How Dental Materials Interact with Magnetic Fields
Magnetic Resonance Imaging (MRI) uses a powerful static magnetic field, radiofrequency pulses, and gradient coils to generate detailed images of the body’s soft tissues. When a patient with dental fillings enters the MRI bore, the metal materials interact with these powerful fields. The interaction is determined by the material’s magnetic susceptibility, which measures how much a substance becomes magnetized in a magnetic field. Dental materials are broadly categorized as diamagnetic, paramagnetic, or ferromagnetic, with the latter causing the most significant reaction.
The alloys used in older dental work, such as some stainless steel components or base metal alloys, may contain ferromagnetic elements like iron or nickel, leading to a strong attraction to the main static magnetic field. Conversely, modern amalgams, gold, and titanium alloys are generally only weakly magnetic, falling into the paramagnetic or diamagnetic categories. These low-susceptibility materials cause far less field distortion and are considered safer and more compatible with MRI technology.
Patient Safety: Movement and Heating Risks
The primary safety concern often raised is the risk of a metal filling being pulled from the tooth by the extreme force of the MRI’s main magnetic field. This risk is extremely low for standard dental restorations like amalgam fillings or gold crowns, because the materials used are not strongly ferromagnetic. They are firmly secured within the non-magnetic structure of the tooth, and their low magnetic susceptibility means the attractive forces generated are negligible compared to the bond strength of the filling.
A more realistic, though still rare, safety issue is the potential for localized heating caused by the radiofrequency (RF) energy pulses used during the scan. These pulses induce electrical currents within conductive metal objects, such as long, thin wires found in orthodontic retainers or older dental implants, leading to a slight rise in temperature in the surrounding tissue. While this heating is rarely dangerous with small, solid fillings, the risk is mitigated by using lower RF power settings for patients with extensive metal dental work.
Diagnostic Impact: Understanding Artifacts
The most common consequence of having metal fillings during an MRI is the degradation of image quality, known as a magnetic susceptibility artifact. This phenomenon occurs because the metal restoration drastically alters the homogeneity of the magnetic field in its immediate vicinity. The resulting distortion causes a loss of signal from the surrounding tissues, which manifests on the image as a dark area or signal void, often larger than the filling itself.
These artifacts also cause geometric distortion, where surrounding anatomical structures appear stretched, compressed, or displaced. The degree of image degradation is directly proportional to the magnetic susceptibility of the material and the strength of the MRI scanner. The diagnostic impact depends heavily on the scan’s target area; for example, an artifact from an amalgam filling can completely obscure the brainstem, sinuses, or temporomandibular joint in a head or neck scan, potentially hiding a lesion or disease.
Pre-Scan Screening and Mitigation Strategies
Before any MRI is performed, patients must undergo a thorough pre-scan screening process, often involving a detailed questionnaire about any metallic objects in their body. For dental work, it is helpful for patients to know the material composition of their fillings, although technicians and radiologists are trained to recognize the likely impact of common restorations. This assessment allows the technologist to anticipate the location and size of the expected artifact.
To minimize image distortion, the technician can implement several technical mitigation strategies. One common approach is to use specific pulse sequences, such as a fast spin-echo (FSE) sequence, which is less susceptible to magnetic field inhomogeneities than standard gradient-echo sequences. More advanced scanners may employ specialized Metal Artifact Reduction Sequences (MARS), like Slice Encoding for Metal Artifact Correction (SEMAC) or Multi-Acquisition Variable-Resonance Image Combination (MAVRIC). These sequences use complex spatial encoding techniques and increased radiofrequency bandwidth to compensate for localized field distortion and recover diagnostic information.