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Exposure Factors or Prime Factors

Exposure Factors or Prime Factors

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Exposure Factors or Prime Factors

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  1. Exposure Factors or Prime Factors

  2. PRIME FACTORS What is “technique” ? How does it affect the “image”

  3. Exposure Factors – 3 or 4 • The four prime exposure factors are: • Voltage = kVp* • Current = mA* • Exposure time = seconds or fractions of a sec* • Source-to-image distance = SID

  4. PRIME FACTORS • KVP • MAS • DISTANCE

  5. kVp • Kilovolts controls how fast the electrons are sent across the tube • Controls, quality, penetrability & contrast • Increasing kVp also increases scattered photons reducing image quality • Does kVp influence OD?

  6. “SHORT” VS “LONG” SCALE

  7. kVp • Low kVp (50 – 60) • Short scale • High contrast • “Bone work”

  8. kVp • High kVp (90 – 120) • Long scale • Low contrast • “Chest images”

  9. mA • Determines the number of photons, radiation quantity, OD & patient dose • Changing mA does not change the kinetic energy of e- • Available mA stations are usually 50, 100, 200, 300, 400 & 600

  10. Exposure Time • Should be kept as short as possible, for most examinations. To minimize the risk of patient motion • mA X s = mAs • mAs controls OD • mAs determines the number of photons in the primary beam

  11. Distance • Affects exposure of the IR because of the Inverse Square Law • SID largely determines the intensity of photons at the IR • Distance has no effect on radiation quality

  12. INTENSITY IS SPREAD OUT…

  13. Inverse Square Law Formula Distance #2 - Squared Intensity #1 Distance #1 - Squared Intensity #2

  14. SID Changes

  15. Direct Square Law • New mAs = New distance 2 Old mAs Old distance 2

  16. Focal-Spot Changes

  17. Producing optimal radiographs … key for diagnosing disease

  18. How much of the radiation received by the patient Actually reaches the IR ?

  19. About 1%

  20. Creating the IMAGE • When x-rays pass through a patient's body, three things can happen: • (1) the x-ray photon is transmitted, passing through the body, interacting with the film, and producing a dark area on the film; • (2) the x-ray photon is absorbed in an area of greater tissue density, producing lighter areas on the film; and • (3) the x-ray photon is scattered and reaches the film causing an overall gray fog.

  21. IMAGES • DENSITY = THE AMOUNT OF BLACKENING “DARKNESS” ON THE RADIOGRAPH • CONTRAST – THE DIFFERENCES BETWEEN THE BLACKS TO THE WHITES

  22. mAs Changes • at least 20 - 30 % mas change needed to see a visible change in density

  23. mAs DOUBLED = DENSITY DOUBLED

  24. + 25% + 50 % mas

  25. mAs Changes

  26. Tube voltage (kVp) • Determines the maximum energy in the beam • spectrum and affects the quality of the output spectrum • Efficiency of x-ray production is directly related to tube voltage

  27. Influencing factors: kVp 15% rule: • 15% kVp = doubling of exposure to the film  15% kVp = halving of exposure to the film 15% rule will always change the contrast of the image because kV is the primary method of changing image contrast. Remember : 15% change ( ) KVP has the same effect as doubling or ½ the MAS on density

  28. kVp Changes • The kVp setting must be changed by at least 4% to produce visual changes an image

  29. kVp Changes

  30. + 15% kvp - 15% kvp

  31. kVp Changes

  32. 4% kVp Changes

  33. Determining Radiographic TechniqueThe Patient Factor • The most difficult task for technologists… evaluating your patient • The patient size, shape, and physical condition greatly influences the technique selection

  34. 4 general states of body habitus

  35. Radiographic Technique • Technique charts are based on the “average patient” • The thicker the part the more x-radiation is required to penetrate. Calipers should be used • Keep in mind not only the measurement but the type of tissue you need to penetrate (fat vs muscle)

  36. Technique • In general, Soft tissue = low kVp and high mAs • Extremity (soft tissue & bone) = low kVp • Chest (high subject contrast) = high kVp • Abdomen (low subject contrast) = middle kVp

  37. Low Subject Contrast

  38. OD = amount of black on image

  39. Film Screen • Overexposed • Referring to a radiograph that is too dark because too much x-radiation reached the image receptor • Underexposed • Referring to a radiograph that is too light because too little x-radiation reached the image receptor

  40. Technique - Pathology • Pathology can severely affect the technologist technique selection • Always question your patients about health status • If prior images are available…check them!

  41. Pathology • Can appear with increased radiolucency or radiopacity • Some pathology is destructive causing tissue to be radiolucent • Others can be additive causing tissue to be radiopaque

  42. OD vs tissue attenuation

  43. Technique selection – Fixed kVp • For each anatomic part there is an optimum kVp • mAs is varied based on part thickness or pathological condition