Definition: AEC stands for Automatic Exposure Control.
Purpose: Helps technologists produce consistent radiographic images across patients despite varying sizes or pathologies.
Location and Functionality
Location:
In the wall Bucky and table Bucky.
Not in the image receptor itself.
Usage: Requires a table or wall Bucky; not usable in portable x-rays.
AEC Operation
Technologist Responsibilities:
Set kilovoltage peak (kVp), milliampere (mA), and select chambers or photocells.
Ionization Chamber: Determines exposure time and terminates exposure when anatomy is fully penetrated.
Minimum Response Time: Shortest possible exposure time is one millisecond.
Advantages of AEC
Decreased exposure repeat rates.
Reduced patient exposure.
Improved departmental efficiency.
Chamber and Photocell Setup
Configurations:
Typically three-cell format (two outer and one center).
Some new machines offer five-cell configurations.
Ionization Chamber:
Located behind the patient but in front of the image receptor.
Interacts with exit radiation (radiation passing through and exiting the patient).
Importance of Proper Usage
Correct chamber selection and positioning are crucial to avoid errors.
Incorrect chamber usage can lead to overexposure or underexposure.
Density Settings
Purpose: Adjust the exposure without changing mAs directly.
Controls: Generally a range from +2 to -2.
Backup Timer
Function: Prevents overexposure, protecting patient and equipment.
Settings: Set to at least 150% - 200% of expected exposure time.
Factor Changes and Their Effects
mA and Exposure Time:
Increasing mA decreases exposure time.
mAs is the product of mA and seconds.
kVp and Exposure Time:
Increasing kVp decreases exposure time.
SID and Exposure Time:
Increasing SID increases exposure time.
Disadvantages of AEC
Errors: Improper use can lead to errors.
Influenced by prosthetics, pathologies, and incorrect collimation.
Exposure Indicator (EI)
Specific to each equipment type (CR vs. DR).
CR: Inverse relationship with exposure.
DR: Direct relationship with exposure.
Terminology
Underexposure: Not enough technique, leading to grainy or noisy images.
Overexposure: Saturation with too many x-ray photons, potentially losing soft tissue detail.
Conclusion
AEC is beneficial but requires proper usage for effectiveness. Awareness of chamber settings, anatomy positioning, and exposure indicators is essential for successful radiographic imaging.