The definition of blurring or un-sharpness on mammography images is: the amount of lateral border spread along the edges of our image. A distance of border spread that is visually acute is unacceptable.
What causes our images to look blurry? Why do the x-rays sometimes look indistinct? What can we do, where can we seek an answer?
There are three distinct causes of mammography blurring:
- Motion
- Geometric
- Receptor
Receptor:
The receptor in mammography refers to the combination of a screen composed of phosphorescent particles imbedded in the screen layer and an emulsion coated acetate based film.
If the quality and characteristics of the film/screen combination are unable to capture the sharp, high contrast image produced by the x-ray unit we still end up with a substandard image.
In the case of the screen, again the blur is caused by a shadow or penumbra produced by the diffusion of light around the particles used to assist image production. In the film, blurring can be initiated by the emulsion, base or quality of the combination.
Once more the problems can be simply demonstrated by geometry. Light and x-rays travel in roughly straight lines, so their path can be tracked and measured.
Screens:
Mammography screens must be high quality thin phosphorescent screens with tiny crystals. The screen must be pristine, unblemished and well seated into the cassette. The screen must be kept in tight contact with all aspects of the film.
- The phosphor layer thickness
- The size of the phosphorescent crystal
- The addition of light absorbing pigments
- Film/Screen contact
The Phosphor Layer Thickness:
Thick Layer:
1. Screen crystal
2. Large penumbra reaches film
Medium Layer:
1. Screen crystal
2. Small penumbra reaches film
Ultra Thin Layer
- Screen crystal
- Virtually no penumbra reached film
Crystal Size:
- Large crystal
- Substantial amount of diffused light reaches image
- Virtually no diffused light reaches image
- Tiny crystal
Light Absorbing Pigments:
1. X-ray beam
2. Light diffusing pigment
3. Screen crystal
4. Amount of light actually reaching film
Film/Screen Contact:
- Area of film/screen loss of contact
- Screen crystal
- Large amount of light reaching film
- Small amount of light reaching film
Mammography Film:
The film we use in breast imaging is very important to the image we produce. High quality mammography film is an extremely detailed and highly technical product. It is essential that we pair our film type to our screen type. This complicated area of expertise is best left to the professionals at your provider. Some of the particulars are easy to grasp.
- Mammography film must be single emulsion film. Double emulsions tend to flash back through the acetate and across the two emulsion layers producing a ‘crossover’ blur on the edges of the image.
- The base of mammography film must be coated with an anti-halation finish to prevent halation (a flash off the shiny base layer).
- The emulsion of mammography film must be fast enough to capture the high contrast, black and white images we need.
- The developed film must produce a sensitometry strip with a steep H&D curve rapidly reaching adequate maximum density.
An Adequate H&D Curve for Mammography Film:
SUMMARY
Blurring is a major cause of error in interpretation of mammography images. It is imperative we understand and address all the causes of this difficulty. This ends the series on blurry, indistinct images. I hope it has given you some insight into some of the causes and cures we can employ in this area. Keep on truckin’.
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