Prominent Susceptibility Artifact in MRI of Total Joint Replacements
The most prominent susceptibility artifact in MRI of total joint replacements is signal loss (signal void) with associated geometric distortion and signal pileup, caused by magnetic field inhomogeneity from the metallic prosthesis, particularly in the frequency encoding direction. 1, 2
Understanding the Artifact Mechanism
Susceptibility artifacts arise from differences in magnetic susceptibility between the metal implant and surrounding tissues, creating inhomogeneity in the B0 magnetic field. 2 This fundamental problem manifests as:
- Signal voids (areas of complete signal loss adjacent to the metal) 1, 2
- Signal pileup (bright areas where signal accumulates) 1, 2
- Geometric distortion (warping of anatomical structures) 1, 2, 3
- In-plane distortions affecting image quality 1
- Through-section distortion compromising slice accuracy 1
- Poor or absent fat suppression in periprosthetic regions 1
Factors Affecting Artifact Severity
The magnitude of susceptibility artifact depends critically on implant composition, with stainless steel producing the largest artifacts, followed by cobalt-chromium alloys, while titanium alloy implants consistently produce the smallest artifacts. 1, 4
Additional factors influencing artifact size include:
- Implant orientation relative to the main magnetic field - artifacts are minimized when the implant is positioned parallel to B0 4
- Magnetic field strength - lower field strength MRI units provide better conditions for artifact reduction 2
- Voxel size - smaller voxels correlate positively with reduced artifact size (R² = 0.88, p < 0.01) 4
Clinical Impact on Joint Replacement Imaging
Without metal artifact reduction techniques, susceptibility artifacts can render periprosthetic soft tissues completely nondiagnostic, obscuring critical pathology such as pseudotumors, infections, tendon injuries, and osteolysis. 5, 1
The American College of Radiology recognizes that:
- MRI with metal artifact reduction sequences (MARS-MRI) has 95.4% sensitivity for detecting osteolytic lesions around hip prostheses, significantly superior to CT (74.7%) or radiographs (51.7%) 5
- MARS-MRI enables assessment of soft tissues that would otherwise be completely obscured by metal artifacts 6
- Radiographs may be completely normal in patients with symptomatic pseudotumors, making MRI essential for diagnosis despite the presence of metal artifacts 6
Modern Artifact Reduction Strategies
Dedicated metal artifact reduction sequences including MARS, SEMAC (slice encoding for metal artifact correction), and MAVRIC (multiacquisition variable-resonance image combination) can transform previously nondiagnostic studies into diagnostically useful examinations. 1, 2, 3
Key optimization techniques include:
- Fast spin-echo sequences with long echo train lengths to minimize susceptibility effects 1, 4
- Short inversion time inversion-recovery (STIR) sequences for reliable fat suppression near metal 1
- High receiver bandwidth to reduce frequency-encoding artifacts 1, 2
- Thin slice selection and increased matrix size to improve spatial resolution 1
- Anterior-posterior frequency-encoding direction which reduces artifacts in knee imaging 4
Common Pitfalls to Avoid
Do not assume MRI is contraindicated or nondiagnostic simply because a patient has a metal joint replacement - modern metal artifact reduction techniques allow visualization of periprosthetic tissues that are invisible on radiographs or CT, particularly for detecting pseudotumors, adverse local tissue reactions, and osteolysis. 5, 6, 1
Do not overlook the importance of implant composition when planning MRI - titanium implants produce significantly less artifact than stainless steel or cobalt-chromium, and this information should guide sequence selection and parameter optimization. 1, 4