A Computed Tomography (CT) scan is an imaging test that uses X-rays and computer processing to create detailed cross-sectional images of the body’s internal structures. This technology provides a view of bones, soft tissues, and blood vessels far more detailed than a standard X-ray. Since the procedure relies on X-rays, the general rule is that all metal objects should be removed from the area being scanned to ensure the highest image quality.
Essential Items to Remove Before the Scan
For a CT scan, you must remove all removable metal items, even if they seem minor or far from the planned scan area. Common items include necklaces, earrings, rings, and watches, which can interfere with the imaging process. Clothing containing metal parts, such as zippers, snaps, or underwire bras, should be avoided; you will often be asked to change into a hospital gown.
Other items like hairpins, hair clips, and removable dental work, such as dentures, must also be taken off before entering the scanner. The requirement for removal is not strictly limited to the direct area being scanned because X-ray scatter can still cause image issues, potentially creating artifacts that degrade the image quality.
The Impact of Metal on CT Image Quality
The need to remove metal stems from the fundamental physics of how X-rays interact with dense materials. Metal is significantly denser than biological tissue, causing it to absorb nearly all the X-ray photons directed at it. This complete absorption prevents the X-ray beam from reaching the detectors, a phenomenon known as photon starvation.
When the computer reconstructs the image from this incomplete data, it results in image degradation called “streak artifacts” or “beam hardening artifacts.” These artifacts manifest as bright and dark streaks radiating outward from the metal object. The bright streaks are often caused by scattered radiation, while the dark streaks are due to the beam hardening effect, where the metal selectively filters out lower-energy X-rays.
These streaks can completely obscure the surrounding anatomy, making it impossible for the radiologist to visualize the area of interest and potentially leading to a misdiagnosis. For example, a small metal earring can obscure a tumor in the neck or a bone fracture in the face. The severity of the artifact is directly related to the density and atomic number of the metal, with high-density metals like iron or platinum causing more pronounced problems than lower-density metals like titanium.
Navigating Non-Removable Jewelry and Implants
While removable jewelry is simply taken off, non-removable metal within the body presents a different challenge. This category includes medical implants like pacemakers, joint replacements, surgical staples, and certain body piercings that are difficult or impossible to remove. These devices will inevitably cause artifacts, but they are generally tolerated if they are far from the area being examined.
If a non-removable metal object is in the direct path of the scan, you must inform the technologist beforehand, as they may be able to adjust the scan settings to minimize the artifacts. For body piercings, especially those that are new or difficult to remove, patients can often substitute the metal jewelry with non-metallic alternatives like glass, plastic, or nylon retainers. These non-metal materials may still cause a minimal artifact, but the effect is significantly less disruptive than that of metal. Modern CT scanners also employ advanced software techniques, such as iterative reconstruction, which can help reduce the appearance of metal artifacts, even those caused by large implants.