Automatic Exposure Control (AEC)

1. Automatic Exposure Control (AEC) S. Guilbaud Education Director School of Radiography

2. AEC’s • Often called photo timers, they were developed for the purpose of achieving more consistent densities while reducing the patient dose & repeat radiographs. S. Guilbaud, Education Director

3. All AEC devices work on similar physical principles. • Based on the radiation detector device’s ability to convert radiant energy to an electrical signal/current. S. Guilbaud, Education Director

4. The two most commonly used types of detectors are the Ion chamber & the Photo multiplier tube. S. Guilbaud, Education Director

5. Ion Chamber • Induces an electrical current when the gas atoms contained inside are ionized by the radiation, freeing electrons from the gas. • The electrons are then attracted to & strike the positively charged anode. • The electrons flow out of the anode plate & down a wire becoming an electrical signal. S. Guilbaud, Education Director

6. Photo multiplier Tube • The process is similar to that of an Ion chamber. • An additional step of using a fluorescent screen to convert X-rays into light. • When the light strikes the photo cathode, the photo cathode induces an electrical current from the light energy. • The induced electrical current is used to charge an electromagnet which pulls the main exposure switch open & terminates the exposure. S. Guilbaud, Education Director

7. Both devices operate on similar mechanisms • Thyristor/Thyratron • A device that is designed to release the capacitor’s charge when a pre-set amount is reached. • This is pre-set by the service engineer to always produce a desired density level on the radiographic image. • This is done with special attention to the film-speed screen. • Thus, when film-speed screen changes, the thyratron must be re-adjusted. S. Guilbaud, Education Director

8. Basic AEC circuit diagram S. Guilbaud, Education Director

9. Time • The AEC only controls the exposure time. • Thus, controlling the mAs (density). • An optimum kVp must be set or pre-programmed into the radiographic unit. • An optimum mA station must be selected or pre-programmed into the unit. • Note: When the kVp selected is less than adequate to penetrate the part, no amount of mAs will compensate for the lack of penetration. • The radiograph will still be too light in spite of the automatic compensation. • Even if the density control is adjusted to darken the overall image, it will only result in excessive mAs & overexposure to the patient. S. Guilbaud, Education Director

10. Minimum Response Time (MRT) • The minimum amount of time it takes the unit to detect and react to the radiation exposure received. • The MRT ranges from 0.002 seconds to 0.02 seconds for older radiographic units. S. Guilbaud, Education Director

11. Minimum Response Time (MRT) • With high-speed screens such as 800 speed rare-earth screens, the exposure times are too short for the AEC circuit to respond. • A delay in response will normally result in an image that is too dark. • Consequently, a smaller mA station should be used until sufficient exposure times are produced. S. Guilbaud, Education Director

12. Example # 1: • Assume that the MRT of a unit is 0.005 seconds. To keep exposure times short, the technologist uses the 300 mA station: The minimum total mAs the unit can produce is 1.5 mAs. S. Guilbaud, Education Director

13. Example # 2: • Suppose as technique called for 1 mAs, if the technologist uses the AEC to produce 1 mAs, the image will be over-exposed. This would occur b/c the AEC cannot shut off in less than 0.005 seconds. • So even if the density control is adjusted to minus 1, the setting would not help. The best thing to do is use a lower mA station. S. Guilbaud, Education Director

14. Minimum Response Time (MRT) • This is always a concern whenever the following factors compound: • High-speed receptor systems are used. • High-power generators are used. • High mA stations are used. • Small anatomical parts are imaged w/ AEC. S. Guilbaud, Education Director

15. Optimum mA • High enough mA at a given focal spot size (FSS) to minimize motion, but not so high that the needed exposure times are shorter than the AEC can handle. S. Guilbaud, Education Director

16. Back-Up Time • This is used to prevent overexposure to the patient en case one of the electrical components of the AEC unit fails. • Many manufacturers now include circuitry that automatically sets an appropriate back-up time or back-up mAs for all AEC exposures. • On older units, it is common to set the back-up time at 1 to 2 seconds on all procedures. S. Guilbaud, Education Director

17. Density Control • The density control is designed to increase or decrease the preset sensitivity of the thyratron by specific percentages so that the exposure time will be automatically extended or shortened by specific amounts. • Settings are for small, average and large patients. • The small setting cuts the average exposure by ½. • The large setting doubles the exposure. • The most common format is to have seven settings from 1 – 7 or from –3 to +3. S. Guilbaud, Education Director

18. Density Control • Remember that the same concept of technique manipulation applies to AEC. • A change of 50% when decreasing the density. • A change of X 2 or doubling, when increasing the density. S. Guilbaud, Education Director

19. Density Control • It is appropriate to adjust the density controls up or down when the position and configuration of the detector cells cannot be adapted to placement of the part of interest or utilize a different detector cell configuration. • E.g.. Frog Lateral of hip. S. Guilbaud, Education Director

20. Remember: • On newer radiographic and fluoroscopic units, the AEC unit is automatically engaged when the X-ray machine is set for fluoroscopy. S. Guilbaud, Education Director

21. Limitations of the AEC • AEC’s were not meant to be used on all procedures. • Major technique constraints when using the AEC: • AEC is not to be used on parts too small to completely cover the detector cell. • Portions of the cell not covered by the part will receive too much radiation & terminate the exposure prematurely. S. Guilbaud, Education Director

22. Limitations of the AEC • AEC’s should not be used on anatomy that is peripheral to the X-ray beam. • Clavicle • Mandible • Lateral Scapula • Sternum • Sinuses • Note: The CR is centered too close to an edge surface, portions of the detector may extend beyond the part & into the raw beam. S. Guilbaud, Education Director

23. Limitations of the AEC • Improper centering of part & film. • CXR – Many factors affect a CXR image: • Differing tissue thickness in the thorax: • Heart, lungs, diaphragm, breast. • Should a portion of the detector overlap any of those other tissues besides the lungs, the exposure time will lengthen resulting in an image that has excessive density. S. Guilbaud, Education Director

24. Improper chest/lung centering S. Guilbaud, Education Director

25. Limitations of the AEC • Parts that cannot be properly positioned. • When a part cannot be positioned correctly, a manual technique should be used as not to cause additional exposure to the patient with repeat examinations. S. Guilbaud, Education Director

26. Limitations of the AEC • Improper beam restriction. • Not enough beam restriction results in an excessive amount of scatter production & too much scatter reaching the film. • This normally results in early termination of the exposure resulting in an image that lacks in appropriate density. S. Guilbaud, Education Director

27. Limitations of the AEC • Using the AEC in the presence of radiopaque foreign objects in the patient. • Orthopedic devices. • Hip replacements. • ORIF devices. • Dental hardware. • Other metallic artifacts that cannot be removed. S. Guilbaud, Education Director

28. Repeating radiographs • Repeating radiographs prior to diagnosing the cause of the improper density. • Whenever there is a technique analysis problem, the radiographer should always utilize a manual technique for the repeated exposure. • Manual techniques assume more predictability than the AEC. S. Guilbaud, Education Director

29. Detector Configuration S. Guilbaud, Education Director

30. Detector Configuration • The specific locations of the detector cells are normally marked on the upright stand face or appear as a shadow in the collimation field. This is normally achieved with the use of an insert at the collimator head. S. Guilbaud, Education Director

31. The actual detectors are located behind the upright stand face or just beneath the table top. • Older machines with photo multiplier tubes were placed behind the Bucky tray so as not to cause artifacts on the film. • With the photo multiplier tubes, special cassettes were used to allow more of the remnant beam to pass through to reach the detectors. • Modern detectors (ionization chambers) are made of thin aluminum. This permits the detectors to be placed directly behind the tabletop in front of the Bucky tray and cassette. S. Guilbaud, Education Director

32. Conclusion • AEC does not obviate the need for the radiographer to set technique factors. • Optimum kVp must still be set manually to ensure adequate penetration and gray scale. • An optimum mA station must be selected in consideration of the MRT for the equipment. • An intelligent choice from the various options of detector cell configurations and density control settings must be made. • Whenever an AEC exposure turns out too light or too dark, the radiographer should have a mental checklist of possible causes. • A second AEC exposure should not be attempted unless the radiographer has identified the cause of the problem and corrected it. • If there is some uncertainty, a manual technique should be utilized. S. Guilbaud, Education Director

33. References Bushberg et al, The Essentials of Physics and Medical Imaging, Williams & Wilkins Publisher. Bushong, S., Radiologic Science for Technologists, Physics, Biology and Protection, 8th Edition, C.V. Mosby Company. Carlton et al, Principles of Radiographic Imaging, An Art and Science, Delmar Publishing. Quinn, B. C., Fuchs’s Radiographic Exposure, Processing & Quality Control, 6th Edition, Charles Thomas Publisher. Selman, J., The Fundamentals of X-Ray and Radium Physics, 8th Edition, Charles C. Thomas Publisher. S. Guilbaud, Education Director