Equis ISSN 2398-2977

Radiography: scattered radiation and grids

Contributor(s): Tim Donovan, Patricia Rose, Jeffrey Watkins, Jonie Watkins

Interaction of X-rays with Tissues

Photoelectric absorption
  • A photon of electromagnetic energy is absorbed and ejects an electron from an inner shell of an atom.
  • The displaced electron can ionise other atoms.
  • An electron from a higher orbiting shell drops into the vacant space releasing energy   Radiation physics: photoelectric absorption    →  Characteristic radiation.

  • Features of photoelectric absorption
  • Important at low energy.
  • Proportional to cube of atomic number Z3   →   small variation in atomic number as in body tissue   →   large contrast in film exposure.
    Photoelectric absorption is important at kV<100 and at high kV reduced film contrast occurs because photoelectric effect less important

  • Compton scatter
  • A photon of electromagnetic energy interacts with a loosely bound electron in the outer shell of an atom.
  • The photon displaces the loosely bound electron which can ionise other atoms.
  • The photon is diverted and continues in a different direction with a lower energy   Radiation physics: photoelectric absorption    →  Scattered radiation.

  • Features of Compton scatter
  • Increases with increasing energy.
  • As energy increases more of scattered radiation is directed in a forward direction ie more likely to reach x-ray film.
  • Independent of atomic number of tissue.
    Compton scatter is significant at kV >70 and in tissue of low atomic number, ie most body tissue

Production of Scatter

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Scatter Reduction

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Grids

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Further Reading

Publications

Refereed papers


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