Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool, but metal implants often raise concerns about safety and image quality. Spinal fusion joins vertebrae using hardware, such as rods, screws, or plates. While an MRI can generally be performed after this surgery, the feasibility of a safe and successful scan depends entirely on the specific materials used and the precise conditions under which the imaging is performed.
Understanding MRI Safety Concerns with Spinal Implants
The primary safety concerns when an implant is exposed to the MRI’s powerful magnetic field are movement and heating. The risk of movement or displacement is extremely low with modern spinal hardware. This is because most current implants are made from non-ferromagnetic materials like titanium alloys or PEEK (Polyetheretherketone). Ferromagnetic materials, which are strongly attracted to magnets, are generally avoided in contemporary spinal surgery, unlike some older stainless steel implants.
Heating is another potential issue, caused by the radiofrequency (RF) energy pulses used during the scan interacting with the conductive metal hardware. While the actual temperature increase is usually minimal with modern implants, the risk can increase with longer implants, specific implant configurations, or higher magnetic field strengths, such as 3 Tesla (3T) scanners. To manage these risks, implant manufacturers categorize their products according to standardized MRI safety terminology.
An implant is labeled “MRI Safe” if it poses no known hazard in any MRI environment. A device labeled “MRI Unsafe” should never enter the MRI room. Most spinal fusion hardware falls into the “MRI Conditional” category, meaning it is safe only if specific conditions are met during the scan. These conditions, defined by the manufacturer, may restrict the maximum magnetic field strength, the specific absorption rate (SAR) related to heating, or the duration of the exam. Strict adherence to these parameters is required to ensure patient safety.
How Spinal Hardware Affects Image Quality
The presence of metallic hardware significantly impacts MRI image quality by creating artifacts. These distortions occur because the metal disrupts the local magnetic field uniformity required for accurate image formation. The most common distortion is a susceptibility artifact, which results in a localized area of signal loss, appearing as a dark void or a distorted bright signal around the implant.
This artifact makes the tissue immediately surrounding the fusion hardware difficult or impossible to evaluate diagnostically. For example, a doctor looking for a recurrent disc herniation or an infection near a pedicle screw might find the area completely obscured. The extent of this obscuration relates directly to the implant material; titanium alloys cause smaller artifacts than stainless steel. Non-metallic materials like PEEK produce almost no artifacts, offering a significant advantage for post-operative imaging.
Although diagnostic quality near the fusion site is compromised, the artifacts are typically localized to that region of the spine. Imaging structures distant from the implant, such as the brain, knee, or spinal levels far above or below the fusion, are usually unaffected. This means an MRI remains valuable for examining other parts of the body or adjacent spinal segments despite the hardware.
Key Factors Determining MRI Feasibility After Fusion
The first and most important step in determining the feasibility of a post-fusion MRI is accurately identifying the specific hardware that was implanted. Healthcare providers must obtain the device manufacturer, model, and material composition, which can often be found on the patient’s surgical records or an implant identification card. Without this information, the hardware must be treated as “MRI Unsafe” as a precaution, which prevents the scan from taking place.
Once hardware details are confirmed, the MRI technologist and the radiologist must review the manufacturer’s specific “MRI Conditional” guidelines to tailor the protocol. These guidelines dictate the maximum allowable magnetic field strength, the maximum SAR, and other technical limits necessary to ensure safety. The relevance of the scan location also plays a role, as a knee MRI is less affected by lumbar spinal hardware than an attempt to image the thoracic spine.
To mitigate the image artifacts and improve diagnostic yield, specialized scanning techniques are frequently employed. These metal artifact reduction sequences (MARS) include methods like increasing the receiver bandwidth, using multi-acquisition variable-resonance image combination (MAVRIC-SL), or slice-encoding for metal artifact correction (SEMAC). These adjustments help minimize the magnetic field distortions, allowing for better visualization of the soft tissues near the implant and making the MRI a more effective tool for post-surgical evaluation.