A computed tomography (CT) scan uses X-rays to create detailed, cross-sectional pictures of the body’s internal structures, providing information about bone, soft tissues, and blood vessels that a standard X-ray cannot capture. The presence of metal, whether external (jewelry) or internal (implants), is highly problematic. While a CT scan is safe with metal, the primary issue is the severe degradation of the resulting image, which can obscure diagnostic information. This necessitates removing metal whenever possible or using special protocols when it cannot be removed.
How Computed Tomography Works
A CT scanner operates by projecting a narrow beam of X-rays through the patient’s body from multiple angles. Inside the doughnut-shaped machine, an X-ray tube rotates rapidly around the patient, while a ring of detectors measures the radiation that passes through. Different tissues absorb or attenuate the X-rays to varying degrees. Dense materials like bone absorb much of the radiation, while less dense tissues like fat or air absorb very little.
The detectors record the intensity of the X-ray beam after it has passed through the body. This measurement is then sent to a powerful computer. The computer uses these numerous measurements, taken from every angle, to calculate the density of each tiny point, or voxel, within the scanned area. Finally, the computer uses these calculated density values to construct a detailed, three-dimensional image of the internal anatomy.
Image Distortion: The Metal Artifact Effect
The primary reason metal is incompatible with a high-quality CT scan is its extreme density, which fundamentally disrupts the X-ray data collected. Metal completely absorbs or strongly scatters nearly all the X-ray photons that strike it, creating a significant void in the data that the detectors receive. This phenomenon is a severe form of what is known as beam hardening, where the initial X-ray beam loses its lower-energy photons after passing through the metal.
The computer’s reconstruction algorithm, which assumes a consistent X-ray beam, cannot accurately process this missing or corrupted data. This data void results in distinct image defects known as streak and shadow artifacts that radiate outward from the metal object. These artifacts appear as alternating bright streaks and dark bands across the image, making the surrounding anatomical structures unreadable.
For example, a dental filling can cause streaks that obscure the jawbone, or a hip replacement can block the view of nearby soft tissues. The resulting artifacts are a computational error caused by the computer attempting to calculate a density value where it received no usable information. The streaks and shadows therefore render a large area of the image diagnostically useless.
Patient Safety and Physical Risks
While image degradation is the most common consequence of metal in a CT scan, there are also non-image-related concerns, particularly with external items. Unlike magnetic resonance imaging (MRI), CT scans do not use powerful magnets that would cause ferromagnetic metal to fly through the room. However, external metal objects like jewelry, zippers, or body piercings can still pose a risk of physical harm.
Though rare, certain metallic objects can absorb energy from the X-ray beam and potentially heat up, creating a risk of localized skin burns or discomfort for the patient. Furthermore, external metal items may shift during the scan, especially if they are loose or bulky. This movement not only exacerbates imaging errors but also creates a minor risk of injury if the item catches on the moving parts of the scanner. Therefore, the removal of all external metal is a standard precaution to ensure both optimal image quality and patient comfort.
Techniques for Scanning Patients with Implants
Many patients requiring a CT scan have internal metal that cannot be removed, such as orthopedic screws, surgical clips, or pacemakers. Technicians manage this challenge through a combination of positioning and advanced software tools. The type of metal matters, as materials like titanium cause less severe artifacts than stainless steel because of their differing atomic structures and X-ray absorption characteristics.
One practical strategy involves adjusting the angle of the CT scanner, known as gantry angulation, or changing the patient’s position to project the metal artifact away from the specific area of diagnostic interest. Modern CT scanners also utilize highly sophisticated software known as iterative reconstruction algorithms. These programs work by computationally modeling the effect of the metal and then systematically minimizing the resulting streaks and shadows from the final image data. This advanced digital processing significantly reduces the severity of the artifacts, allowing physicians to visualize the anatomy adjacent to the implant more clearly.