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Iterative techniques for reducing metal artifacts in CT scanning

White Paper
Philips CT Clinical Science Philips Healthcare • USA

Background
Streak artifacts from high-attenuation objects are a common problem in computed tomography (CT). This type of artifact typically occurs from metallic implants like joint replacement, osteosynthesis, dental implants, or surgical clips.

Metal Artifacts Reduction (MAR) is an iterative technique that reduces artifacts due to beam hardening, scatter, Poisson noise, motion, and edge effects. Improved image quality can increase diagnostic confidence.
 X-ray projection data through metal is unavailable due 
to the severe attenuation of X-rays by the metal, causing 
streaking or dark bands.
X-ray projection data through metal is unavailable due to the severe attenuation of X-rays by the metal, causing streaking or dark bands.

Metal Artifact Reduction (MAR)
Philips Access Dual CT uses an iterative technique to reduce the metal artifacts.

The first iteration is reconstructed using uncorrected projection data. Metal and bone are then detected using a Hounsfield unit cutoff, and these are forward projected to determine how much bone and metal are present in each detector measurement.

The following several iterations are comprised of detecting anatomical structures in the original CT Filtered Back Projection image and raw data: 
  • Adjusting the raw data
  • Re-projecting the artifact creating object based on metal noise model
  • Interpolating replacement data for the missing projection data
  • Reconstructing the final CT image 

The interpolation-based methods do not increase the computational cost and enhances image quality for the case of hip prosthesis and dental fillings.

This image and graph show a metal artifact and its projection data, which demonstrate the severe attenuation of X-rays by metal (highlighted by the red circle on the graph).



Enhanced visualization
Images reconstructed using the Philips Access Dual CT scanner with MAR show a reduction in metal artifacts in both the axial and coronal planes, enhancing visualization of surrounding anatomy.

Summary
In literature, many different techniques have been proposed to reduce metal artifacts in CT scans. Some techniques suggest replacing the metal implants with less attenuating materials or to use higher energy x-ray beams to overcome metal artifacts. Others use image windowing techniques to reduce the appearance of artifacts in the images. However, these case-by-case solutions are not ideal for most clinical applications. The most efficient methods work on the raw projection data, i.e. the matrix of ray attenuations related to different angles acquired by the CT scanner. In iterative reconstruction methods, the projection data associated with metal objects are disregarded and reconstruction is applied only for non-corrupt data. Briefly, in these methods, an initial guess of the reconstructed image is made and then the projections obtained of this initial image are compared to the raw projection data. By iteratively reconstructing projection ratios and applying an appropriate correction algorithm for the initial image, an improved estimate of the image is obtained. In projection interpolation based methods, the projection data corresponding to rays through the metal objects is considered missing data. A prior art technique manually identified the missing projections and replaced them by interpolation of non-missing neighbor projections.

MAR uses a polynomial interpolation technique to bridge the missing projections. Most metallic implants like joint replacements, osteosynthesis, dental implants, or surgical clips can be prevented by using this algorithm in scanner technology.


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White Paper
Access CT
artifacts, beam hardening correction, Body, bone, dental, dental fillings, FBP reconstruction, Head, hip, image quality, lumbar spine, MAR, metal artifact reduction, Pelvis
 

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