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What are factors influencing the image quality?

What are factors influencing the image quality?

With regard to image quality, the term frequently used is “sensitivity”. Sensitivity determines the extent to which a radiograph is able to clearly show (anomaly) details of a certain size. Sensitivity in this sense must not be confused with the sensitivity or “speed” of the film. (see section 7.5).

Digitised and enhanced image of a radiograph of the (American) Liberty Bell

 

Image contrast is affected by :

  • differences in thickness of the specimen
  • the radio-opacity (radiation transparency) of the specimen and its defects
  • the shape and (depth)location of the defects
  • the quality (hardness) of the radiation
  • the amount and effects of scattered radiation
  • the effect of filters used

Discernibility of defects on a radiograph depends in general on:

  • the quality of the radiation
  • the properties of the film
  • the film viewing conditions Image quality is governed by contrast, sharpness and film graininess.

Film contrast depends on:

  • the type of film
  • the density level

Sharpness of an image is governed by:

  • the (effective) size of the focal spot or radiation source
  • the source-to-object distance
  • the object-to-film distance
  • the contact between film and intensifying screens
  • the type of intensifying screens used
  • the radiation energy used

The last factor, graininess, depends on :

  • the thickness of the emulsion layer
  • the concentration of silver crystals in the emulsion (silver/gelatine ratio)
  • the size of the silver crystals
  • the radiation energy used
  • the developing process employed

The radiation energy level is the only factor that can be influenced by the radiographer; the other factors are determined by the film making process.