Species: Feline | Classification: Miscellaneous
- To produce an accurate portrayal of area under examination
- Accurate positioning.
- Minimal distortion.
- Adequate contrast and detail.
- No artifacts Radiography: film faults .
- Blurring of outline of the object under examination prevents accurate interpretation.
- Also known aspenumbra effect.
- This is dependent on:
- Size of focal spot :
- 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 .
- Increasing the film focus distancereduces unsharpness but the ability to do this is limited by the power of the tube.
- Size of focal spot :
Film object distance
- Increasing the distance between the film and the object allows the x-ray beam to diverge .
- 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.
- 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.
- Phosphor crystals in intensifying screens Radiography: cassette and intensifying screen emit light in all directions when stimulated by an incident photon .
- 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.
- 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 and unsharpness appear to be linked.
- If there is high contrast the image appears sharper.
Contrast and sharpness are completely independent.
- Minimize by increasing film focal distance and reducing object film distance.
- Occurs if
- X-ray beam oblique .
- Occurs if object and film are not parallel .
- 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
- The use of different types of films and screens affects image quality Radiography: cassette and intensifying screen .
- Initial exposure of the film must be correct as it cannot be corrected for later Radiography: x-ray film .
- Poor processing technique Radiography: processing can ruin an otherwise perfect image.