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CT Artifacts

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A brief introduction to CT Artifacts, with definitions, example images, and solutions.

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CT Artifacts

  1. 1. CT Artifacts Definitions and Solutions
  2. 2. CT Artifacts • Artifacts can degrade image quality, affect the perceptibility of detail, or even lead to misdiagnosis. An artifact is a distortion or error in an image that is unrelated to the subject being imaged. (Morgan 1983). • CT artifacts can arise from a number of sources, including the patient. Inappropriate selection of protocols, reconstruction processes, equipment malfunction or imperfections, and the limitation of physics.
  3. 3. CT Artifacts • Ring Artifact – Ring artifact is caused by a miscalibrated or defective detector element, which results in rings centered on the center of rotation. This can often be fixed by recalibrating the detector. • Noise – Noise is due to the statistical error of low photon counts, and results in random thin bright and dark streaks that appear preferentially along the direction of greatest attenuation. Also known as photon starvation. • Metal artifact – Metal streak artifacts are caused by multiple mechanisms, including beam hardening, scatter, Poisson noise, motion, and edge effects. • Beam Hardening - Beam hardening and scatter both produce dark streaks between two high attenutation objects (such as metal or bone), with surrounding bright streaks. • Out of field “artifact” - Out of field “artifacts” are due to a suboptimal reconstruction algorithm, and can be fixed using a better algorithm. • Motion - (patient, cardiac, respiratory, bowel) causes blurring and double images, as well as long range streaks
  4. 4. CT Artifacts • Ring Artifact - A miscalibrated or defective detector element creates a bright or dark ring centered on the center of rotation. This can sometimes simulate pathology. Usually, recalibrating the detector is sufficient to fix this artifact, although occasionally the detector itself needs to be replaced.
  5. 5. CT Artifacts • Noise (photon starvation) – noise is influenced partially by the number of photons that strike the detector. Photon starvation can occur as a result of poor patient positioning or poor selection of exposure techniques. The image below gives a representation of how photon starvation visualized. Todays modern scanners virtually eliminate this effect due to mA modulation. Smart mA, Sure-exposure…etc. However for those scanners that still employ manual techniques, this effect can be reduced by an increase in mA.
  6. 6. CT Artifacts • Metal Artifact - Metal streak artifacts are caused by multiple mechanisms, including beam hardening, scatter, Poisson noise, motion, and edge effects. Metal artifacts can be reduced using iterative reconstruction.
  7. 7. CT Artifacts • Beam Hardening - Beam hardening in CT arises due to the polychromatic (varied energies) X-ray beam. As the beam passes through the body, lower energy photons are attenuated more easily, and the remaining higher energy photons are not attenuated as easily causing what we see as an artifact. So by increasing the overall energy of the beam (kV) we can reduce this artifact.
  8. 8. CT Artifacts • Motion – motion artifacts can be anything from voluntary motion (caused by the patient ie Head movement, improper breathing etc.) to involuntary motion such as peristalsis or cardiac motion. Proper patient education can only reduce voluntary motion or by applying immobilization techniques. Improved techniques such as faster pitch can reduce the presence of involuntary motion, or as in cardiac CT a technique that monopolizes the “at rest” portion of the QRS complex (generally the ST segment). Look for a future presentation solely devoted to cardiac CT.
  9. 9. CT Artifacts • Motion Images :
  10. 10. CT Artifacts • Well there we have it, CT artifacts, certainly this is not a complete documentation of all CT artifacts but it addresses the ones that technologists as a whole will see. For a more in depth understanding of CT Artifacts please follow this link. • http://www.edboas.com/science/CT/0012.pdf

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