Species: Feline   |   Classification: Miscellaneous

Interaction of x-rays with tissues

Photoelectric absorption
  • A photon of electromagnetic energy with energy just slightly greater than the electron binding energy ejects an electron from an inner shell of an atom.
  • The photon gives all its energy to the electron in displacing it and is truly absorbed.
  • The displaced electron can ionize 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   →   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

  • An electromagnetic photon with energy greater than the binding energy of the electron interacts with an electron in one of the shells 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   Radiation physics: photoelectric absorption    →  scattered radiation.

Features of Compton absorption

  • Predominates at higher beam energies than those susceptible to photoelectric effect.
  • As energy increases more of 'scattered radiation' is directed in a forward direction ie more likely to reach x-ray film (effectively not 'scattered' therefore).
  • Independent of atomic number of tissue, for a given beam energy.

Compton scatter is significant at kV >70.

Production of scatter

  • Scatter can be produced when x-rays interact with matter.
  • Lower energy than primary beam.
  • Travel in any direction.
  • Very important in large animal radiography.
  • At high kV a higher percentage of the radiation is incident on the film. Since more of the scattered radiation is moving in a forward direction towards the film there is an increasing fogging of the film at high kV and contrast is reduced.
  • 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 (by displacing tissue mass outside primary beam).
  • Can be achieved using Bucky band - a webbing strap which can be tightened around the body (particularly abdomen).

Reduce kV

  • Reduces the amount of scatter produced.
  • Reduces the amount of scatter reaching the film.

Reduce scatter affecting film

Grids

  • Placed between film and patient to absorb scatter.
  • Most scatter is traveling 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 traveling obliquely misses film.
    • Important in large animal radiography where film is often some distance from object.
    • Air gap increases magnification and reduces image sharpness   Radiography: image quality  so need to increase focal film distance to compensate.
  • Filter between patient and film.

Not practical since primary beam also significantly attenuated by filter.

Lead backing to film cassettes

  • Absorbs radiation penetrating film and screen.
  • Prevents back scatter.

Reduce effects of scatter on film

Grids

  • Increase film contrast by reducing general fog of film by scatter.

Construction

  • Parallel strips of lead separated by radiolucent interspacers (plastic with aluminum surround).
  • With 30-60 strips per cm.
  • Primary rays are traveling virtually parallel and pass through interspaces.
  • Scattered radiation, traveling obliquely, cannot penetrate lead   Radiation physics: grid construction 

Parallel grid

  • Lead strips are parallel and equal height across the grid.
  • At edge of grid, effect of diverging beam is apparent and some primary rays are absorbed.
  • Reduced exposure density at periphery of film.

Focused grid

  • Lead strips are angled from center   Radiation physics: grid - focused and pseudofocused  .
  • This accommodates 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!
  • Pseudofocused gridshave shorter lead strips at edges than in middle   Radiation physics: grid - focused and pseudofocused  .

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.

Characterisitcs

Grid ratio

  • The higher the ratio the more scatter is absorbed   Radiation physics: grid construction  .

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 film density.
  • Usually 2.5-3.
  • Greater for parallel than for focused.

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.