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Density (Film)
Density is directly related to the quantity of x-rays produced (mAS)
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Contrast (radiographic)
Different densities (black and white). High contrast (short scale) very white and very black; low contrast (long scale) more shades of gray.
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What is contrast inversely related to?
kVp
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What happens to contrast kV is increased?
Decrease contrast/ more grays
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What happens to contrast kV is decreased?
Increased contrast/ more black and white
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What do geometric factors include?
- Focal spot size
- FFD
- OFD
- xray machine alignment
- patient placement
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Distortion
Misrepresentation of the size or shape of an object.
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Detail
Degree of film sharpness
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How do you improve detail?
- Smaller focal spot
- Collimation
- Increase FFD
- Decreased OFD
- grids
- patient positioning
- machine alignment
- shorter exposure time
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Penumbra
Area of unsharpness or fuzziness
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What happens when penumbra is increased?
- Larger focal spot size
- lack of collimation
- decrease FFD
- increase OFD
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Primary radiation
The purpose of exposing the film (the useful beam).
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* Scatter radiation
Secondary radiation produced by an absorber (body), produced film fog.
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FFD Focal Film Distance
(Also TFD, SID, SFD) Distance between the focal spot and the film.
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If you increase FFD what happens?
Decreases magnification
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OFD Object Film Distance
Always decrease as much that is practical to decrease magnification.
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Inverse square law
The intensity of the radiation varies inversely with the square of the distance.
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Inverse square law: what happens if you double the distance (FFD)?
Decrease of intensity of the radiation to 1/4 and requires a compensatory increase of the mAs by 4x's.
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Anode heel effect
- Due to the angle (bevel) intensity.
- INCREASES slowly towards CATHODE; and DECREASES rapidly towards the ANODE.
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Air gap technique
Reducing scatter radiation. between 10-15cm from the film.
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What is the disadvantage of the air gap technique?
increases magnification
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Rectification
converting AC current to DC current
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15% Rule
Increase kVp by 15% double the mAs.
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Wolff's Law
Bone deposition occurs along the lines of increased mechanical stress.
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Hueter-volkmann Law
Pressure on a growth plate retards the rate of growth, traction accelerates.
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