Species: Feline   |   Classification: Miscellaneous

Aim

  • To produce an accurate portrayal of area under examination

Unsharpness

  • Blurring of outline of the object under examination prevents accurate interpretation.

Geometric unsharpness

  • Also known aspenumbra effect.
  • This is dependent on:
    • Size of focal spot   Radiation physics: unsharpness - focal spot size  :
      • Reducing the size of the focal spot is difficult to achieve as this represents the target area of the anode   Radiography: x-ray generation  .
      • The size of the effective focal spot can be reduced by using an angled target area   Radiation physics: focal spot size  .
      • Increasing the film focus distancereduces unsharpness but the ability to do this is limited by the power of the tube.

Film object distance

  • Increasing the distance between the film and the object allows the x-ray beam to diverge   Radiation physics: unsharpness - film object distance  .
  • The cassette should be kept in contact with the object if possible.
  • In large animal radiography, however, this effect can be beneficial in reducing the amount of scattered radiation affecting the film.

Patient effects

  • Movement unsharpnessalso known asmovement blur.
  • Active movement eg struggling, and essential movement eg respiration or peristalsis.
  • Can be reduced by:
    • Reducing exposure time (may need to increase kV if mA limited).
    • Reducing focus-film distance.
  • Patient unsharpnessis inherent in the structure of biological tissues.
  • Most organs get thinner towards the perimeter therefore attentuation decreases and their contrast with surrounding tissues is reduced.

Screen unsharpness

  • Phosphor crystals in intensifying screens   Radiography: cassette and intensifying screen  emit light in all directions when stimulated by an incident photon   Radiation physics: unsharpness (screen)  .
  • Good film -screen contact should reduce this effect, by minimizing light divergence.
  • Cross-over effect where light from each screen passes through the film base and exposes the emulsion on the opposite side.

Film unsharpness

  • Also known asparallax unsharpness.
  • Most x-ray film is double-sided   Radiography: x-ray film  and so the viewer is essentially looking at 2 superimposed images.
  • If the film emulsion is swollen ie the film is not fully dried - the effect is increased.

X-ray film should not be finally read until it is completely dry.

Other factors affecting image quality

Contrast
  • Contrast and unsharpness appear to be linked.
  • If there is high contrast the image appears sharper.

Contrast and sharpness are completely independent.

Magnification

  • Minimize by increasing film focal distance and reducing object film distance.
  • Occurs if
    • Object film distance is large.
    • X-ray beam continues to diverge after being incident on object   Radiation physics: magnification  .
  • X-ray beam oblique .

Distortion

  • Occurs if object and film are not parallel   Radiation physics: geometric distortion  .
  • Occurs in large animal radiography where cassette hand held at end of pole and difficult to retain in constant position.
  • Occurs in small animal radiography if, for instance, a joint is not fully extended.

Film and screen quality

Exposure

Processing