What Is a MARS MRI for Metal Implants?

A MARS MRI is a specialized magnetic resonance imaging scan designed to produce usable images in patients who have metal implants such as joint replacements, spinal hardware, or fracture fixation devices. Standard MRI sequences struggle around metal because the implant distorts the magnetic field, causing dark voids, bright flares, and warped anatomy that can hide the very problems a doctor needs to see. MARS protocols use modified pulse sequences and acquisition tricks to suppress those distortions, giving clinicians a clearer view of the bone, soft tissue, and fluid surrounding the hardware. The technique has become especially important for monitoring hip and knee replacements, diagnosing infections around prosthetic joints, and evaluating the spine after fusion surgery.

Why Metal Implants Ruin a Normal MRI

MRI works by placing the body inside a strong, uniform magnetic field and reading radio signals that hydrogen atoms in your tissues send back. Metal disrupts that uniformity. Different materials respond to the magnetic field with different intensities, and the mismatch between a metal implant and the tissue next to it throws off the signals the scanner relies on. The result is a combination of signal loss (dark areas where tissue should be visible), geometric distortion (anatomy appears shifted or stretched), and failure of fat-suppression techniques that radiologists use to highlight inflammation or fluid.¹ In practical terms, this can make the area within several millimeters of the implant completely unreadable on a conventional scan.

Not all metals are equally problematic. Stainless steel, once commonly used in orthopedic hardware, produces the largest artifacts. Titanium generates significantly less distortion, which is one reason it became the preferred material for many implants.² Cobalt-chromium, widely used in bearing surfaces of hip and knee prostheses, falls somewhere in between but still creates substantial distortion. One study of spinal instrumentation found that cobalt-chromium rods produced artifact zones roughly 3.5 mm larger on average than titanium rods across standard imaging sequences.³ Even newer biodegradable magnesium alloys generate fewer artifacts than both stainless steel and titanium in some imaging conditions.⁴

The size and shape of the implant matter too. A small screw produces a smaller field distortion than a massive total hip stem. And the orientation of the implant relative to the scanner’s main magnetic field changes how severely the artifact spreads. All of this means that every patient’s scan presents a somewhat different challenge, which is part of why MARS protocols need to be flexible rather than one-size-fits-all.

How MARS Techniques Reduce Metal Artifacts

The term “MARS” can refer to a specific branded sequence or, more broadly, to any set of MRI techniques designed to minimize metal-related distortion. In clinical use, several approaches are often combined. The most widely deployed advanced techniques go by the names VAT, SEMAC, and MAVRIC SL, and understanding what each does helps explain why MARS scans look so much better than standard ones.

View angle tilting, or VAT, tackles the distortion that metal causes within each imaging slice. It adds an extra magnetic gradient during signal readout that compensates for the way metal shifts signal positions sideways across the image. VAT is effective at correcting these in-plane distortions, though it can introduce some image blurring as a trade-off.⁵ Think of it as straightening out a picture that metal has warped sideways, even if it slightly softens the image in the process.

SEMAC (Slice Encoding for Metal Artifact Correction) addresses a different problem: distortion between slices, called through-plane artifact. Metal can warp the shape of the imaging slice itself, bending what should be a flat pancake of tissue into a curved surface. SEMAC corrects this by encoding extra spatial information that lets the scanner figure out where each signal truly originated and reassemble the slices properly.⁶ Combined with VAT, SEMAC handles both in-plane and through-plane distortions. One study on orthopedic tumor implants confirmed that while VAT reduced in-plane artifacts, through-plane artifacts were mainly reduced only when SEMAC was added on top.⁷

MAVRIC SL (Multi-Acquisition Variable-Resonance Image Combination Selective) is a competing approach developed by a different MRI manufacturer. It works on a similar principle, acquiring data at multiple frequency offsets to capture signal that metal has pushed off-resonance, then combining those acquisitions into a cleaner image. Studies have compared MAVRIC SL head-to-head with conventional fast spin-echo sequences in patients with hip replacements and found substantial artifact reduction.⁸ In practice, whether a facility uses SEMAC-based or MAVRIC-based protocols depends largely on which brand of MRI scanner it has.

Beyond these advanced sequences, radiologists can also tune basic scan parameters. Using a higher receiver bandwidth, for instance, compresses the artifact zone. One study on cobalt-chromium knee implants found that standard high-bandwidth images had artifact zones roughly 46 to 51 percent larger than SEMAC images, illustrating the additional benefit the advanced sequence provides beyond simple parameter tweaks.⁹ Phantom testing at ultra-high-field strengths has confirmed that combining optimized bandwidth with VAT or SEMAC produces the best results, significantly shrinking both signal voids and geometric distortion compared to non-optimized scans.¹⁰

Where MARS MRI Gets Used Clinically

The biggest clinical role for MARS MRI is in evaluating problems around hip and knee replacements. After a total hip arthroplasty, patients can develop a range of complications that need soft-tissue imaging to diagnose, and plain X-rays or CT scans may not provide enough detail about muscles, fluid collections, or the tissue reaction around the prosthesis.

Metal-on-metal hip replacements, which were widely implanted in the 2000s before concerns about tissue reactions emerged, are a particularly important use case. These devices can shed tiny metal particles that trigger adverse local tissue reactions, sometimes forming masses called pseudotumors. MARS MRI has become the standard follow-up tool for these patients, allowing clinicians to identify who may need revision surgery. A longitudinal study tracking patients with metal-on-metal hips over time found that while adverse tissue reactions were common, clinically significant progression was rare, and MARS MRI remained valuable for picking out the patients whose reactions were worsening enough to warrant intervention.¹¹

Spinal hardware is another common application. Standard MRI after a spinal fusion can be nearly impossible to read around the screws and rods. Early MARS sequences demonstrated clear improvements in image quality compared to conventional T1-weighted scans, reducing the tissue-obscuring artifact produced by spinal fixation hardware.¹² This matters when a patient with prior spinal surgery develops new symptoms and the surgeon needs to distinguish between recurrent disc herniation, scar tissue, infection, or hardware loosening.

Telling Infection from Loosening

One of the more clinically impactful uses of MARS MRI is distinguishing periprosthetic joint infection from aseptic loosening in patients with painful hip or knee replacements. Both conditions can cause similar symptoms, but they require very different treatments: infection usually means long courses of antibiotics and staged revision surgery, while aseptic loosening typically calls for a simpler single-stage revision. Getting the diagnosis right before operating changes the surgical plan substantially.

A recent meta-analysis pooling 11 studies found that MARS MRI demonstrated high accuracy for diagnosing periprosthetic infection, with pooled sensitivity around 88 percent and specificity around 95 percent. Soft-tissue changes on the scan, such as edema and synovial abnormalities, had the strongest diagnostic performance, while changes in bone alone were less reliable.¹³ Specific MRI findings help clinicians tell the two conditions apart. Synovial layering and muscle edema, for example, have been identified as hallmarks of infection, while a pattern called granulomatous synovitis points more toward aseptic failure.¹⁴ Soft-tissue edema and abnormalities at both the acetabular and femoral components have also been highlighted as accurate differentiators.¹⁵

This diagnostic capability matters because the alternative approaches, such as blood tests for inflammatory markers or joint aspiration, are imperfect. Having a non-invasive imaging tool that can characterize the tissue reaction pattern around the implant gives surgeons more information before they commit to a surgical plan.

MARS MRI Versus Ultrasound

Ultrasound is sometimes used as a cheaper, faster alternative for checking on metal hip implants, especially for metal-on-metal devices. It has the obvious advantage of being completely unaffected by metal, since it uses sound waves rather than magnetic fields. But comparison studies show that the two modalities have different strengths.

MARS MRI is better at detecting pseudotumors and muscle atrophy around hip replacements, while ultrasound is actually superior for spotting joint effusions and tendon problems.¹⁶ In longitudinal follow-up, ultrasound performs reasonably well at tracking changes over time. One study found that ultrasound had good agreement with MARS MRI in detecting changes in lesion size at follow-up, with high sensitivity and specificity for identifying adverse tissue reactions.¹⁷

In practice, many clinics use ultrasound for routine surveillance and reserve MARS MRI for cases where symptoms are concerning, ultrasound findings are ambiguous, or the surgeon needs detailed information about deep soft-tissue structures before planning a revision. The choice also depends on access: not every facility has an MRI scanner with MARS capability, and the scans are considerably more expensive and time-consuming than a bedside ultrasound.

Why 1.5T Is Usually Preferred Over 3T

MRI scanners come in different field strengths, commonly 1.5 Tesla (1.5T) and 3 Tesla (3T). In general radiology, 3T scanners offer higher signal and sharper images, which is why they are popular for brain, musculoskeletal, and cardiac imaging. But around metal implants, the relationship flips. Metal artifacts scale with field strength: the stronger the magnet, the bigger the distortion zone.

The theoretical advantage of double the signal at 3T is partly offset by increased artifact severity, higher radiofrequency energy deposition, and greater safety concerns around implants. For these reasons, many centers prefer to scan patients with large implants at 1.5T, where artifacts are smaller and easier to control with MARS techniques. That said, 3T MARS scanning is feasible and is used in practice, especially in facilities that only have a 3T magnet. Studies have shown that advanced artifact-reduction sequences such as MAVRIC SL can produce diagnostic-quality images even at 3T in hip arthroplasty patients.⁸

The energy deposited in tissue during an MRI scan, measured as the specific absorption rate or SAR, is another factor. Metal implants can concentrate radiofrequency energy, causing localized heating. Measurements around artificial hip joints have recorded SAR values reaching the mid-to-upper 20s of watts per kilogram near the implant surface on both 1.5T and 3T systems, with slightly higher values at 3T.¹⁸ Modern scanners have safety systems that limit SAR automatically, but the consequence for MARS scans is that the scanner may throttle its performance, pause between sequences, or require longer scan times to stay within safe limits.

Scan Time and the SAR Trade-Off

One practical downside of MARS MRI is that it takes longer than a standard MRI. The additional encoding steps in SEMAC or the multiple frequency acquisitions in MAVRIC add scan time substantially. A conventional knee or hip MRI might take 20 to 30 minutes; a MARS protocol can push well beyond that, sometimes requiring the patient to lie still for 45 minutes or more. For patients with painful joints, this is not trivial.

Compressed sensing, a technique that reconstructs images from fewer data samples, has been applied to MARS sequences to speed things up. Some implementations have achieved significant acceleration factors while preserving image quality. The SAR issue also contributes to longer scan times: when the scanner’s safety system detects that radiofrequency energy is approaching limits, it inserts cooling pauses between pulse sequences. Researchers have developed variable flip angle techniques specifically for metal artifact reduction scans that reduce SAR while maintaining diagnostic image contrast, achieving scan-time reductions of around 15 to 32 percent in some protocols while preserving the majority of tissue contrast.¹⁹ These technical refinements are gradually making MARS more practical for routine clinical use rather than a special-order exam.

Limitations and Quirks to Know About

MARS MRI is not a magic eraser. Even with the best artifact-reduction sequences, some residual distortion remains close to the implant surface, particularly with large stainless steel or cobalt-chromium prostheses. The image quality is dramatically better than a standard MRI, but it still will not look as clean as imaging tissue without any metal present. Radiologists reading MARS scans need specific training to distinguish residual artifact from actual pathology.

An underappreciated quirk is that MARS algorithms can sometimes overcorrect. Because these sequences are tuned to compensate for the strong field distortions that metals produce, applying them to implants made from non-metallic or weakly magnetic materials can actually introduce artifacts that were not there before. An in-vitro study using dental implants found that MARS substantially reduced artifacts around titanium and titanium-zirconium implants but paradoxically increased artifact volumes around zirconia (ceramic) implants, likely because the algorithm overcorrected for a material that was not causing much distortion in the first place. This reinforces that MARS protocols need to be tailored to the specific implant material, not applied as a blanket setting for every scan.

For head and neck imaging, different MARS implementations have distinct strengths. SEMAC-based approaches are better at handling strongly magnetic materials, reducing artifact volume by about 28 percent compared to alternative methods in that context, while MSVAT-SPACE-based approaches offer higher resolution and work comparably well around less magnetic hardware. Radiologists choosing a protocol consider both the type of implant and where in the body it sits.

What Patients Should Expect

If your doctor orders a MARS MRI, the experience from a patient’s perspective is similar to any MRI. You lie on a table that slides into the scanner bore, and you hear loud knocking and humming sounds. The main differences are behind the scenes: the technologist selects specialized sequences, and the scan usually runs longer. You may be asked about your implant type beforehand, since the specifics of the metal alloy influence how the radiologist sets up the protocol.

Safety screening is the same as for any MRI. Modern joint replacements and spinal hardware are virtually always MRI-conditional, meaning they have been tested and cleared for scanning under certain conditions (field strength, SAR limits, specific positioning). Older implants, pacemakers, and certain types of surgical clips require individual screening. The MARS designation does not change the safety requirements; it only changes how the images are acquired once you are inside the scanner.

You might notice that your MARS scan takes longer than an MRI you have had before. Asking how long the exam will run before you start is reasonable, especially if you have pain that makes lying still difficult. Some facilities offer music, padding, or brief repositioning breaks to make longer scans tolerable. If claustrophobia is a concern, open or wide-bore MRI scanners are sometimes available, though advanced MARS sequences may not be installed on every machine.

Insurance coverage for MARS MRI follows the same pathway as standard MRI: if the clinical indication is approved, the scan is typically covered. There is no separate billing code for the MARS technique itself, though specialized protocols at high-volume arthroplasty centers may carry higher facility fees. If you are being seen at a center that specializes in joint replacement complications, they almost certainly have MARS capability and experience reading the results.