Magnetic Resonance Imaging (MRI) uses strong magnetic fields and radio waves to create detailed images of organs and soft tissues. Harrington rods are a historical type of spinal instrumentation, typically made of stainless steel, used primarily from the 1960s to the 1990s to correct scoliosis and other spinal deformities. Because MRI relies on a strong magnetic field, the presence of older metallic implants like these rods raises concerns about patient safety and image quality. This article addresses the compatibility of this hardware with modern MRI technology.
Understanding the Interaction Between Metal Implants and MRI
The fundamental concern when introducing any metal implant into an MRI environment stems from the machine’s static magnetic field and radiofrequency (RF) energy. The static field exerts a force on ferromagnetic materials (metals attracted to magnets), creating a risk of implant displacement or torque, which is a twisting motion.
Another major risk is radiofrequency-induced heating. The RF energy used to create images can induce electrical currents within the metal implant, especially in long components like spinal rods. This process, known as the antenna effect, can cause localized heating at the ends of the implant, potentially leading to tissue damage. Stainless steel, typically used in Harrington rods, is considered weakly ferromagnetic. This means it is susceptible to these effects, though generally to a lesser degree than highly magnetic metals.
The third significant interaction is the creation of signal interference, or artifact. This occurs because the metal disrupts the uniformity of the magnetic field in its immediate vicinity. The resulting magnetic field inhomogeneities cause the signal data to be misinterpreted, severely degrading the image quality near the implant.
Safety Status of Harrington Rods During MRI
Harrington rods are generally designated as MR Conditional, meaning they can be safely scanned only if specific parameters are followed. Because the original stainless steel composition is weakly magnetic, the primary safety precaution revolves around the strength of the MRI machine’s main magnet, measured in Tesla (T). Most safety guidelines recommend that patients with these rods undergo imaging only in a 1.5 Tesla scanner.
Scanning at higher field strengths, such as 3 Tesla, may increase the potential for displacement force and RF heating, sometimes requiring exclusion. Although the risk of a spinal rod displacing is low after many years of securing by scar tissue and bone growth, the potential for localized heating remains a consideration. Modern spinal implants are often made of non-ferromagnetic titanium, which allows for imaging at higher field strengths with fewer restrictions. The conditional safety status for Harrington rods hinges on adhering to the lower field strength and monitoring for any sensation of warmth or discomfort during the procedure.
Impact on Diagnostic Imaging Quality (Artifacts)
While the patient may be safe to scan, the stainless steel rods significantly impact the diagnostic utility of images near the spine. The metal causes a susceptibility artifact, appearing as a large area of signal void, distortion, or blurring. This visual interference is due to the metal’s magnetic properties creating a localized distortion of the main magnetic field.
Stainless steel produces substantially larger artifacts compared to non-ferromagnetic materials like titanium or newer cobalt-chromium alloys. For patients with Harrington rods, the artifact typically obscures the soft tissues immediately adjacent to the rods, including the spinal cord and nerve roots. This limitation means that while the rest of the body can be imaged clearly, the specific area of the spine containing the hardware may not be adequately evaluated for conditions like disc herniation or nerve compression. Radiologists can sometimes use specialized sequences, such as fast spin-echo, to mitigate some blurring and signal loss, but complete elimination of the artifact is not possible.
Essential Pre-MRI Screening Procedures
Before any MRI procedure, a rigorous screening process must be completed to ensure patient safety. The patient must accurately disclose the presence of the Harrington rods, including the approximate date of the implantation surgery. This information allows the radiology team to confirm the material composition and age of the implant.
The MRI technologist must verify that the scanning parameters are compatible with the known conditional safety limits for stainless steel spinal hardware. This confirmation includes ensuring the machine is set to a safe field strength (typically 1.5 Tesla or less) and that the Specific Absorption Rate (SAR)—the measure of RF energy deposition—is within acceptable limits. The patient should be instructed to report any unusual sensation, such as localized burning or heating, immediately during the scan. The final decision to proceed is a risk-benefit assessment made by the supervising radiologist, balancing the diagnostic need against the potential risks associated with the implant.