Technical Aspects: the machine the image

1. Technical Aspects:the machinethe image LeeAnn Pack DVM

4. Exposure settings: 4 components • kVp • mAs • Time • Focal film distance (FFD) • All can be changed alone or in combination

5. Make it just right • Make a film more black • Increase the kVp, mAs or time • Decrease the FFD • Make a film less black • Decrease the kVp, mAs or time • Increase the FFD

6. Questions to Ponder • Is there adequate penetration of the part imaged? • How much contrast is needed on the film (some inherent?)? • Is motion a strong possibility during the exposure? • Does the tube move?

7. Inverse Square Law • Decrease distance by ½ intensity is increased by 4 times • Increase distance by ½ intensity is increased by 4 times • Technical applications • Radiation safety

8. Cassettes • Contain single or double screen • Plastic, cardboard, metal • Must be durable • Film placed between the screens in the cassette • Foam between cassette and screen • Keeps out unwanted light

10. Intensifying Screens • Absorb X ray photons and convert them to light • Reduces amount of radiation needed • Contain phosphors • Calcium tungstate  blue light • Rare earth phosphors  green light • More efficient

11. Screen Speed • Determined by the efficiency of X ray absorption and conversion • Thicker phosphor layer and larger crystal produce a greater amount of light  faster • Slow  detail   photonsresolution • Medium  par  average • Fast less resolution but not as many photons needed

12. Screen Speed/Resolution • 100,200,400,800 • Smaller number being a slower high resolution screen and the higher numbers are a faster screen • Screen speed and resolution are inversely proportional

14. Screen Cleaning • Keep free of debris • Sharp white artifact • Screen cleaner • Soft lint free cloth • Never use soap and water • Stand on end to dry

15. Radiographic Film • Composition • Base supports the emulsion – blue • Adhesive – base to emulsion • Emulsion – gelatin with silver halide crystals which is sensitive to radiation • Protective coat protects from damage during handlin and processing

17. Radiographic Films • Single or double emulsion • Light sensitivity • Film speed • Film speed is inversely related to exposure needed to produce a given degree of blackness • 50 speed is half as fast as a 100 speed

18. Radiographic Film • Film latitude • Range of exposures which can be used to achieve an acceptable film density • Wide range = high latitude • Image contrast suffers • Narrow range = low latitude • Good image contrast

19. Radiographic Density • Subject density • Additive – thick parts absorb more • Summation – know definition • Relative • What are the surroundings? • Silhouette sign – know definition • Over exposed = film to black • Under exposed = film not black enough

20. Factors that affect film density • Subject density – we cant change this • mAs – biggest factor • kVp – increases the penetrability • Distance – ISL • Development time and temp • Scatter and fog

21. Radiographic Contrast • Difference of densities of the different parts on the image • Subject contrast • Film contrast • Scatter and fog

22. Turn that dial: Density • The 16-20 rule • kVp by 20% will double film density • kVp by 16% will half film density • Double mAs  double density • Half mAs  half density • Combos: • Inc kVp by 20% and dec mAs by half • Dec kVp by 16% and double mAs

23. Contrast • Short scale  lot of contrast few grey • High mAs and low kVp • Long scale  many shades of grey • Low mAs and high kVp • Low contrast = long scale = high latitude = low mAs = high kVp • High contrast = short scale = low latitude = high mAs = high kVp

24. Magnification • Enlargement of the image relative to the actual size • Causes loss of detail and blurring • Subject film distance • Dec SFD  decreases magnification • Focal film distance • Inc FFD  decreases magnification

25. Distortion • Image which does not represent the true shape of the object • Unequal projection of an object • Femurs • Location of an object within the beam • spine

26. Scatter radiation • What is scatter? • Bounce around undergo numerous interactions • Scatter increases with: • Increased thickness of the patient • Increased X ray beam energy • Amount of patient exposed (collimate)

27. Beam Limiting Devices • Smaller fields = less scatter and less radiation expsoure • Cones, cylinders, shuttered collimators • They do not shape the beam only exclude the part we don’t want to use • Light to illuminate field

29. Filtration • Inherent • Glass housing, oil and tub housing window • Filter only low energy photons • Added • Aluminum filter • Will not remove the high energy photons • Polychromatic beam • Reduces patient dose

30. Grids • Used to keep scatter from hitting film • Perpendicular radiation gets through, off angle photons do not • Used on parts > 10-11cm • Film contrast is improved • Required more X ray exposure

31. Grid ratio • Height of lead strips:width between the strips • The higher the GR, the greater the ability of the grid to remove scatter • 4:1 – 16:1 • Lines per inch • Good and bad

33. Grid Cutoff • Grid must be precisely under the center of the central ray of the X ray beam along the central axis of the grid • Know what the types of grid cutoff look like • Bucky-Potter moving grid

34. Fog • Light fog • Storage fog • Safelight fog • Chemical fog