Canis ISSN: 2398-2942

Radiography: scattered radiation and grid use

Interaction of X-rays with tissues

Photoelectric absorption
  • A photon of electromagnetic energy ejects an electron from an inner shell of an atom.
  • The displaced electron can ionize other atoms.
  • An electron from a higher orbiting shell drops into the vacant space releasing energy ’Characteristic radiation.
Features of photoelectric absorption
  • Important at low energy.
  • Proportional to cube of atomic number ’ small variation in atomic number ie 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 absorption
  • 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 ionize other atoms.
  • The photon is diverted and continues in a different direction with a lower energy ’Scattered radiation.
Features of Compton absorption
  • 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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