Chapter 8

• Automatic Exposure Control (AEC)


  A system used to consistently control the amount of radiation reaching the image receptor by terminating the length of exposure

  
• AEC system operation


  1. Radiation is transmitted through the patient and converted into an electrical signal and travels to the timer circuit where the exposure is terminated

  2. A predetermined level of radiation must be reached before exposure termination

  3. Service personnel calibrate the predetermined level of radiation

  
• Phototimers


  • Light paddles, coated with a fluorescent material, serve as the detectors, and the radiation interacts with the paddles, producing visible light
  • This light is transmitted to remote photomultiplier tubes or photodiodes that convert this light into electricity
  • The timer is tripped, and the radiographic exposure is terminated when a sufficiently large charge has been received

  
• Ionization Chamber System


  • A hollow cell that contains air and is connected to the timer circuit via an electrical wire
  • Ionization chamber is exposed to exit radiation, the air inside the chamber becomes ionized, creating an electrical charge
  • Charge travels along the wire to the timer circuit
  • The timer is tripped, and the radiographic exposure is terminated when a sufficiently large charge has been received
• Radiation Detectors


  • AEC systems use a set of three radiation-measuring detectors, arranged in some specific manner
  • The radiographer selects the configuration of these devices, determining which one(s) of the three actually measures radiation exposure reaching the image receptor

  
• AEC System Features


  • mAs readout is the actual amount of mAs used for that image is displayed immediately after the exposure
  • Minimum response time refers to the shortest exposure time that the system can produce
  • Backup time refers to the shortest exposure time that the system can produce
  • Exposure Adjustment allows the radiographer to adjust the amount of preset radiation detection values
• Kilovoltage Selection


  • Set the kVp as needed to ensure adequate penetration while producing the appropriate level of subject contrast
  • Determines the length of exposure time when using AEC
  • A low kVp requires more exposure time to reach the predetermined amount of exposure
  • A high kVp decreases the exposure time to reach the predetermined amount of exposure and reduces the overall radiation exposure to the patient
• If a digital image receptor is underexposed or overexposed, the computer adjusts for the exposure error, but the image quality and/or patient exposure is compromised

  
• Underexposure


  May result in the visibility of quantum noise
• Overexposure


  Will increase patient exposure and may decrease contrast
• Exposure Adjustment


  Allows the radiographer to adjust the amount of preset radiation detection values

  
• Alignment/Positioning Considerations


  • Select the detector(s) that will be superimposed by the anatomic structures that are of greatest interest
  • The anatomic area of interest must be centered properly over the selected detector(s)
  • The patient's anatomic area of interest must adequately cover the detector combination
  • Improper collimation may affect termination of the exposure time
• Evaluation


  • It is important for radiographers to evaluate image quality, the applied mAs, and the exposure indicator when using the AEC device
  • Errors in selecting the appropriate combination of AEC detectors, detector size, or positioning errors may not be visually apparent due to computer adjustment

  
• AEC Response


  • Increasing patient thickness - Exposure time increased
  • Increasing mA - Exposure time decreased
  • Decreasing kVp - Exposure time increased
  • Decreasing SID - Exposure time decreased
  • Increasing OID - Exposure time increased
  • Anatomic part has excessive gas - Exposure time decreased
  • Different types of image receptor - No effect on exposure time, but image quality may be affected
  • Excessive collimation - Exposure time increased
  • Increased exposure adjustment - Exposure time increased
• Anatomically Programmed Techniques


  • A radiographic system that allows the radiographer to select a particular button on the control panel that represents an anatomic area for which a preprogrammed set of exposure factors are displayed and can be selected
  • Once an anatomic part and projection or position has been selected, the radiographer can adjust the exposure factors that are displayed
  • When anatomically programmed techniques are used in conjunction with AEC on some radiographic units, the system not only selects and displays manual exposure factors but also selects and displays the AEC detectors to be used for a specific radiographic examination
• Exposure Technique Charts


  • Technique charts standardize the selection of exposure factors for the typical patient so that the quality of radiographic images is consistent
  • Designed for the average or typical patient
  • Accurate measurement of part critical
  • Types: Variable kVp-fixed mAs, Fixed kVp-variable mAs

  
• Variable kVp-Fixed mAs


  • Based on the concept that kVp can be increased as the anatomic part size increases
  • Advantages: Easy to construct and simple to use, At low kVp levels, small changes in kVp more effective
  • Disadvantages: Anatomic part penetration not guaranteed, Changing kVp less effective for range of procedures, Less accurate for part size extremes
• Fixed kVp-Variable mAs


  • Optimal kVp value for each part is indicated, and mAs is varied as a function of part thickness
  • Advantages: Part measurement less critical, Increased accuracy with range of procedures, More consistent image contrast and overall quality, Part penetration assured
  • Disadvantage: Optimal kVp values not standardized
• Pediatric patients


  • Small size may require a reduction in exposure
  • Quicker exposure times may be necessary
• Geriatric patients


  May require decreased exposure techniques, lower kVp values, and increased mA to decrease exposure time
• Bariatric patients


  • A high BMI (greater than 30) can be an indicator of Obesity
  • Need increase in kVp and mAs, require a grid, may need to be imaged in quadrants
• Projections and positions

  May require changes in exposure technique due to changes in tissue thickness
• Casts and splints


  • Plaster casts require increase in exposure factors
  • Splints may require adjustment in exposure factors due to type of material if in the path of the x-ray beam
• Pathology

  Additive versus destructive diseases
• Soft tissue


  • May need exposure factor adjustment (decreased mAs) to visualize soft tissues
  • Digital imaging systems allow visualization of soft tissues without changing the exposure technique
• Contrast media


  • Used to increase radiographic contrast when imaging areas of low subject contrast
  • Positive agents versus negative agents
• Automatic Exposure Control (AEC)


  A system used to consistently control the amount of radiation reaching the image receptor by terminating the length of exposure

  
• Phototimers


  1. Light paddles, coated with a fluorescent material, serve as the detectors, and the radiation interacts with the paddles, producing visible light

  2. This light is transmitted to remote photomultiplier tubes or photodiodes that convert this light into electricity

  3. The timer is tripped, and the radiographic exposure is terminated when a sufficiently large charge has been received

  
• Ionization Chamber System


  1. A hollow cell that contains air and is connected to the timer circuit via an electrical wire

  2. Ionization chamber is exposed to exit radiation, the air inside the chamber becomes ionized, creating an electrical charge

  3. Charge travels along the wire to the timer circuit

  4. The timer is tripped, and the radiographic exposure is terminated when a sufficiently large charge has been received
• Radiation Detectors


  • AEC systems use a set of three radiation-measuring detectors, arranged in some specific manner
  • The radiographer selects the configuration of these devices, determining which one(s) of the three actually measures radiation exposure reaching the image receptor

  
• Kilovoltage Selection


  • Set the kVp as needed to ensure adequate penetration while producing the appropriate level of subject contrast
  • A low kVp requires more exposure time to reach the predetermined amount of exposure
  • A high kVp decreases the exposure time to reach the predetermined amount of exposure and reduces the overall radiation exposure to the patient

  
• If a digital image receptor is underexposed or overexposed, the computer adjusts for the exposure error, but the image quality and/or patient exposure is compromised

  
• Alignment/Positioning Considerations


  • Select the detector(s) that will be superimposed by the anatomic structures that are of greatest interest
  • The anatomic area of interest must be centered properly over the selected detector(s)
  • The patient's anatomic area of interest must adequately cover the detector combination
  • Improper collimation may affect termination of the exposure time
• It is important for radiographers to evaluate image quality, the applied mAs, and the exposure indicator when using the AEC device
• Errors in selecting the appropriate combination of AEC detectors, detector size, or positioning errors may not be visually apparent due to computer adjustment
• Anatomically Programmed Techniques


  A radiographic system that allows the radiographer to select a particular button on the control panel that represents an anatomic area for which a preprogrammed set of exposure factors are displayed and can be selected
• When anatomically programmed techniques are used in conjunction with AEC on some radiographic units, the system not only selects and displays manual exposure factors but also selects and displays the AEC detectors to be used for a specific radiographic examination

  
• Exposure Technique Charts


  Technique charts standardize the selection of exposure factors for the typical patient so that the quality of radiographic images is consistent

  
• Variable kVp-Fixed mAs


  Based on the concept that kVp can be increased as the anatomic part size increases
• Fixed kVp-Variable mAs


  Optimal kVp value for each part is indicated, and mAs is varied as a function of part thickness