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 CIRCUITS TIMERS:
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.

30. 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.

31. DETECTOR SELECTION

32. LAT. L-SPINE

33. C-SPINE

34. C-SPINE -AP

35. SHOULDER

36. KNEE

37. HIP

38. PELVIS

39. ABDOMEN

40. CHEST PA

41. LAT. CHEST

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

43. MINIMUM RESPONSE TIME
SHORTEST EXPOSURE TIME POSSIBLE

44. EFFECTS OF mA, kVp, SID ON AEC
mA AEC TIME
kVp AEC TIME
SID AEC TIME

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

46. CERTAIN PATHOLOGIES ALTER AEC READING

47. PROSTHESIS GIVES FALSE AEC READING

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

49. 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.

50. WRONG SPEED OF IMAGE RECEPTOR!!!