Species: Canine   |   Classification: Miscellaneous

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

  • Scatter is produced when x-rays interact with matter.
  • Lower energy than primary beam.
  • Travel in any direction.
  • Very important inlarge animal radiography.
  • At high kV less of the primary beam is converted to scatter but more scattered radiation is moving forward towards the film.
  • Increases with increasing volume of tissue irradiated.
Effects of scatter
  • Increases radiation exposure to personnel.
  • Increases radiation dose to patient.
  • Reduces film contrast (increases overall film density in a non-specific way).

Scatter reduction

Reduce scatter production

Collimate x-ray beam
  • Reduces the radiation dose reaching the patient and therefore the volume of tissue being irradiated.
Compress patient
  • Reduces volume of tissue irradiated.
  • Can be achieved using Bucky band - a webbing strap which can be tightened around the body (particularly abdomen).
Reduce kVReduce scatter affecting filmGrids
  • Placed between film and patient to absorb scatter.
  • Most scatter is travelling in an oblique direction and therefore is unable to pass through grid.
  • Results in increased exposure factors required.
  • Grid lines can appear on film.
Alternative filtration devices
  • Air gap between patient and film:
    • Radiation travelling obliquely misses film.
    • Important in large animal radiography where film is often some distance from object.
    • Air gap increases magnification and reduces image sharpness.
  • Filter between patient and film:
    • Intensifying screens act as a filter.
      Not practical since primary beam also significantly attenuated by filter.

Lead backing to film cassettes

  • Absorbs any radiation penetrating film.
  • Prevents back scatter.

Reduce effects of scatter on film

  • Intensifying screens (particularly rare earth) intensify primary photons more than scatter.
  • Screens also increase gamma so that film contrast is enhanced and effect of scatter is reduced.

Grids

  • Increase film contrast by reducing general fog of film.
Construction
  • Parallel strips of lead (0.05 mm wide) separated by radiolucent interspacers (plastic with aluminium surround).
  • With 20-28 strips per cm.
  • Primary radiation beam is travelling vertically and passes through interspaces.
  • Scattered radiation, travelling obliquely, cannot penetrate lead.
Linear grid
  • Lead strips are parallel and equal height across the grid.
  • At edge of film radiation beam is diverging and obliquely moving photons are absorbed.
  • Reduced exposure density at periphery of film.
Focussed grid
  • Lead strips are angled from center outwards.
  • This accomodates for the diverging pattern formed by the primary beam.
  • Center of x-ray beam must pass through center of grid.
  • Must be used right way up!
  • Pseudofocussed gridshave shorter lead strips at edges than in middle.
Crossed grid
  • Two grids of low grid ratio at right angles to one another.
  • Central axis of incident beam perpendicular to the grids.
Moving grid
  • Blurs grid lines by moving grid across film during exposure.
  • Grid must move 3-4 interspaces to be effective.
  • Need special x-ray table and Bucky linked to machine so that movement of grid linked to exposure.
Single stroke
  • Grid pulled at uniform speed across film.
  • Moves about 2.5 cm in 0.2-15 seconds.
Oscillating
  • Grid moves backwards and forwards during exposure.
  • Moves 1.25 cm either side of center over 0.5 seconds.
CharacteristicsGrid ratio
  • The higher the ratio the more scatter is absorbed.
Grid factor
  • How much the exposure factors must be raised to compensate for use of grid.
  • Depends on:
    • Number of lead strips per cm.
    • Thickness of strips.
  • Determined by taking x-ray and noting exposure ’ add grid and increase exposure to get back to same intensity.
  • Usually 2.5-3.

Contrast improvement ratio

  • Measures the improvement in contrast created by using a grid.
  • Calculated by dividing film contrast with a grid by that without.
  • Usually 1.5-3.5.