Spinal Cord Stimulators (SCS) are implanted medical devices used primarily to manage chronic pain, such as that associated with failed back surgery syndrome or complex regional pain syndrome. The system consists of an implanted pulse generator and thin wires called leads that deliver low-voltage electrical impulses near the spinal cord. These impulses modify pain signals before they reach the brain, providing relief. In medical imaging, an “artifact” is any unwanted distortion or signal change that does not represent the actual anatomy, often caused by a non-biological source. Because SCS devices contain metallic components, they interact with the strong magnetic fields and radiofrequency pulses used in common imaging techniques, leading to predictable interferences.
Understanding Artifact Generation in Stimulator Devices
The physical cause of artifacts from SCS devices centers on the material science of their components and their interaction with the strong magnetic field of an imaging scanner. Most SCS hardware, including the metallic casing and conductive leads, possesses a different magnetic susceptibility than the surrounding human tissue. Magnetic susceptibility describes how much a material becomes magnetized when exposed to an external magnetic field. When a material with high magnetic susceptibility is placed inside a uniform magnetic field, it locally distorts the field lines.
This disruption creates a steep gradient, or inhomogeneity, in the magnetic field immediately around the device, which is the primary mechanism of artifact generation. This non-uniform field prevents the accurate spatial encoding of the signal from nearby protons, leading to signal loss or distortion. The size and composition of the metal directly influence the extent of the interference.
Identifying Common Artifact Types in Imaging
The interference caused by the SCS hardware manifests as several distinct visual distortions in medical images. The most prominent type in Magnetic Resonance Imaging (MRI) is the magnetic susceptibility artifact, which appears as a large, dark region or signal void surrounding the metallic components. This signal dropout occurs because the disrupted magnetic field causes rapid dephasing of the proton spins, meaning the signal cannot be accurately measured.
Another common distortion is geometric distortion, where the size and shape of anatomical structures near the device are misrepresented, appearing stretched, compressed, or shifted. While MRI is most affected by magnetic susceptibility, Computed Tomography (CT) scans also experience specific artifacts. For example, beam hardening creates dark bands or streaks radiating from the dense metal components. These visual anomalies make it challenging to accurately assess the tissue immediately adjacent to the implanted device.
Clinical Impact on Diagnostic Accuracy
The presence of imaging artifacts significantly hampers a clinician’s ability to accurately diagnose new or evolving medical conditions. The signal voids produced by the SCS often obscure critical anatomical structures in the spinal region, such as the spinal cord, nerve roots, and surrounding soft tissues. If a patient with an implanted stimulator develops new neurological symptoms, the artifact can make it nearly impossible to visualize the source of the problem.
This interference is problematic in scenarios requiring detailed spinal imaging, such as assessing for post-operative infection, tumor recurrence, or hardware complications like lead migration or fracture. The inability to rule out pathology within the obscured region of interest may necessitate using less ideal imaging modalities or surgical exploration. Even with modern “MR Conditional” devices, image quality degradation around the implant remains a possibility, potentially delaying a diagnosis.
Methods for Minimizing Artifact Interference
Radiologists and technologists employ various techniques to manage or reduce the artifact burden created by SCS devices. A primary approach involves technical adjustments to the imaging sequence. This includes using Fast Spin Echo (FSE) pulse sequences, which are less susceptible to magnetic field inhomogeneities than gradient echo sequences.
Adjusting the frequency-encoding direction, often from anterior-to-posterior, can shift the artifact away from the area of diagnostic interest. Increasing the receiver bandwidth is another parameter adjustment that helps reduce the magnitude of certain artifacts. Furthermore, specialized metal artifact reduction software is increasingly used to process raw image data and computationally mitigate some distortions. Procedural precautions include ensuring the use of modern MRI-conditional devices, which are designed with materials that minimize magnetic susceptibility.