High-yield overview
Imaging patients with orthopaedic hardware
1.5TPreferred over 3T
~50-60%Artefact area reduced by MARS at hip
TitaniumLeast artefact material
SEMAC / MAVRICCore 3D MARS techniques
Metal Artefact Mechanisms
Susceptibility: Different magnetic properties cause local field distortion
Signal void: No signal from metal itself
Geometric distortion: Spatial mismapping of signal
Pile-up artefact: Signal displaced and concentrated
Key: MARS techniques address susceptibility and geometric distortion
Critical Must-Knows
- Metal artefact = signal void + geometric distortion
- Titanium causes less artefact than stainless steel or cobalt-chrome
- 1.5T preferred over 3T for metal artefact reduction
- MARS sequences: SEMAC, MAVRIC, VAT (View Angle Tilting)
- Increase bandwidth, use spin echo over gradient echo
Clinical Pearls
- “Susceptibility artefact proportional to field strength
- “Stainless steel: 10x more artefact than titanium
- “Short tau inversion recovery (STIR) better than fat-sat near metal
- “Thinner slices and higher matrix reduce artefact
- “ALTR assessment around MoM hips requires MARS MRI
Clinical Warning
Understanding metal artefact reduction is increasingly important with the rising number of patients with orthopaedic implants. Know which materials cause most artefact (stainless steel worst), why 1.5T is preferred, and the basic MARS techniques. ALTR assessment around metal-on-metal hips is a common application.
Physics of Metal Artefact
| Artefact Type | Cause | Appearance |
|---|---|---|
| Signal void | No mobile protons in metal | Black region at metal location |
| Susceptibility artefact | Local field inhomogeneity | Blooming, signal distortion around metal |
| Geometric distortion | Frequency mismapping | Spatial displacement of anatomy |
| Pile-up artefact | Signal misregistration | Bright bands adjacent to void |
| Fat-saturation failure | Off-resonance effects | Incomplete fat suppression near metal |
Mnemonic
MARS = Material, Alignment, Resolution, SequenceFactors Affecting Artefact Severity
M
Material: Stainless steel greater than cobalt-chrome greater than titanium
A
Alignment: Long axis parallel to B0 reduces artefact
R
Resolution: Higher matrix, thinner slices help
S
Sequence: Spin echo better than gradient echo
Hook:Artefact severity is proportional to field strength - use 1.5T over 3T when imaging around metal
Implant Material Properties
| Material | Susceptibility | Artefact Severity | Common Uses |
|---|---|---|---|
| Titanium alloy | Low | Minimal | Plates, screws, stems, spinal implants |
| Cobalt-chrome | Moderate | Moderate-high | Femoral heads, tibial trays, bearing surfaces |
| Stainless steel | High | Severe | Older implants, some screws, wires |
| Tantalum | Low | Minimal | Trabecular metal, acetabular augments |
| Oxinium (Zr) | Very low | Minimal | Ceramic-like bearing surfaces |
| PEEK | None | None | Spinal cages, radiolucent |
Practical Implications
When possible, choose titanium implants if post-operative MRI is anticipated. For hip surveillance (ALTR), cobalt-chrome MoM bearings cause significant artefact requiring MARS sequences. Total knee replacements with cobalt-chrome femoral components also challenging to image.
Standard Protocol Optimisation
| Parameter | Adjustment | Effect |
|---|---|---|
| Field strength | Use 1.5T over 3T | Artefact proportional to B0 |
| Receiver bandwidth | Increase (wide bandwidth) | Reduces geometric distortion |
| Slice thickness | Decrease (thin slices) | Reduces through-plane distortion |
| Matrix size | Increase (high resolution) | Improves spatial resolution |
| Sequence type | Spin echo over gradient echo | Less susceptibility-sensitive |
| Fat suppression | STIR over chemical fat-sat | STIR works despite field inhomogeneity |
| Echo train length | Optimise (not too long) | Balance SNR and blurring |
Mnemonic
BLAST = Bandwidth up, Low field, Avoid gradient echo, Slices thin, STIRProtocol Levers to Cut Metal Artefact
B
Bandwidth up: wide receiver bandwidth reduces geometric distortion
L
Low field: 1.5T over 3T (artefact proportional to B0)
A
Avoid gradient echo: use spin/fast spin echo (less susceptibility-sensitive)
S
Slices thin + high matrix: reduce through-plane distortion
S
STIR: inversion-recovery fat suppression survives field inhomogeneity
Hook:These simple levers alone often suffice for small titanium implants; reserve full 3D SEMAC/MAVRIC for large cobalt-chrome arthroplasty.
Why 1.5T?
Metal artefact is directly proportional to field strength. 3T produces approximately twice the artefact of 1.5T for the same implant. For routine imaging around metal, 1.5T is preferred despite generally lower SNR.
Why STIR Not Fat-Sat?
Chemical fat saturation relies on uniform magnetic field to selectively excite fat. Metal disrupts field homogeneity causing fat-sat failure. STIR uses inversion recovery (T1 property) which works regardless of field uniformity.
Advanced MARS Techniques
VAT Principle
Applies additional gradient during readout that tilts the view angle. This corrects in-plane geometric distortion caused by metal. Effective for reducing distortion but increases scan time. Often combined with increased bandwidth.
| Technique | Mechanism | Scan Time | Availability |
|---|---|---|---|
| VAT | In-plane distortion correction | Moderate increase | Widely available |
| SEMAC | Through-plane encoding | Significant increase | GE, Siemens, Philips |
| MAVRIC | Multi-frequency acquisition | Significant increase | GE |
| MAVRIC-SL | Combined SEMAC + MAVRIC | Long | GE |
| WARP | VAT + optimisation | Moderate increase | Siemens |
Clinical Imaging Applications
Adverse Local Tissue Reaction (ALTR)
Metal-on-metal hip bearings can cause ALTR (metallosis, pseudotumour). MRI with MARS essential for assessment. Look for: solid or cystic masses, fluid collections, muscle oedema/atrophy, tendon damage, osteolysis. Compare with blood metal ion levels for management decisions.
| Finding | MRI Appearance | Significance |
|---|---|---|
| Pseudotumour | Cystic or solid mass adjacent to hip | May compress neurovascular structures |
| Fluid collection | T2 bright, may have debris | Periarticular, trochanteric bursa |
| Muscle damage | Oedema (T2 high) or atrophy (T1 fat) | Abductors commonly affected |
| Tendon disruption | Discontinuity, retraction | May affect surgical approach |
| Metallosis | Low signal debris, synovial thickening | Metal particle deposition |
Protocol Selection
| Indication | Field Strength | Key Sequences | MARS Technique |
|---|---|---|---|
| MoM hip surveillance | 1.5T | PD fat-sat, STIR, T1 | MARS (MAVRIC, SEMAC, WARP) |
| THA periprosthetic soft tissue | 1.5T | PD, STIR | VAT or MARS |
| Post-fusion spine | 1.5T | T1, T2, STIR sagittal/axial | VAT, MARS if available |
| Fracture fixation | 1.5T | STIR (oedema), T1 | Standard optimisation often sufficient |
| Shoulder arthroplasty | 1.5T | PD fat-sat, STIR | MARS if available |
When Standard MRI Is Adequate
Small titanium screws and plates often produce acceptable images with standard optimised protocols (increased bandwidth, thin slices, spin echo). MARS sequences add scan time and are most valuable for large cobalt-chrome implants like hip replacements.
Differential Diagnosis of Periprosthetic Masses on MARS MRI
| Entity | Typical Patient | MARS MRI Features | Discriminators |
|---|---|---|---|
| ALTR / pseudotumour | MoM hip, raised Co/Cr ions | Peri-articular cystic or solid mass, variable wall thickness, synovitis | Communicates with joint, abductor/tendon damage, ion levels elevated |
| Prosthetic joint infection | Pain, raised CRP/ESR, sinus | Lamellated synovitis, fluid collections, reactive marrow oedema, sinus tract | Systemic inflammatory markers, positive aspirate; ions normal |
| Particle-disease osteolysis (polyethylene wear) | Older non-MoM bearing | Expansile low-signal synovial masses, focal osteolysis around implant | Bearing is metal-on-poly/ceramic; ions normal |
| Haematoma / seroma | Recent surgery or anticoagulation | Well-defined collection, blood-degradation signal, no enhancing solid component | Temporal relation to surgery, resolves over time |
| Trochanteric bursitis | Lateral hip pain | Fluid in greater trochanteric bursa, no intra-articular mass | Confined to bursa, no abductor destruction |
| Soft-tissue sarcoma | No bearing-related cause, growing mass | Heterogeneous enhancing mass, may not respect joint planes | Independent of implant, biopsy required if atypical |
Key Principle
A solid or cystic periprosthetic mass is not automatically an ALTR pseudotumour. Always correlate with bearing type, serum metal ions, inflammatory markers and joint aspiration. Infection and (rarely) sarcoma are the dangerous mimics that change management entirely.
Evidence Base
Evidence
SEMAC — Original Slice-Encoding Technique
Lu W, Pauly KB, Gold GE, Pauly JM, Hargreaves BA • Magnetic Resonance in Medicine (2009)
Key Findings:
- SEMAC extends a view-angle-tilting (VAT) spin-echo sequence with additional z-phase encoding, resolving distorted excitation profiles that cause through-plane distortion.
- VAT suppresses in-plane distortion while z-phase encoding corrects through-plane distortion, so spins are repositioned to their true spatial locations.
- The method requires no additional hardware and was validated in phantom and in vivo spine and knee studies with feasible scan times.
Clinical implication: SEMAC is the foundational 3D metal-artefact-correction technique and the conceptual basis for modern MARS sequences (and the hybrid MAVRIC-SL) used around arthroplasty and spinal implants.
Limitation: Resolving multiple slice-encoding steps lengthens acquisition and lowers SNR, the trade-offs that later acceleration methods (parallel imaging, compressed sensing) were designed to address.
Source: Lu W et al. Magn Reson Med 2009;62(1):66-76
Evidence
MAVRIC vs Conventional FSE After Arthroplasty
Hayter CL, Koff MF, Shah P, Koch KM, Miller TT, Potter HG • AJR American Journal of Roentgenology (2011)
Key Findings:
- In 122 patients (74 hip, 27 shoulder, 21 knee arthroplasties), MAVRIC showed significantly better visualisation of synovium and periprosthetic bone than metal-artefact-reduction FSE at all three joints.
- Synovitis and periprosthetic osteolysis were detected only on MAVRIC images in a substantial proportion of subjects.
- Supraspinatus tendon tears in 44% of relevant subjects were seen only on MAVRIC and not on FSE.
Clinical implication: 3D multispectral MARS (MAVRIC) materially increases detection of synovitis, osteolysis and tendon pathology around metal implants and is a valuable adjunct to standard high-bandwidth FSE.
Limitation: Subjective grading by readers, prototype sequence, and a referral cohort that may over-represent symptomatic joints.
Source: Hayter CL et al. AJR Am J Roentgenol 2011;197(3):W405-11
Evidence
MAVRIC-SL at 3T in Hip Arthroplasty
Choi SJ, Koch KM, Hargreaves BA, Stevens KJ, Gold GE • AJR American Journal of Roentgenology (2015)
Key Findings:
- In 21 hips, MAVRIC-SL reduced measured artefact area versus 2D FSE by 59.9% at the level of the hip and 31.3% at the femur (both significant).
- Joint capsule and obturator externus/iliopsoas attachment sites were better depicted, and abnormal findings were significantly better shown with MAVRIC-SL.
- MAVRIC-SL increased diagnostic confidence even at 3T, a field strength normally avoided around large metal implants.
Clinical implication: Modern multispectral MARS can make even 3T imaging of hip arthroplasty diagnostically useful, though 1.5T remains the default where artefact is the limiting factor.
Limitation: Small single-centre cohort; not all implant alloys or bearing types represented.
Source: Choi SJ et al. AJR Am J Roentgenol 2015;204(1):140-7
Controversies and Areas of Uncertainty
When to Revise an MoM Hip
There is no universal threshold. Imaging (pseudotumour size, solid component, muscle destruction) and serum metal ions are combined, but ALTR lesions can regress (Goldstein 2016) and asymptomatic patients with low ions may still harbour cystic lesions. Decisions remain individualised against symptoms, trajectory and surgeon judgement.
3T vs 1.5T for MARS
The teaching that artefact is proportional to field strength favours 1.5T, yet modern multispectral MARS (MAVRIC-SL) can make 3T diagnostically useful (Choi 2015). Where only 3T is available, MARS sequences are increasingly acceptable rather than an absolute contraindication.
Scan Time and Access
Full 3D SEMAC/MAVRIC historically added many minutes. Compressed sensing and deep-learning reconstruction now compress this to a few minutes (Fritz 2016), but these sequences are not universally available, creating real-world variation in image quality between centres.
MRI vs CT vs Nuclear Medicine
No single modality is definitive for the painful arthroplasty. MARS MRI excels for soft tissue and ALTR, CT for bone/component detail and lysis, and labelled-leukocyte/marrow imaging for infection. The optimal pathway is debated and resource-dependent.
Guidelines, Registries & Global Practice
| Body | Region | Position on Imaging |
|---|---|---|
| MHRA Medical Device Alert | UK | Risk-stratified follow-up of MoM hips; cross-sectional imaging (MARS MRI or ultrasound) indicated for symptomatic patients or rising/raised metal ions |
| FDA guidance on MoM hips | US | Recommends clinical follow-up and considers cross-sectional imaging (MRI/US/CT) in symptomatic patients or with abnormal ion levels |
| AAOS / arthroplasty society guidance | US | Supports a combined algorithm of symptoms, examination, radiographs, serum metal ions and MARS MRI for evaluating the painful MoM hip |
| BOA / BHS consensus | UK | Endorses surveillance pathways using metal ions plus MARS MRI or ultrasound for soft-tissue/pseudotumour assessment |
| EFORT / European consensus | Europe | Aligns with stratified surveillance; cross-sectional imaging for symptomatic or high-risk implants |
Global Epidemiology & Registry Signal
National joint registries (NJR England/Wales, AOANJRR Australia, Swedish and Norwegian registries) documented higher revision rates for large-head MoM and resurfacing implants, driving worldwide curtailment of MoM bearings and the surveillance pathways that rely on MARS MRI. Pseudotumours/ALTR are reported in a substantial minority of MoM hips, many asymptomatic.
High- vs Limited-Resource Practice
In well-resourced centres, 1.5T MARS MRI with SEMAC/MAVRIC and serum metal ion assays are standard. Where MRI access, MARS sequences or ion assays are limited, ultrasound becomes the primary cross-sectional tool for periprosthetic collections and pseudotumours, with radiographs and clinical assessment carrying more weight.
Material Selection — A Global Principle
Where post-operative MRI is anticipated (e.g. spinal instrumentation, peri-articular fixation, oncology), choosing titanium over stainless steel improves subsequent image quality everywhere, independent of scanner platform or vendor MARS implementation.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
Viva scenarioChallenging
Clinical prompt
“A patient with a painful metal-on-metal hip replacement is referred for MRI. Blood cobalt level is 12 ppb (elevated). You are asked about optimal imaging.”
Practical approach
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.
Key clinical points
1.5T preferred over 3T (less metal artefact)
MARS sequences essential for MoM hips
STIR not fat-sat (field inhomogeneity)
Look for: pseudotumour, fluid, muscle damage
Elevated metal ions correlate with ALTR risk
Common pitfalls
Using 3T (more artefact)
Using chemical fat-sat (fails near metal)
Not requesting MARS sequences
Further questions
“What blood metal ion levels are considered concerning? How would you counsel this patient about their options?”
Viva scenarioStandard
Clinical prompt
“A patient 2 years post lumbar fusion with persistent leg pain is referred for MRI. The spine surgeon wants to assess for recurrent disc herniation.”
Practical approach
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.
Key clinical points
Titanium screws: less artefact than stainless
1.5T, spin echo, increased bandwidth
STIR for oedema (not fat-sat)
Focus on areas away from screw heads
CT myelogram is alternative if MRI inadequate
Common pitfalls
Using gradient echo (more susceptibility artefact)
Not increasing bandwidth
Expecting perfect visualisation adjacent to screws
Further questions
“When would you recommend CT myelogram instead of MRI? How does adjacent segment disease appear on imaging?”
Viva scenarioStandard
Clinical prompt
“You are asked to explain why MRI around metal implants is challenging. An orthopaedic trainee asks what material causes the least artefact.”
Practical approach
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.
Key clinical points
Susceptibility difference causes field distortion
Signal void: no protons in metal
Geometric distortion: frequency mismapping
Titanium: least artefact (low susceptibility)
Stainless steel: most artefact (high susceptibility)
Common pitfalls
Not explaining susceptibility physics
Confusing material properties
Not mentioning practical implant choice implications
Further questions
“What protocol adjustments reduce metal artefact? Why is 1.5T preferred over 3T?”
Exam day cheat sheet
MARS MRI Quick Reference
Material Artefact Severity
- Titanium: Least artefact
- Tantalum/Oxinium: Low artefact
- Cobalt-chrome: Moderate-high
- Stainless steel: Severe (10x titanium)
Protocol Optimisation
- 1.5T over 3T (artefact proportional to B0)
- Spin echo over gradient echo
- Increase receiver bandwidth
- STIR not chemical fat-sat
- Thin slices, high matrix
MARS Techniques
- VAT: In-plane correction
- SEMAC: Through-plane encoding
- MAVRIC: Multi-frequency acquisition
- WARP: Siemens combined technique
Clinical Applications
- MoM hip ALTR surveillance
- Post-fusion spine assessment
- Periprosthetic soft tissue
- PJI soft tissue extent