MRI Metal Artefact Reduction (MARS)

For metal-on-metal hip surveillance with elevated metal ions, I would request MRI with MARS (metal artefact reduction sequences) at 1.5T (not 3T - artefact is proportional to field strength). Protocol would include: STIR for fluid collections and oedema (not chemical fat-sat which fails near metal), PD sequences for soft tissue detail, T1 for anatomy. I would look for ALTR features: pseudotumour (cystic or solid mass), fluid collections, muscle oedema/atrophy (particularly abductors), tendon damage, and bone changes. The elevated cobalt level increases concern for ALTR requiring revision.

Post-fusion spine MRI presents challenges from pedicle screw artefact, though titanium screws cause less artefact than stainless steel. I would optimise the protocol: 1.5T field strength, spin echo sequences (not gradient echo), increased receiver bandwidth, thin slices, STIR for oedema assessment. If available, MARS sequences (WARP, SEMAC) improve visualisation. For recurrent disc herniation assessment, I would focus on sagittal and axial T2-weighted images at and above the fusion level. The neural foramina and thecal sac should be assessable away from the screw heads. CT myelogram remains an alternative if MRI quality is inadequate.

Metal artefact occurs because metals have different magnetic susceptibility than surrounding tissue, causing local magnetic field distortion. This results in: signal void from the metal itself (no mobile protons), geometric distortion from frequency mismapping, and pile-up artefact from signal misregistration. Regarding materials, magnetic susceptibility determines artefact severity: titanium alloys cause the least artefact (low susceptibility), cobalt-chrome causes moderate-high artefact, and stainless steel causes severe artefact (high susceptibility). Tantalum and oxinium also have low susceptibility. PEEK (polymer) causes no artefact. When post-operative MRI is anticipated, choosing titanium implants improves subsequent imaging quality.