MRI artifacts are features in the image that do not correspond to real anatomy. Understanding artifacts is essential for correct image interpretation and for optimising acquisition protocols.

Gibbs Ringing (Truncation Artifact)

Sharp edges in the image (e.g. brain-CSF interface) produce ringing stripes parallel to the edge, caused by truncation of k-space at finite matrix size. Mitigated by acquiring more k-space lines (larger matrix) or applying k-space apodisation filters (at cost of resolution).

Chemical Shift Artifact

Water and fat protons resonate at slightly different frequencies (3.5 ppm ≈ 450 Hz at 3 T). In the frequency-encoding direction, fat and water signals are spatially misregistered by this frequency offset, creating a bright/dark band at fat-water interfaces (e.g. kidney-perinephric fat). Mitigated by wider receiver bandwidth or fat suppression.

Motion Artifacts

Patient movement during acquisition causes ghosting (copies of moving structures) in the phase-encoding direction. Respiratory motion causes abdominal blurring. Mitigated by breath-holding, respiratory triggering/gating, navigator echoes, and radial k-space trajectories (more motion-robust).

Susceptibility Artifacts

Interfaces between materials of different magnetic susceptibility (metal implants, air-tissue, bone-tissue) distort the local B0 field, causing signal loss and geometric distortion. Worst near metallic implants. Mitigated by reducing TE, using spin echo (refocuses static inhomogeneity), or MARS (Metal Artifact Reduction Sequences).

RF Inhomogeneity (B1+ Artifact)

At 3 T and above, wavelength effects cause spatially non-uniform B1⁺ fields, resulting in flip angle variations across the image. Manifests as signal brightening in the centre and darkening at the periphery of large body parts at 3 T, and complex interference patterns at 7 T. Mitigated by adiabatic pulses, parallel transmit, and B1 mapping-based pulse calibration.