Off-resonance

Off-resonance = spins precess at a slightly "wrong" frequency because of local B0 inhomogeneity

In fat suppression context = "wrong fat frequency" → errors/artefacts

Certain readouts (EPI) are very sensitive to B0 inhomogeneity, so off-resonance artefacts are more pronounced.

Causes

• B0 magnet inhomogeneity, poor shimming → more pronounced in large FOV
• Local B0 inhomogeneity due to susceptibility differences along interfaces between tissues/air/bone/metal
• Local hardware imperfection (e.g., coil)

Effects

• Fat suppression
  • CHESS → patchy (incomplete) fat suppression
  • Dixon → water-fat swap, weird ugly artefacts
• Banding artefact
• Geometric distortion/Signal pile-up in EPI