Exposure factors2


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Exposure factors2

  1. 1. EXPOSURE FACTORS DR Hussein Ahmed Hassan
  2. 2.  Exposure factors are factors that control density (blackening) and contrast of radiographic image. They are some of the tools that technologists use to create high-quality radiographs
  3. 3. Exposure Factors Controlled by the Operator  kVp  mA times Exposure Time = mAs  Determines the quality and quantity of the exposure  FFD (SID), Focal Spot and Filtration are secondary factors
  4. 4. 1- EXPOSURE FACTORS:  KVP. :  It controls the quality of the beam, i.e. PENETRATION.  It influences : a: penetration power, i.e. beam quality; kVp. α penetration power. b: Radiographic contrast; kVp. α 1/radiographic contrast.    c: Radiation dose to patient.
  5. 5. KVP  kVp controls radiographic contrast.  kVp determines the ability for the beam to penetrate the tissue.  kVp has more ef fect than any other factor on image receptor exposure because it af fects beam quality.
  6. 6. KVP  To a lesser extent it also influences the beam quantity.  As we increase kVp, more of the beam penetrates the tissue with higher energy so they interact more by the Compton ef fect.  This produces more scatter radiation which increases image noise and reduces contrast.
  7. 7. KVP  50 kV 79% is photoelectric, 21% Compton, < 1% no interaction  80 kVp 46% is photoelectric, 52% Compton 2% no interaction  110 kVp 23% photoelectric, 70% Compton, 7% no interaction  As no interaction increases, less exposure is needed to produce the image so patient exposure is decreased.
  8. 8. High kVp. low radiographic contrast Low kVp. High radiographic contrast
  9. 9.  MA.: 1 Ampere = 1 C/s = 6.3 x 1018 electrons/ second.  The mA selected for the exposure determines the number of x-rays produced.  The number of x-rays are directly proportional to the mA assuming a fixed exposure time.  100 mA produced half the x-ray that 200 mA would produce.
  10. 10. MA  Patient dose is also directly proportional to the mA with a fixed exposure time.  A change in mA does not af fect kinetic energy of the electrons therefore only the quantity is changed.
  11. 11. MA  Many x-ray machines are identified by the maximum mA or mAs available.  A MP 500 has a maximum mAs of 500 mAs.  A Universal 325 has a maximum mA of 300 and maximum kVp of 125
  12. 12.  MA  More expensive three phase machines will have a higher maximum mA.  A General Electric MST 1050 would have 1000 mA and 150 kVp.
  13. 13.  EXPOSURE TIME  The exposure time is generally always kept as short as possible.  This is not to reduce patient exposure but to minimize motion blur resulting from patient movement.  This is a much greater problem with weight bearing radiography.
  14. 14. EXPOSURE TIME  Older machine express time as a fraction.  Newer machines express exposure time as milliseconds (ms)  It is easy to identify the type of high voltage generation by looking at the shortest exposure time.
  15. 15. EXPOSURE TIME  Single phase half wave rectified fasted exposure time is 1/60 second 17 ms.  Single phase full wave rectified fastest exposure time is 1/120 second or 8 ms  Three phase and high frequency can provide exposure time down to 1 ms.
  16. 16. (4) MAS. :  It af fect the total number of x-ray produced by the tube during exposure, i.e. QUANTITY.  It is the product of two quantities; mA. the tube current; s. the exposure time;
  17. 17. MAS  mA and exposure time is usually combined and used as one factor expressed as mAs.  mAs controls radiation quantity, optical density and patient dose.  mAs determine the number of xrays in the beam and therefore radiation quantity.  mAs does not influence radiation quality.
  18. 18. MAS  Any combination of mA and time that will give the same mAs should provide the same optical density on the film. This is referred to as the reciprocity law.  As noted earlier for screen film radiography, 1 ms exposure and exposure longer than 1 seconds do not follow this rule.
  19. 19. MAS  On many modern machines, only mAs can be selected. The machine automatically gives the operator the highest mA and shortest exposure time.  The operator may be able to select mA by what is referred to as Power level.
  20. 20. MAS  mAs is one way to measure electrostatic charge. It determines the total number of electrons.  Only the quantity of the photons are af fected by changes in the mAs.  Patient dose is therefore a function of mAs.
  21. 21. Ampere is 1 coulomb (C) of electrostatic charge flowing each second. 1A = 1C/s = 6.3 X 10 18 electron/s 20 mAs = 0.2 Amperes. This charge releases this No. of electrons: 6.3 X 10 18 X 0.2 = 1.26 X 10 18 electron/s 20 mA. mAs 40 mA. mAs 80 mA. 200 mA. X 1.0 s = 20 X 0.5 s = 20 X X 0.25 s 0.1 s = 20 mAs = 20
  22. 22. (5) Focal spot:  Most x-ray tubes of fer two focal spot sizes: a. Fine focus: b. Broad focus:
  23. 23. a/ Fine focus: (0.3 – 0.6 mm 2 )  It records fine details.  It can not withstand too much heat.  Its usage may require long exposure time.  Used whenever geometric factors are more (long subject-film distance, short FFD ... etc).
  24. 24. a/ Broad focus: (0.6 – 1.2 mm 2 )  It can withstand too much heat.  Always used in combination with short (s) and fast film/screen system.  Used whenever voluntary or involuntary motion is highly expected.  Used when radiosensitive organ is within exposed area or 10 cm from
  25. 25. Two focal spot
  26. 26. FOCAL SPOT SIZE  The focal spot size limits the tube’s capacity to produce xrays. The electrons and resulting heat are placed on a smaller portion of the x-ray tube.  The mA is therefore limited for the small focal spot. This
  27. 27. FOCAL SPOT SIZE  If the mA is properly calibrated, the focal spot will have no impact on the quantity or quality of the beam.
  28. 28. (6) F.F.D. :  The intensity of x-ray beam reduces with increased FFD.  It follows the Inverse Square Law ( I.S.L.) . I α 1/d 2 .
  29. 29. DISTANCE  Distance af fects the intensity of the x-ray beam at the film but has no ef fect on radiation quality.  Distance af fects the exposure of the image receptor according to the inverse square law.
  30. 30. INVERSE SQUARE LAW  mAs (second exposure) SID2 2nd exposure  ---------------------------- = ----------------------- mAs (first exposure) exposure SID2 1st
  31. 31. DISTANCE  The most common source to image distances are 40” (100 cm) and 72”(182 cm)  Since SID does not impact the quality of the beam, adjustments to the technical factors are made with the mAs.  To go from 40” to 72” increase the mAs 3.5 time.
  32. 32. DISTANCE  Increasing the distance will impact the geometric properties of the beam.  Increased SID reduces magnification distortion and focal spot blur.  With the need to increase the mAs 3.5 times for the 72” SID, tube loading becomes a concern.
  33. 33. DISTANCE  72” SID is used for Chest radiography and the lateral cervical spine to reduce magnification.  72” SID used for the full spine to get a 36” beam.
  34. 34. (7) FILTERATION:  Thin sheet of Al (aluminum) 1mm or 2mm thick added to the pathway of radiation to filter the low energy radiation.  Increasing filtration will increase the quality and reduce the quantity of the beam.  It removes low energy radiation:  Reduce skin dose;  Harden the beam;
  35. 35. FILTRATION  All x-ray beams are af fected by the filtration of the tube. The tube housing provides about 0.5 mm of filtration.  Additional filtration is added in the collimator to meet the 2.5 mm of aluminum minimum filtration required by law.  2.5 mm is required for 70 kVp.
  36. 36. FILTRATION  3.0 mm is required for at 100 kVp.  3.2 mm is required for operations at 120 kVp.  Most machines now are capable of over 100 kVp operation.  We have no control on these filters.
  37. 37. FILTRATION  3.0 mm is required for at 100 kVp.  3.2 mm is required for operations at 120 kVp.  Most machines now are capable of over 100 kVp operation.  We have no control on these filters.
  39. 39. THE END