12: MRI Artifacts Flashcards by Sherrie Nigito

Aliasing artifact is also called…

Wrap artifact

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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

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Wrap or Aliasing artifact: Appearance:

area of slice that would be cut would be folded back into the image in the phase direction

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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)

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Adding “no phase wrap” adds….

time

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Truncation artifact is also called

Ringing artifact

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Ringing / Truncation artifact:

Incomplete sampling of high contrast borders

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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

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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 )

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Gibbs Artifact:

When Fourier transform samples phase and frequency separate from another

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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

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Gibbs Artifact: Compensation

Does not warrant the need to fix and repeat

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Chemical Shift artifact:

Lack of resonant frequency in hydrogen atoms in fat compared to hydrogen atoms in water

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Chemical Shift artifact: Appearance

Shifting of fat based tissue over water based tissue in the FREQUENCY direction

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Chemical Shift artifact: Compensation

Increase bandwidth (indirect relationship with chemical shift)

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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

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Magnetic Susceptibility:

Extreme localized dephasing due to different areas being magnetized

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Magnetic Susceptibility: Appearance

1: extreme signal loss
2: Geometric distortion

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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)

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RF Zipper Artifact

Stray RF waves entering the room from mobile or navigation devices

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RF Zipper Artifact: Appearance:

severe distortion with signal disorientation (zipper lines) in the phase direction

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RF Zipper Artifact: Compensation

1: closing the scanner door completely
2: removing any mobile devises from vicinity of control room

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Motion Artifact: Flow/movement:

Study These Flashcards

movement of the image being sampled by either patient motion or blood flow

Motion Artifact: Flow/movement: Appearance:

Study These Flashcards

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

12: MRI Artifacts Flashcards

(55 cards)