12: MRI Artifacts Flashcards
Aliasing artifact is also called…
Wrap artifact
Wrap or Aliasing artifact: Cause -
when the area of anatomy exceeds the FOV margins in the phase direction with no/limited oversampling and no phase wrap option off
Wrap or Aliasing artifact: Appearance:
area of slice that would be cut would be folded back into the image in the phase direction
Wrap or Aliasing artifact: Compensation:
1: select “no phase wrap/ oversampling” before scanning
2: increase FOV to extend past anatomy in the phase direction
3: Place Saturation pulse on anatomy extending past FOV (least effective)
Adding “no phase wrap” adds….
time
Truncation artifact is also called
Ringing artifact
Ringing / Truncation artifact:
Incomplete sampling of high contrast borders
Ringing / Truncation artifact: Appearance-
Oscillating waves parallel with high contrast border
Commonly found on SAG T2 of C spine due to high contrast edges between the thecal sac and spinal cord
Ringing / Truncation artifact: Compensation:
1: Increase matrix ( increase matrix = decrease truncation )
2: Select a raw data filler ( 2D Sequential Filling ) to smooth out incomplete sampling
3: Fat Suppression ( if high contrast border is due to fat )
Gibbs Artifact:
When Fourier transform samples phase and frequency separate from another
Gibbs Artifact: Appearance:
Dark circumference surrounding anatomy followed by brighter circumference surrounding the dark circumference
Imagine a dark circle surrounded by a white circle on the edge of an image
Differs from Truncation by that is does NOT have a wave light appearance and is present along the entire circumference of an image
Gibbs Artifact: Compensation
Does not warrant the need to fix and repeat
Chemical Shift artifact:
Lack of resonant frequency in hydrogen atoms in fat compared to hydrogen atoms in water
Chemical Shift artifact: Appearance
Shifting of fat based tissue over water based tissue in the FREQUENCY direction
Chemical Shift artifact: Compensation
Increase bandwidth (indirect relationship with chemical shift)
Magnet shift = Magnet strength (T) x Hertz/T
149 Hertz/T in 1.0 Tesla Magnet = Increase Chemical Shift
224 Hertz/T in 1/5 Tesla Magnet = Decrease Chemical Shift
Magnetic Susceptibility:
Extreme localized dephasing due to different areas being magnetized
Magnetic Susceptibility: Appearance
1: extreme signal loss
2: Geometric distortion
Magnetic Susceptibility: Compensation
1: Remove metallic object (hairpins, bra, glasses, etc. )
2: Metal protocol (increase bandwidth + increase FOV + tailored RF)
3: Metal Suppression sequences (MAVRIC - GE; WARP - Seimens)
RF Zipper Artifact
Stray RF waves entering the room from mobile or navigation devices
RF Zipper Artifact: Appearance:
severe distortion with signal disorientation (zipper lines) in the phase direction
RF Zipper Artifact: Compensation
1: closing the scanner door completely
2: removing any mobile devises from vicinity of control room
Motion Artifact: Flow/movement:
movement of the image being sampled by either patient motion or blood flow
Motion Artifact: Flow/movement: Appearance:
Flow - pulsation artifact along the phase direction
motion - blurry anatomy in the phase direction
Motion Artifact: Flow: Compensation:
Saturation band inferior or superior to area of interest
select “flow compensation option”
Motion Artifact: Movement: Compensation
Sponges to keep patient still
motion correction imaging techniques such as Propeller/BLADE
Partial Volume Averaging:
With thick slices, more area gets sampled and also gets averaged which drops signal contrast in certain pieces of anatomy
Partial Volume Averaging: Appearance
decrease signal contrast on thicker slices
Partial Volume Averaging: Compensation
Use thinner slices
Crosstalk Artifact:
Multiple overlapping slices or acquisitions
Crosstalk Artifact: Appearance
loss of signal in area of overlapping slices or acquisitions
Crosstalk Artifact: Compensation:
Separate acquisitions
increasing spaces of slices
Moire Pattern:
too large of a FOV while using a gradient pulse sequence and magnet body coil
Moire Pattern: Apprearance
Severe banding or wave-line pattern at the periphery of image
Moire Pattern: Compensation
Use a different pulse sequence for a localizer rather than a gradient
use a dedicated coil rather than the body coil
Annefact:
direction of signals being generated outside of desired FOV
Commonly found when scanning thoracic spine and your coil elements are tuned on outside the FOV
Annefact: Appearance
bright, ghosting signals smeared across the phase direction of an image
Annefact: Compensation
Proper selection of the receiver coil
verify the phase/frequency directions
IDEAL Artifact *NOT ON REGISTRY *
too large of a FOV with too high of an acceleration rate
IDEAL Artifact: Apprearance
moire pattern on the periphery with a mid-sagittal pixelations
IDEAL Artifact: Compensation
Decrease FOV and decrease ACCL rate
use another sequence such as inversion recovery
Imperfect Fat Saturation:
range of frequencies exceed the FOV selected for fat saturation or anatomy in direct contact with coil
Imperfect Fat Saturation: Appearance
immediate fat saturation drop at the periphery of the FOV
Imperfect Fat Saturation: Compenstion
use traditional IR Recovery (STIR)
Corduroy Artifact
K space corruption in a particular data point which causes an error during fourier transformation
Corduroy Artifact: Appearance
Narrow angular lines throughout the entire sequence resulting in a corduroy appearance
Corduroy Artifact: Compensation
choose another sequence from protocol management; this sequence may be corruption
Dielectric Effect:
when a patient’s body diameter measures wider than the RF pulse wavelength be distributed
Dielectric Effect: Appearance:
Results in a drastic decrease in signal resulting in a dark shadowing in the center of the image
Can also happen in people very skinny, Low fat high water
Normally found in 3T Body MR
Dielectric Effect is normally found on__________________
3T Body MR
Dielectric Effect: Compensation
Move to a lower field magnet (most effective)
Use of dielectric pads
Drainage of ascites or other body fluids
Wrap occurs in the ________ direction.
Phase
Wrap folds the image back into the ____________ direction.
Phase direction
A 8mm slice will have _________ partial volume averaging artifact than a slice with a 4mm slice thickness.
more
NOT HALF
By turning “No Phase Wrap” and increasing FOV to cover anatomy, your scan time will be _________.
shorter
