Undergoing a Magnetic Resonance Imaging (MRI) scan with an implanted peripheral nerve stimulator (PNS) requires understanding the interaction between powerful magnetic fields and electronic medical devices. PNS is a form of neuromodulation that uses mild electrical impulses to manage chronic pain. MRI is a non-invasive diagnostic tool that uses strong magnetic fields and radiofrequency waves to create detailed images of soft tissues. Because PNS devices contain metallic and electronic components, their exposure to the MRI environment introduces specific safety concerns, requiring careful risk assessment and adherence to strict protocols.
Understanding Peripheral Nerve Stimulation
Peripheral nerve stimulation systems treat chronic pain by delivering targeted electrical impulses to specific peripheral nerves, located outside the brain and spinal cord. This stimulation modulates the central processing of pain. It works by stimulating large-diameter nerve fibers, which interrupts the transmission of pain signals from smaller fibers before they reach the spinal cord and brain. This offers relief for conditions like regional neuropathies or complex regional pain syndrome.
A complete PNS system has two primary parts: the lead, a thin wire with electrodes placed near the target nerve, and the pulse generator, which supplies power and controls the electrical pulses. The pulse generator may be fully implanted beneath the skin, or it may be an external, wearable component that transmits power wirelessly to an implanted receiver. The metallic and electronic materials used in the leads and circuitry create the potential conflict when exposed to an MRI scanner.
The Conflict Between Magnetic Fields and Implanted Devices
The powerful forces generated during an MRI scan create three main risks for patients with implanted electronic devices like a PNS.
Radiofrequency (RF) Induced Heating
The long, thin leads of the stimulator can act like an antenna. RF energy waves used by the MRI system induce electrical currents along these leads, causing the tissue at the electrode tip to heat up significantly. This localized temperature increase can cause thermal injury to the surrounding nerve tissue, potentially leading to permanent neurological damage or exacerbating the patient’s pain.
Magnetic Field Force and Torque
The powerful static magnetic field of the MRI scanner exerts force on any ferromagnetic components within the device. This force can cause magnetic torque, potentially resulting in the movement or dislodgement of the implanted pulse generator or the lead itself. Although modern PNS systems often use non-ferromagnetic materials, the risk of lead migration remains a concern, particularly with older systems.
Electronic Interference and Malfunction
The strong magnetic and rapidly changing gradient fields can interfere with the electronic components of the pulse generator. This interference may cause temporary or permanent device failure, or it could result in unintended electrical stimulation, which can be painful to the patient during the scan. External components must be strictly kept out of the magnetic field, as they are unsafe and pose a projectile hazard.
Determining Your Device’s MRI Compatibility Status
Regulatory bodies classify medical devices regarding the MRI environment into three categories: MR Safe, MR Unsafe, and MR Conditional. MR Safe devices pose no known hazard in any MRI environment. MR Unsafe devices contain ferromagnetic materials that pose a definite hazard and must never enter the scanner room.
Most modern PNS systems are classified as MR Conditional. This means the device is safe only if specific, predefined conditions are met, based on rigorous manufacturer testing. These conditions typically limit the static magnetic field strength (e.g., 1.5T or 3.0T) and the maximum radiofrequency power level, known as the Specific Absorption Rate (SAR). Conditions may also restrict the body region that can be scanned or the type of radiofrequency coil used. Patients must present their device identification card, which allows radiology staff to consult manufacturer guidelines and confirm that all complex conditions for a safe scan can be met.
Essential Safety Protocols for Scanning
When a patient with an MR Conditional PNS device requires an MRI, a strict procedural protocol must be followed to mitigate risks.
Pre-Scan Preparation
The neurostimulator must first be reprogrammed or placed into an “MRI Mode” by a trained clinician or the implanting physician. This special mode typically disables the stimulation and adjusts the device’s internal settings. This prevents unwanted stimulation or damage to the device during exposure to the magnetic field.
During the Scan
The patient must be closely monitored for any signs of discomfort or adverse effects, such as unusual warmth or painful nerve stimulation. Specific operational parameters of the MRI machine, including field strength and SAR limits, must be strictly adhered to as specified in the device’s labeling. Insulating padding is often used between the skin and the scanner bore to prevent direct contact that could increase the risk of heating.
Post-Scan Check
Following the MRI scan, a post-scan check is required. The clinician who initiated the MRI Mode must test the device and return it to its original therapeutic settings. This ensures the system is functioning correctly and the patient’s pain management therapy is resumed.