A Computed Tomography (CT) scan captures detailed cross-sectional images of the body’s internal structures. This diagnostic tool uses X-rays, which pass through the body and are measured by detectors to create these “slices.” The presence of metal, whether worn externally or implanted internally, interferes with the accurate collection of X-ray data. Metal does not pose a safety risk in the CT environment, as the technology relies on radiation, not the powerful magnetic fields used in MRI. The main concern is image quality degradation, which can obscure crucial diagnostic information.
How Metal Creates CT Image Artifacts
The problem metal causes is a significant image distortion known as an “artifact.” CT scanners rely on the principle that tissues absorb X-rays consistently based on their density. Metal is extremely dense and absorbs nearly all X-rays, leading to two major physical effects: beam hardening and photon starvation.
Beam hardening occurs because the X-ray beam contains a spectrum of energies. As the beam passes through dense metal, lower-energy photons are filtered out, increasing the average energy of the remaining beam. This “hardens” the beam, causing the computed attenuation value to be inaccurate. Since the reconstruction software assumes a consistent X-ray energy, it cannot correctly interpret the data, resulting in dark streaks between dense objects and bright streaks adjacent to them.
Photon starvation occurs when metal completely blocks the X-ray beam, preventing photons from reaching the detectors behind it. This data gap causes the reconstruction algorithm to generate prominent streaking lines that radiate outward from the metallic object. This streaking pattern, often described as a starburst shape, can severely obscure the surrounding anatomy.
If the obscured area contains a small tumor, fracture line, or abscess, the artifact can hide the pathology completely, leading to a missed diagnosis. For instance, a small dental filling can create streaks that conceal a mass in the jaw, while a hip replacement can block the view of nearby lymph nodes or pelvic organs.
Essential Preparation: Removing External Metal
Preparation for a CT scan involves removing any metallic objects near the area being scanned to ensure optimal image quality. The technologist will ask the patient to remove these items to prevent unnecessary artifacts from appearing in the final images.
Common external items that must be removed include:
- All jewelry, such as necklaces, earrings, rings, and watches.
- Clothing accessories like belt buckles, metal zippers, snaps, and underwire bras, which must be taken off or replaced with a metal-free gown.
- Hair accessories like metal clips, barrettes, and bobby pins for head or neck scans.
- Removable dental appliances, such as dentures or retainers.
Removing external metal eliminates a major source of image distortion and ensures the radiologist has the clearest possible images for interpretation.
Addressing Internal Metal Implants and Safety
Internal metal, such as surgical implants, cannot be removed, so its presence must be disclosed to the scanning team. Safety concerns are minimal for devices like orthopedic screws, plates, joint replacements, and most modern pacemakers, as the CT process does not involve strong magnetic forces. While the patient is safe, the internal metal will still create image artifacts, often more significant than those caused by external metal.
To manage these artifacts, CT technologists use specialized software known as Metal Artifact Reduction (MAR) techniques. These algorithms identify data gaps caused by the metal and use interpolation methods to estimate the missing information, effectively reducing streaking. Newer scanners also use techniques like dual-energy CT, which helps mitigate beam hardening by acquiring images at two different X-ray energy levels. This technological intervention helps recover diagnostic detail near the implant, making it necessary to inform the technologist about any internal hardware.