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TIMERS

TIMERS. BASIC X-RAY SCHEMATIC. TIMER CIRCUIT:. IS SEPARATE FROM THE OTHER MAIN CIRCUITS OF THE IMAGING SYSTEM . . TIMER. TIMER. A MECHANICAL OR ELECTRONIC DEVICE WHOSE ACTION IS TO MAKE OR BRAKE THE HIGH VOLTAGE ACROSS THE X-RAY TUBE. TYPES OF TIMING CIRCUITS TIMERS:. MECHANICAL

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TIMERS

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  1. TIMERS

  2. BASIC X-RAY SCHEMATIC

  3. TIMER CIRCUIT: • IS SEPARATE FROM THE OTHER MAIN CIRCUITS OF THE IMAGING SYSTEM. TIMER

  4. TIMER • A MECHANICAL OR ELECTRONIC DEVICE WHOSE ACTION IS TO MAKE OR BRAKE THE HIGH VOLTAGE ACROSS THE X-RAY TUBE

  5. TYPES OF TIMING CIRCUITSTIMERS: • MECHANICAL • SYNCHRONOUS • ELECTRONIC • mAs • BACKUP • AUTOMATIC EXPOSURE CONTROL

  6. MACHANICAL TIMER • OPERATE BY CLOCKWORK. • SPRING MECHANISMS IS WOUND UP. • DURING EXPOSURE THE SPRING UNWINDS AND TERMINATES EXPOSURE.

  7. SYNCHRONOUS TIMER • IN SYNCH WITH FREQUENCY OF THE A/C CURRENT (60 Hz in the US). • MINIMUM EXPOSURE TIME IS 1/60 s. • TIMING INTERVALS INCREASE BY 1/60 s. • MUST BE RESET AFTER EACH EXPOSURE

  8. ELECTRONIC TIMER • THE MOST SOPHISTICATED. • THE MOST ACCURATE • IT’S CIRCUITRY CONSISTS OF THYRATRON TUBE OR THYRISTOR. • ALLOW WIDE RANGE OF TIME INTERVALS. • ALLOW SERIAL EXPOSURES. • NOWADAYS CONTROLLED BY MICROPROCESSOR

  9. THYRATRON CIRCUIT

  10. mAs TIMER • MONITORS THE PRODUCT OF mA & EXPOSURE TIME AND TERMINATES EXPOSURE WHEN THE DESIRED mAs VALUE IS ATTAINED.

  11. BACKUP • SET TO TERMINATE EXPOSURE IN THE EVENT OF FAILURE IN THE AEC CIRCUIT. • MAX. mAs ALLOWABLE – 600 mAs UNLESS TUBE POTENTIAL IS LESS THAN 50 kVp – 2,000 mAs IS THE LIMIT.

  12. AUTOMATIC EXPOSURE CONTROL Automatic exposure control devices can assist the radiographer in producing consistent radiographic images from patient to patient, regardless of size or presence of pathology. The advantages of this consistency are numerous and include: decreased repeat rate; decreased patient exposure; and increased department efficiency. The most important benefit being decreased repeat rate

  13. When an AEC device is used to terminate an exposure, the technologist sets the kVp and mA, but the time of the exposure is automatically determined by the machine. The AEC device differs from a manual timer because the AEC does not stop the exposure until the film has reached an appropriate density. Unlike manual timers, which simply stop the exposure after the preset time has elapsed.

  14. A major benefit of the AEC device is its ability to consistently obtain accurately exposed radiographs, even in the presence of pathology. While manual timers terminate the exposure at the preset time, regardless of pathology or achievement of proper film density. The following example demonstrates the difference

  15. AUTOMATIC EXPOSURE CONTROL (AEC) • FLAT IONIZATION CHAMBER • PHOTOTIME

  16. IONIZATION CHAMBER AEC X-RAY TUBE X-RAY TABLE BUCKY

  17. IONIZATION CHAMBERS ARE SITUATED • BETWEEN THE GRID MECHANISM & THE CASSETTE

  18. CHAMBERS

  19. CHAMBER SELECTION ON THE CONSOLE DETECTOR SELECTION Along with proper

  20. AEC

  21. IONIZATION CHAMBER AEC X-RAY TUBE X-RAY TABLE CASSETTE SCREEN

  22. PM TUBE LIGHT ELECTRONS

  23. PHOTOTIME MECHANISM IS SITUATED • BEHIND THE CASSETTE • CASSETTE NEEDS SPECIAL DESIGN IT HAS OPENING IN THE BACK

  24. PHOTOTIME CASSETTE

  25. PHOTOTIME SCREEN X

  26. Even though the ionization chamber and the phototimer operate differently, they both have the same function: convert radiation into an electrical signal which will be used to automatically stop the exposure when the film has reached the proper density

  27. FACTORS INFLUENCING AEC • PATIENT POSITIONING • DETECTOR SELECTION

  28. POSITIONING!!!! • AEC USES DETECTORS TO RECEIVE THE RADIATION THAT PASSED THROUGH THE PATIENT WE MUST POSITION THE ANAOMY OF INTEREST DIRECTLY ABOVE THE DETECTORS.

  29. Without a technologist who is very knowledgeable about anatomy and positioning, automatic timers are worthless. In fact, they may actually decrease department efficiency because of the increased amount of repeat radiographs that will result if used improperly.

  30. DETECTOR SELECTION

  31. LAT. L-SPINE

  32. C-SPINE

  33. C-SPINE -AP

  34. SHOULDER

  35. KNEE

  36. HIP

  37. PELVIS

  38. ABDOMEN

  39. CHEST PA

  40. LAT. CHEST

  41. +4 +3 +2 +1 0 -1 -2 -3 -4 +100 +75 +50 +25 0 -25 -50 -75 -100 DENSITY SETTING D. SETTING % DENSITY CHANGE

  42. MINIMUM RESPONSE TIME • SHORTEST EXPOSURE TIME POSSIBLE

  43. EFFECTS OF mA, kVp, SID ON AEC • mA AEC TIME • kVp AEC TIME • SID AEC TIME

  44. PROBLEMS WITH AEC • PATIENT PATHOLOGY • PROSTHETIC DEVICE • COLLIMATION • STROBOSCOPIC EFFECT • WRONG IMAGE RECEPTOR

  45. CERTAIN PATHOLOGIES ALTER AEC READING

  46. PROSTHESIS GIVES FALSE AEC READING

  47. NOT ENOUGH COLLIMATION ALLOWS TOO MUCH SCATTER REACHING THE DETECTOR AND PREMATURE TERMINATION OF THE EXPOSURE TAKES PLACE

  48. STROBOSCOPIC EFFECT • TOO SHORT OF THE EXPOSURE TIME IS SELECTED BY THE AE, SHORTER THAN THE SPEED OF THE OSCILLATION OF THE GRID AND IT FREEZES THE GRID MOVEMENT.

  49. WRONG SPEED OF IMAGE RECEPTOR!!!

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