MRI MRI Artifacts 1 — Questions and Answers
Question 1: Which MRI artifact appears as a ghosting or smearing of bright structures in the phase-encoding direction?
- Chemical shift artifact
- Gibbs (truncation) artifact
- Motion (ghosting) artifact (Correct answer)
- Susceptibility artifact
Correct answer: Motion (ghosting) artifact
Motion artifact manifests as ghost images displaced in the phase-encoding direction because phase-encoding samples data over many TR intervals while frequency encoding is instantaneous.
Question 2: Chemical shift artifact of the first kind occurs at interfaces between fat and water and manifests as:
- A bright band and a dark band at the fat-water interface in the frequency-encoding direction (Correct answer)
- Signal voids around metallic implants
- Zebra-stripe banding artifact
- Image wrap-around from the opposite side
Correct answer: A bright band and a dark band at the fat-water interface in the frequency-encoding direction
First-kind chemical shift artifact creates a bright rim on one side and a dark void on the other side at fat-water interfaces in the frequency-encoding direction due to the 3.5 ppm resonance frequency difference.
Question 3: Gibbs (truncation) artifact is caused by:
- Patient motion during scanning
- Undersampling of k-space — insufficient high-frequency data to represent sharp tissue interfaces (Correct answer)
- Field inhomogeneity near air-tissue interfaces
- Aliasing from a FOV smaller than the patient
Correct answer: Undersampling of k-space — insufficient high-frequency data to represent sharp tissue interfaces
Gibbs artifact arises from the Fourier series representation of sharp edges when high spatial-frequency k-space data is truncated, producing ringing lines parallel to the interface.
Question 4: Wraparound (aliasing) artifact in MRI is caused by:
- Structures outside the FOV contributing signal that is folded back into the image (Correct answer)
- Gradient non-linearity at the edges of the bore
- T2* dephasing from metallic objects
- Phase errors from pulsatile flow
Correct answer: Structures outside the FOV contributing signal that is folded back into the image
When anatomy extends beyond the selected FOV in the phase-encode direction, it is aliased and appears superimposed on the opposite side of the image.
Question 5: Which technique is most effective for reducing susceptibility artifact from a metallic implant in MRI?
- Increase TE
- Use a gradient-echo sequence
- Use a spin-echo or fast spin-echo (FSE) sequence with narrow bandwidth (Correct answer)
- Increase the slice thickness
Correct answer: Use a spin-echo or fast spin-echo (FSE) sequence with narrow bandwidth
Spin-echo and FSE sequences use 180° refocusing pulses that compensate for susceptibility-induced dephasing, and a narrow bandwidth reduces chemical shift but a wide bandwidth reduces susceptibility — MARS protocols use high bandwidth FSE.
Question 6: Zipper artifact on an MRI image appears as a line of alternating bright and dark pixels across the image, most often caused by:
- Patient movement
- External RF interference from inadequate shielding or open RF cage door (Correct answer)
- Gradient switching noise
- Coil coupling artifact
Correct answer: External RF interference from inadequate shielding or open RF cage door
Zipper artifact is caused by external radiofrequency interference entering the scan room, typically due to an open waveguide, broken shielding, or improperly closed scanner room door.
MRI MRI Artifacts 1
Which MRI artifact appears as a ghosting or smearing of bright structures in the phase-encoding direction?