The potential for metal in the body to interact with the powerful magnetic fields of an MRI machine is a common concern, especially regarding surgical steel, a material widely used for implants and medical instruments. The question of whether surgical steel is safe during a Magnetic Resonance Imaging (MRI) scan has no simple yes or no answer, as the outcome depends entirely on the specific metallic composition of the alloy used.
Understanding Surgical Steel Composition
The term “surgical steel” refers to a category of corrosion-resistant alloys, most commonly austenitic stainless steel. These alloys are chosen for medical use due to their strength and biocompatibility, meaning the body generally accepts the material without adverse reaction. The primary components include Iron, Chromium (typically 16–18%), Nickel (10–14%), and often Molybdenum (2–3%).
The presence of these elements directly influences the material’s magnetic properties, which is the most important factor in MRI safety. Modern surgical steel, such as grade 316L, has an austenitic crystal structure that is non-ferromagnetic. This means the steel is not strongly attracted to a magnet, unlike older grades that may contain higher amounts of ferromagnetic elements. The “L” in 316L signifies low carbon content, which stabilizes the non-magnetic properties and enhances corrosion resistance.
The Physics of MRI and Implant Interaction
An MRI scan involves two primary physical risks when a patient has metal present: magnetic deflection and radiofrequency (RF) heating. The machine uses a powerful static magnetic field that interacts with ferromagnetic materials, causing a force known as translational attraction. For strongly magnetic objects, this force can result in movement or rotation, potentially dislodging the implant or causing injury to surrounding tissue.
The second risk involves the pulsed RF fields used by the MRI system. These oscillating fields induce electrical currents within conductive materials, leading to localized heating. While the temperature increase is generally insignificant for modern orthopedic implants, the risk is greater for devices that form a loop or are long and thin, like wires or leads. The heat generated near the metal can potentially cause thermal damage to the adjacent soft tissue.
Safety Assessment: Specific Grades and Applications
Surgical steel safety depends on its specific alloy and classification by regulatory standards (e.g., ASTM) as “MRI Safe,” “MRI Conditional,” or “MRI Unsafe.” Modern surgical steel, particularly 316L, is generally classified as “MRI Conditional.” This status means that while the material is not completely non-magnetic, it can be scanned safely under specific parameters, usually up to 3-Tesla (T) field strength. Studies show that while minor magnetic interactions like slight deflection may occur, they are generally acceptable for contemporary orthopedic implants. Older implants or external items like certain jewelry may use different alloys that could pose a higher risk, especially if they are not made of the non-magnetic austenitic grades.
Patient and Clinical Safety Protocols
Even with “MRI Conditional” surgical steel implants, a thorough patient screening process remains paramount. Clinicians must obtain the exact manufacturer and model information for the implanted device to confirm its specific MRI safety profile and conditions. This documentation is necessary because safety guidelines can vary based on the implant’s size, shape, and how long it has been secured within the body.
Technologists play a role by adjusting scan parameters to mitigate any residual risk. This may include limiting the magnetic field strength of the scanner (often preferring 1.5T systems) or adjusting the power settings to reduce the specific absorption rate (SAR). While the risk of severe heating is low for passive implants, patients must be instructed to report any unusual sensation, such as warmth or discomfort, immediately during the procedure. Furthermore, the presence of metal will almost certainly cause imaging artifacts, which appear as signal loss or distortion on the scan, potentially obscuring diagnostic information in the area near the implant.