MRI Metal Artefact Reduction (MARS)

The protocol levers repeatedly say "spin echo over gradient echo," but the mechanism is exactly what an examiner probes. (General sequence physics is developed in the mri-imaging-principles topic; here is the metal-specific reason.) The dominant problem near an implant is the large static off-resonance (field inhomogeneity) from the metal's susceptibility, and whether the sequence can undo that dephasing decides the artefact.

The physics tables name "geometric distortion: frequency mismapping" and "increase bandwidth reduces geometric distortion," and list VAT and SEMAC separately, but never assemble the directional logic that ties them together - the unifying concept the exam wants.

Metal off-resonance mismaps signal along the frequency-encode (readout) direction in-plane and along the slice-select direction through-plane, because both of those axes use frequency to localise signal; the phase-encode direction is largely spared. Three consequences follow:

• Receiver bandwidth. A higher receiver bandwidth spreads more hertz across each pixel, so a given off-resonance shift displaces signal by fewer pixels - less in-plane distortion and a smaller void. The price is signal-to-noise, which falls roughly with the square root of bandwidth; this is the trade-off behind "increase bandwidth when SNR allows."
• Steer the artefact. Because the in-plane displacement runs along the frequency-encode axis, swapping the frequency- and phase-encode directions moves the pile-up and void off the structure you care about.
• This is why VAT and SEMAC are different tools. View-angle tilting corrects the in-plane (readout-direction) distortion with an extra readout gradient; SEMAC adds slice-direction (z) phase encoding to correct the through-plane distortion; combining them (and MAVRIC's multi-frequency acquisition) covers both directions - the reason these are the definitive techniques for large cobalt-chrome arthroplasty.