MRI MRI Artifacts 2 — Questions and Answers
Question 1: Pulsatile flow artifact from a vessel typically appears as ghost images displaced in which direction?
- Slice-selection direction
- Frequency-encoding direction
- Phase-encoding direction (Correct answer)
- Both frequency and phase directions equally
Correct answer: Phase-encoding direction
Pulsatile flow ghosts appear in the phase-encoding direction because pulsatile variations in spin phase occur between TR intervals during phase-encode data collection.
Question 2: Which of the following best describes the 'magic angle' artifact and at what angle it occurs?
- T2 darkening of tendons at 45° to B0
- Artifactual T1/PD signal increase in ordered collagen structures at ~55° to B0 (Correct answer)
- Susceptibility dropout at 45° to B0
- Chemical shift banding in fat at 55°
Correct answer: Artifactual T1/PD signal increase in ordered collagen structures at ~55° to B0
When ordered collagen fibers (tendons, ligaments) are oriented at ~55° to B0, dipole-dipole interactions average to zero, artifactually increasing signal on short TE sequences.
Question 3: A 'DC offset' or 'center dot' artifact seen as a bright spot in the center of the image is related to:
- Patient positioning off-isocenter
- A small DC current offset in the receiver causing a spike in the center of k-space (Correct answer)
- Excessive TR causing saturation effects
- B1 field inhomogeneity
Correct answer: A small DC current offset in the receiver causing a spike in the center of k-space
A DC offset artifact occurs when a constant voltage offset in the MR receiver chain introduces a spike at the origin of k-space (DC component), appearing as a bright dot at image center.
Question 4: EPI (echo planar imaging) sequences are particularly prone to which artifact caused by off-resonance spins accumulating phase during the long readout?
- Truncation artifact
- Chemical shift (N/2 ghost or Nyquist ghost) and geometric distortion (Correct answer)
- Magic angle artifact
- Susceptibility artifact only in GRE
Correct answer: Chemical shift (N/2 ghost or Nyquist ghost) and geometric distortion
EPI's long readout makes it sensitive to field inhomogeneity, causing geometric distortion along the phase-encode direction and N/2 ghosting from alternating echo phase inconsistencies.
Question 5: Which artifact appears as parallel bright and dark bands in the phase-encoding direction when the FOV is too small and the patient extends beyond it?
- Gibbs ringing
- Aliasing (wraparound) (Correct answer)
- Chemical shift type 2
- Susceptibility artifact
Correct answer: Aliasing (wraparound)
When anatomy outside the FOV is phase-encoded, it aliases (wraps around) and appears as a superimposed copy on the opposite edge of the image, sometimes appearing as bands.
Question 6: Cross-excitation (cross-talk) artifact in multislice MRI occurs because of:
- Imperfect slice profiles causing RF excitation of adjacent slices (Correct answer)
- Gradient switching between adjacent slices
- Patient breathing between slice acquisitions
- Coil sensitivity overlap between two adjacent coil elements
Correct answer: Imperfect slice profiles causing RF excitation of adjacent slices
Real RF pulses have imperfect rectangular profiles; the tails of adjacent slice profiles overlap, causing partial saturation of neighboring slices — reduced by interleaved acquisition or adding gaps.
MRI MRI Artifacts 2
Pulsatile flow artifact from a vessel typically appears as ghost images displaced in which direction?