of aliasing
-Undersampling along the phase axis
-Using an active coil that is much larger than the anatomy of interest or in the FOV
-Using a phase FOV smaller than the anatomy of interest or the active Coil
What are common methods for compensating for Aliasing/wrap around artifacts?
- Enlarge FOV
- Applying phase over sampling (no wrap)
- Properly center anatomy to the RF coil
- Disengage RF coils that are not needed
- Avoid the use of oversized coils
- Place pre saturation pulses over unwated anatomy.
What imaging artifact appears as bands of high and low signal intensities running parallel to tissue interfaces?
Truncation/Gibbs artifact
Common in sagittal spine imaging
Truncation/ Gibbs artifacts mimic what in the sagittal cervical spine
Syrinxes
Truncation/Gibbs artifact are caused by
Undersampling
Truncation artifacts can be compensated for by increasing what
NEX or Phase encoding matrix
As magnetic field strength ________, Chemical shift artifact ________
Increases / Increases
As magnetic field strength ________, the processional frequency difference between fat and water ____________
Increases / Increases
At 1T, the precessional frequency difference between fat and water is:
147 Hz
220 @ 1.5T
440@ 3T
As receive bandwidth _______, chemical shift artifact _________.
Narrows / Increases
Chemical Shift artifact can be compensated for by
- Increasing the receive bandwidth
- Decreasing the pixel size
- Scanning at lower field strengths
Another name for chemical misregistration artifact is
Out of phase artifact
What image artifact appears as rings of dark signal around organs where fat and water interface within the same voxels?
Chemical misregistration artifact
Chemical misregisration artifacts are caused by
Fat and water precessing at different frequencies where they are out of phase with eachother at certain intervals
Chemical misregistration artifact or out-of-phase artifact is common during what pulse sequences
Gradient Echo pulse sequences
TE Values in intervals of ______ ms can results in chemical misregisration artifacts
2.1
Chemical mis registration artifacts can be compensated for by _________ or __________.
Using SE sequence / Applying a TE in intervals of 4.2ms
Dixon Technique
Out of phase imaging
What artifact appears as dark signal void image distortions near the presence of metallic objects?
Magnetic susceptibility artifact
Magnetic susceptibility artifacts can be caused by
- Metallic/ferromagnetic objects
- Iron content from hemorrhages
- Areas of air-tissue interface
Magnetic susceptibility artifacts can be compensated for by:
- Using a FSE sequence with a long ETL
- Decreasing the TE
- Removing all metallic/ferromagnetic objects
Can be decreased by FSE/TSE pulse sequences
Radiofrequency/Zipper artifacts occur in _________ direction
Phase direction
What imaging artifact appears as a los of spatial resolution
Partial volume averaging
Present within all image to some degree
Partial volume averaging is caused by
Large FOV
Small Matrix
Thick slices
Partial volume averaging can be compensated by
Small FOV
Large Matrix
Thin Slices
The amount of partial volume averaging is directly related to :
Voxel Volume Size
Cross excitation artifacts can be compensated for by
Using a gap space of at least 30% / Using the interleaving imaging option
Moire pattern artifacts are only present in what pulse sequence
Gradient echo pulse sequence
Moire pattern artifacts are caused by :
Undersampling
Moire pattern artifacts can be compensated for by
- Using a spine echo pulse sequence
- Keeping patient anatomy away from the walls of the bore
- Making sure that all anatomy fits within the FOV
Parallel imaging artifacts can be compensated for by _________ or __________.
Reducing the R factor / the proper use of a calibration scan
What imaging artifact appears as areas of high signal intensity near collagen dense anatomy
Magic Angle Artifact
Magic angle artifacts can sometimes occur around tendons positioned at what angle to the main magnetic field
55 degrees
