A Computed Tomography (CT) scan uses X-ray beams and computer processing to generate detailed, cross-sectional images of the body, allowing medical professionals to visualize internal structures like bone and soft tissue. However, the presence of metal objects, whether worn externally or embedded internally, directly interferes with how the X-rays are transmitted and recorded. This interference compromises the quality of the resulting images, potentially making the scan difficult or impossible for a radiologist to interpret.
The Mechanism of Image Artifacts
The primary consequence of wearing metal during a CT scan is the creation of image imperfections known as artifacts. Metal, particularly dense metals like steel or titanium, absorbs nearly all the X-ray photons directed at it, preventing them from reaching the detectors on the opposite side of the scanner. This phenomenon is often termed “photon starvation” and introduces a severe data gap into the image reconstruction process.
When the computer attempts to reconstruct a cross-sectional image from this incomplete and distorted data, it produces visual errors. The most common of these is the “streaking” or “starburst” artifact, which appears as bright and dark lines radiating outward from the metallic object. These streaks are caused by a combination of effects, including beam hardening, where the lower-energy X-rays are filtered out by the metal, and scatter, where X-rays are deflected in random directions.
Because metal density is outside the normal range of human tissue, standard mathematical algorithms fail to create an accurate map. These streaking artifacts do not just appear directly on the metal but extend far into the surrounding soft tissue, effectively obscuring any underlying pathology. This degradation means that a tumor, fracture, or blood clot located near the metal object can become completely invisible, rendering the scan diagnostically ineffective for that specific region.
Addressing Physical Safety Concerns
A common concern when considering a CT scan with metal present is the risk of physical danger. CT scanners use X-rays and do not employ powerful magnetic fields, meaning there is no risk of external metallic objects being forcefully pulled into the scanner or internal implants moving from their position.
The risk of metal heating up, which is a concern in other scanning modalities, is also not a factor in CT imaging. Therefore, internal metal, such as joint replacements, surgical clips, or dental implants, is considered safe for the patient within a CT environment.
Internal metal implants are not a physical safety hazard, but they remain a significant source of image artifacts. The primary safety focus for a CT scan is not on the movement of metal but on managing the imaging quality. Newer software techniques, such as Metal Artifact Reduction (MAR), are now used to digitally minimize the streaking caused by permanent implants, improving the clarity of the surrounding tissue.
Essential Preparation Before the Scan
To ensure the best possible image quality, patients are asked to remove any external metal objects before entering the CT room. Common items that must be removed include jewelry, watches, hearing aids, and eyeglasses, as these are positioned directly in the path of the X-ray beam. Clothing containing metal components, such as zippers, snaps, underwire, or belt buckles, should also be avoided, and patients are often provided with a metal-free gown.
Any removable dental work, like retainers or dentures, should be taken out if the scan is focused on the head or neck area. This step prevents the creation of needless artifacts that could interfere with the visualization of facial bones or brain tissue.
It is also important to communicate any non-removable metal within the body to the technologist before the scan begins. This includes permanent piercings, pacemakers, aneurysm clips, or shrapnel. While these items are safe, alerting the staff allows them to adjust the scanning parameters, such as the angle or the X-ray energy level, to try and reduce the resulting image distortion.