Engineers have solved the problem of distorted imaging scans that plague surgeons who need to use them to assess the placement of metal implants. They have developed a simple and practical solution to “steer” a scanner and direct its imaging beam so that most of the distortion problem is remedied.
Metal implants are commonly used to stabilize bones and structures in the body. During an operation, surgeons use a C-arm X-ray or CT (computed tomography) scanner to see if an implant is correctly positioned, has caused hem-orrhaging, or is impinging on adjacent nerves. Problems with the implant can necessitate second surgeries that are expensive and often have poor outcomes.
Metal transmits X-rays differently than tissue or bone, often yielding distortions, or artifacts, that appear on imaging scans. Artifacts may make the implant appear larger than it really is and form streaks throughout the image, blocking the surgeon's ability to see potential problems. Currently, scanners have built-in systems to analyze the X-ray signals and reduce the impact of metal artifacts. These systems, however, are not very effective and the artifacts are worse with larger and denser implants.
The solution involves steering the C-arm scan to give data that contains the fewest errors that could cause image artifacts. The researchers developed a computer algorithm that assesses the location of a metal implant and determines the best position of the CT scanner to avoid artifacts. The method is called “metal artifact avoidance” since it avoids errors in the original data, unlike other techniques that try to correct them later.
The researchers found that their tests of the algorithm and tilting of the C-arm reduced the magnitude of errors in the X-ray data and resulting artifacts in the image. In one example, a 5-millime-ter-diameter metal screw was implanted into the spine of a cadaver. Without the metal artifact avoidance algorithm, the image of the screw was distorted so that it appeared to be 8-12 millimeters in diameter. With the algorithm, the screw appeared as its real size. The key is to compute a scan orbit in a way that can be easily run by the C-arm.
To enable the computer algorithm to work on most C-arms, manufacturers would need to calibrate their scanners to be tilted in the operating room. The metal artifact avoidance method can be paired with different algorithms that correct other types of artifacts.
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