Image Reconstruction, Sampling, and Artifacts
Core concepts
Reconstruction applies the inverse Fourier transform to k-space samples. The Nyquist criterion governs sampling: readout sampling must satisfy bandwidth, and phase encoding must cover enough k-space to avoid aliasing. Undersampling causes wraparound (aliasing).
Common artifacts and causes
- Aliasing/wrap: FOV too small; fix by enlarging FOV or oversampling.
- Chemical shift: fat–water frequency difference causes misregistration; increases with field strength; fix with fat saturation or higher bandwidth.
- Ghosting: motion or phase-encode inconsistency; fix with flow compensation, gating.
- Truncation (Gibbs): ringing from abrupt k-space edges.
- Susceptibility: signal loss near metal/air; worse with GRE and long TE.
- Zipper: RF interference; herringbone: vibration or gradient issues.
Traps
Questions often ask which direction an artifact appears. Aliasing occurs along the phase-encode direction unless oversampling is used; chemical shift occurs along the frequency-encode direction.
Rule of thumb
If an artifact moves when you change phase-encode direction, it is likely a sampling or motion artifact. If it is fixed to anatomy, suspect susceptibility or chemical shift.