Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool that creates detailed images of the body’s internal structures using an extremely strong magnetic field and radio waves. Because of the intense magnetic environment, the presence of metal can pose a significant safety risk or interfere with the diagnostic quality of the scan. This concern applies directly to stainless steel, a material frequently used in medical implants, surgical instruments, and external devices. The safety of a stainless steel object in an MRI depends entirely on the specific composition of the alloy.
The Variable Nature of Stainless Steel Grades
The term “stainless steel” refers to a large family of iron-based alloys containing at least 10.5% chromium for corrosion resistance. The magnetism of the steel is determined by the other elements mixed into the alloy and the resulting crystal structure of the metal. These compositional differences lead to varying magnetic responses in the high-field MRI environment.
Some grades of stainless steel, such as the 400 series, possess a crystal structure that makes them highly magnetic, or ferromagnetic. These alloys are strongly attracted to the static magnetic field of the scanner. Conversely, modern medical devices often use austenitic stainless steels, commonly the 300 series like 316L, which are engineered for biocompatibility and low magnetic attraction.
The addition of elements such as nickel gives austenitic stainless steel a structure that renders the material essentially non-magnetic, or weakly magnetic (paramagnetic). This makes it significantly safer for use in an MRI. Even these weakly magnetic grades can sometimes become slightly more ferromagnetic if they have been mechanically stressed, such as through bending or hammering during surgery.
Potential Risks in the MRI Environment
If a piece of stainless steel is significantly ferromagnetic, it introduces three distinct physical hazards when exposed to the powerful magnetic fields of an MRI scanner. The first is the translational force, often referred to as the projectile effect. This force can violently pull a ferromagnetic object toward the center of the magnet bore at high speed, posing a severe risk of injury to the patient or staff and potentially damaging the scanner itself.
A second concern is torque, which is the twisting or rotational force exerted on a magnetic object within the body. This is particularly dangerous for elongated or oddly shaped implants, such as rods or screws, which could rotate or shift within the tissue. Even a minor rotation can cause pain, tissue damage, or the malfunction of an implanted device.
The third significant hazard involves radiofrequency (RF) heating, which is independent of the steel’s magnetic properties and relates to its electrical conductivity. The RF pulses used to generate the images induce electrical currents, known as eddy currents, within any conductive metallic object. This induced current can lead to localized heating of the implant and the surrounding tissue, potentially causing thermal burns or tissue necrosis.
Standardized Safety Classification Systems
To provide clarity on these complex risks, medical device manufacturers and regulatory bodies use a standardized classification system for objects entering the MRI environment, often based on the ASTM International standards. This system divides compatibility into three distinct categories to provide actionable information for patients and medical staff.
Classification Categories
- MR Safe: These devices pose no known hazard in any MRI environment, regardless of field strength. These items are entirely non-metallic, or made from materials like certain plastics or titanium. Stainless steel devices are almost never labeled as MR Safe because of their conductive properties and potential for magnetic interaction or heating.
- MR Unsafe: These devices present a definite hazard and must never enter the MRI scanner room. This category includes highly ferromagnetic materials, such as many older or non-medical grades of stainless steel. If a device has an MR Unsafe classification, the risk of violent movement or malfunction is too high to permit a scan under any circumstances.
- MR Conditional: This is the third and most common classification for modern stainless steel implants. This label means the device is safe only under clearly defined and specified conditions, which are determined by extensive testing. These conditions typically include limits on the static magnetic field strength (e.g., safe up to 3.0 Tesla), the maximum radiofrequency power output (Specific Absorption Rate or SAR), and the specific location of the implant.
It is important for anyone with a metallic implant to know the exact make, model, and manufacturer of their device, as this information contains the specific MR Conditional parameters. Before undergoing an MRI, patients must thoroughly review their medical records and inform the technologist about any metallic implants or foreign bodies. The technologist must then verify the device’s compatibility using the manufacturer’s documentation to ensure the scan can be performed safely within the established limits.