ANSWERS

1. What is the output of most portable x-ray machines?
   20-60 mA.
2. What is the maximum output of most mobile x-ray machines?
   300 mA.
3. What is the maximum output of most fixed x-ray machines?
   1250 mA.
4. What is the purpose of the line voltage compensator?
   To control fluctuations in normal mains electricity which may cause inconsistent radiographic output.
5. Larger x-ray machines may possess a device which automatically controls electrical fluctuations; what is this device called?
   An autotransformer.
6. What is the minimum distance away from the tube head that the exposure button may be situated?
   At least 2 metres or can be moved to that distance via a cable.
7. What is the function of the kilovoltage (kV) control?
   Selection of the penetrating power of the x-ray beam produced. This works by determination of the speed and kinetic energy with which the electrons hit the target. Remember that kV affects both the contrast and blackening of a radiograph.
8. What is another term for the kilovoltage applied across the tube?
   Potential difference.
9. When would it be sensible to select a high kV setting?
   • For radiography of tissues with a high atomic number.
   • For radiography of tissues with a high specific gravity.
   • For radiography of very thick or deep tissues.
10. What is the function of the milliamperage (mA) control?
    Determination of the quantity of x-rays produced (a measure of the number of electrons crossing the tube from cathode to anode during an exposure). Remember that mA affects only the blackening of the radiograph. This works by alteration of the amounts that the filament is heated.
11. What is the milliamperage also known as?
    The tube current.
12. What unit denotes the product of the tube current (mA) and the length of exposure?
    Milliampere seconds (mAs).
    Exposure time (in seconds) = mAs divided by mA
13. In small x-ray machines, the kV may be directly linked to the mA. Why might this be disadvantageous?
    Longer exposure times are necessary if both a high kV and mAs are required.
14. If the tube current (mA) is increased, how is the exposure time (mAs) affected?
    A shorter exposure time (mAs) is necessary.
    If mA is decreased, a longer exposure time will be required.
15. Why is it sensible to use the largest possible mA for a particular kV setting?
    The longer the exposure time, the more likely you are to encounter movement blur.
16. If the kV were increased by 10, by how much would you need to reduce the mAs in order to produce a film of similar diagnostic quality to the exposures used initially?
    The mAs should be halved.
17. What is meant by the film focal distance (FFD)?
    The distance between the focal spot and the radiographic film. The greater the FFD, the lower the intensity of the x-ray beam.
    A longer FFD therefore requires a higher mAs in order to prevent underexposure.
18. State the inverse square law.
    The intensity of the x-ray beam varies inversely as the square of the distance from the source.
    New mAs = Old mAs x New distance squared divided by old distance squared.
19. What is a suitable FFD for most portable x-ray machines?
    75cm.
20. In addition to FFD, what other parameters must be kept constant when calculating exposures?
    • Line voltage.
    • Film-screen combination.
21. What device is used for measuring the depth of tissues?
    Callipers.
22. What is the purpose of a grid?
    To absorb scattered radiation.
23. How does a grid work?
    The grid is a flat plate sited between the patient and cassette. It consists of narrow strips of lead alternating with strips of a radiolucent material such as plastic or aluminium. Undeflected photons passing through the patient pass through the interspaces, whilst oblique scattered radiation is absorbed largely by the lead strips.
24. List the 4 main types of stationary grid.
    • Parallel.
    • Focussed.
    • Pseudo-focused.
    • Crossed.
25. What is the name of the expensive, but very efficient moving grid which is built into the radiography table-top?
    The Potter Bucky diaphragm.
26. Why is a focussed grid preferable to a parallel grid?
    A parallel grid may cause "grid cut off" around the edges of the film, since x-ray photons at the perimeter of the primary beam may be absorbed by the lead strips. A focussed grid possesses gradually sloping strips on either side to allow for the divergence of the primary beam, therefore helping to avoid grid cut off.
27. What important factors must be taken into account when a grid is used?
    • Ensure that the grid is the right way up.
    • The mAs must be increased in order to compensate for the fact that some of the primary beam will be absorbed in addition to scattered radiation.
28. What is meant by the "grid factor"?
    The amount by which the mAs is increased to compensate for losses from the primary beam absorbed by the grid. The grid factor is usually 2.5-3, but each grid has its own particular specification.
29. Define grid ratio.
    The ratio of the height of the strips to the width of the radiolucent interspace. The larger the grid ratio, the more efficient it will be. Usual grid ratios are 5:1 - 10:1.
30. What is the main disadvantage of grid use (apart from cut off) and how might this be rectified?
    Coarse grid lines on a film can cause image disruption. The use of a grid with a greater number of lines per cm and subsequently finer grid lines will cause less distraction when assessing an exposed image.
31. What is the purpose of intensifying screens?
    As the name implies, these are responsible for intensifying the effect of x-rays on the radiographic film. Some x-ray photons emerging from the patient are converted into visible light photons by crystals of phosphorescent material contained within the screens. This enables the reduction of exposure times.
32. Where are the intensifying screens located?
    Inside the x-ray cassette. The front screen is sited between the front radiolucent material and the x-ray film, whilst the rear screen is sited between the film and a felt pressure pad.
33. What colour light is emitted by calcium tungstate screens?
    Blue light.
34. What colour light is emitted by rare earth phosphor screens?
    Blue or green light.
35. Why are rare earth screens preferable to those of calcium tungstate?
    Rare earth screens are more sensitive to the primary beam, which means that exposure factors may be significantly reduced.
36. Describe how you would clean an intensifying screen.
    With care. Avoid scratches, abrasions, greasy fingerprints and chemical splashes. Always use a proprietary cleaner according to the manufacturer's instructions. Finish off with lens tissue to remove any traces of dust.
37. What film type is suitable for use with calcium tungstate screens or blue light emitting rare earth phosphors?
    Monochromatic film.
38. What film type is suitable for use with green or blue light emitting rare earth phosphors?
    Orthochromatic film.
39. When would non-screen film be utilised?
    For intra-oral radiography where very fine detail is required.
40. What is the main disadvantage of non-screen film?
    Longer exposure times are required in order to obtain a diagnostic image.
41. What is the main advantage of a fast film-screen combination?
    Lower exposure factors are required than a slow system.
    A fast film-screen combination is suitable for thoracic and abdominal radiographs.
42. What is the main disadvantage of a fast film-screen combination?
    Blurring of the image.
43. What is the main advantage of a slow film-screen combination?
    Higher definition images are produced than with a fast system.
    A slow film-screen combination is suitable for skull and limb radiographs.
44. What is the main disadvantage of a slow film-screen combination?
    Higher exposure factors are necessary than with a fast combination.
45. List 6 important factors regarding the storage of unexposed radiographic film.
    • Radiographic film must not be exposed to light.
    • Store well away from the controlled area (ideally in a lead-lined cupboard).
    • Keep in a clean, cool, dry place.
    • Store in an upright position, and within the manufacturer's packaging.
    • Never use film that has passed its expiry date.
    • Film may be refrigerated if it is exceptionally hot weather.