Metal Artifact Reduction Sequence Magnetic Resonance Imaging (MARS MRI) is a specialized scanning technique designed to provide clear images of soft tissues located near metallic orthopedic implants. Standard MRI is severely hindered by the presence of metal, which causes significant image distortion that can obscure the area of interest. MARS MRI uses a combination of modified hardware settings and advanced software algorithms to significantly reduce these distortions. This allows doctors to accurately evaluate the tissues surrounding the implant and diagnose complications like infection, loosening, or soft tissue reactions.
The Challenge of Metal Artifacts in Standard MRI
Standard magnetic resonance imaging relies on a uniform magnetic field to accurately map the body’s tissues. Metallic implants disrupt this homogeneity due to magnetic susceptibility, which is the degree to which a material becomes magnetized in an external field. The substantial difference between the metal and surrounding soft tissue creates a powerful local magnetic field around the implant.
This localized field disruption leads to two primary imaging problems: geometric distortion and signal loss. Geometric distortion is a warping of the image where tissue is incorrectly mapped, creating signal pile-up. Signal loss appears as a large, dark void around the implant because the field inhomogeneity causes proton signals to rapidly dephase. The resulting image near the hardware is largely uninterpretable, preventing evaluation for complications.
Technical Strategies to Minimize Artifacts
MARS MRI implements specialized imaging protocols to counteract the effects of metallic implants. One foundational strategy uses Fast Spin-Echo (FSE) pulse sequences, which employ 180-degree radiofrequency pulses to repeatedly rephase dephasing proton signals. This action corrects much of the signal loss caused by rapid magnetic field variations near the metal, restoring image clarity.
Another adjustment involves significantly increasing the receiver bandwidth during scan acquisition. A higher bandwidth minimizes the spatial displacement resulting from frequency shifts caused by the metal. To further tackle geometric distortion, techniques like View Angle Tilting (VAT) are integrated into the MARS protocol. VAT applies an additional magnetic gradient during signal readout to correct pixel misregistration and reduce in-plane distortion.
For distortions occurring across different slices, known as through-plane artifacts, advanced methods like Slice-Encoding for Metal Artifact Correction (SEMAC) may be used. SEMAC adds extra phase-encoding steps in the slice direction to correct signal misplacement. While these specialized techniques are highly effective, they often increase the overall scan time, requiring a balance between artifact reduction and patient comfort.
Specific Clinical Uses
The ability of MARS MRI to visualize soft tissues near metal is valuable in orthopedic medicine, especially for patients experiencing persistent pain after joint replacement surgery. The technique is frequently used to evaluate complications around hip and knee arthroplasty, providing detailed images that help distinguish between benign post-surgical changes and serious issues requiring intervention.
A primary use is assessing periprosthetic soft tissues for signs of infection or implant loosening, which are difficult to detect with standard imaging. MARS MRI can also effectively identify Adverse Local Tissue Reactions (ALTR), such as pseudotumors or bursae, which are inflammatory responses to wear debris. Furthermore, the scan evaluates the integrity of tendons, muscles, and ligaments near the metal hardware to diagnose causes of post-operative pain. Beyond joint replacements, MARS MRI is applied to assess spinal instrumentation, allowing for better visualization of the spinal cord and nerve roots.
What Patients Should Expect
Patients undergoing a MARS MRI should plan for a longer examination time than a typical MRI, with procedures often lasting between 30 and 60 minutes. The patient must remain very still throughout the scan to ensure the highest image quality, as cooperation is essential for the advanced algorithms to successfully correct the magnetic field distortions.
Before the scan, patients must inform the radiologist about the exact type and location of the metallic implant. The implant must be confirmed as “MR Conditional” or “MR Safe” to ensure there is no risk of movement or overheating. Even with artifact reduction techniques, some residual distortion may still be visible, requiring interpretation by specialized radiologists trained to work with these images.