Automatic Exposure Control (AEC)

Automatic Exposure Control (AEC)

• AEC is a tool used for determining radiation exposure to produce quality images
• In operation, the x-ray exposure terminates once a radiation amount transmits through a patient
• AEC helps determine exposure time and radiation exposure delivered to the image receptor (IR)
• AEC radiation detectors convert radiation to electrical signals
• Exposure time terminates when the electrical signal indicates a predetermined radiation amount is reached
• Most AEC systems use three radiation detectors set in a specific arrangement
• The radiographer selects the detector configuration to measure radiation exposure reaching the IR
• Detecting devices: sensors, chambers, cells, or detectors

Types of AEC Devices

• Phototimers and ionization chambers are the two main types of AEC devices
• Phototimers were in early AEC systems, hence the term "phototiming"
• Modern systems no longer use phototimers making "phototiming" an outdated term
• Phototiming uses an AEC device with photomultiplier tubes or photodiodes
• Photomultiplier tubes (PMT) detect and amplify low light levels
• Solid-state photodiodes are photodetectors converting light to electrical current through the photovoltaic effect
• Solid-state photodiodes operate using solid semiconductors

Phototimers

• Phototimers use a fluorescent light-producing screen with a device that transforms light into electricity
• A photodiode is a solid-state device converting light into an electrical signal
• Phototimer AECs were exit-type devices, placed behind the IR to measure the radiation after exiting the IR
• Light paddles coated with fluorescent material served as detectors
• Once radiation interacted, light was converted to electricity by photomultiplier tubes/photodiodes
• When enough electric charge was reached, the exposure timer stopped the exposure

Ionization Chambers

• Ionization chambers or solid-state systems are used more than phototimers today
• Remnant radiation from the patient creates ionizations or electron pairs in a sensing area
• The ionizations are proportional to the x-ray exposure
• Ion pairs create an electrical current, generating an amplified signal
• The electrical signal tells the x-ray generator to end the exposure
• An ionization chamber is a hollow cell containing air connected to the timer circuit by electrical wire
• Ionization chamber AEC devices are entrance type devices, positioned in front of the IR
• Radiation interacts with the sensing detectors before reaching the IR
• When radiation hits the ionization chamber, the air ionizes and creates an electrical charge
• The charge flows to the timer circuit, and when sufficient exposure is achieved, the timer is tripped
• The electrical charge is proportional to the radiation the ionization chamber is exposed to

Milliampere-Seconds (mAs) Readout

• AEC systems determine the total radiation (mAs) needed for the proper IR exposure automatically
• Many units display the mAs immediately after exposure
• Radiographers should note mAs readouts to help switch to manual techniques, when needed

Kilovoltage Peak (kVp) and mAs Readout

• AEC solely manages the quantity of radiation reaching the IR, not influencing image characteristics like contrast
• kVp selection should be independent of AEC use and based on required penetration and subject contrast
• The selected kVp affects exposure time, as higher kVp needs less mAs for an earlier termination, reducing patient dose
• Lower kVp values need larger mAs values, extending exposure time and increasing patient dose
• In systems allowing independent mA and time settings, select mA irrespective of AEC usage
• Chosen mA has an inverse relationship with exposure time: low mA means longer exposure and vice versa for AEC

Minimum Response Time

• Refers to the shortest possible exposure duration that a system can produce
• Generally, minimum response time is longer in systems with AEC than other radiographic timers
• Modern AEC systems feature minimum response times of 1ms
• If the required time to produce quality exposure falls under the AEC's minimum response time, overexposure can occur
• Example: Pediatric X-rays use short times to decrease movement but if settings are less than 1ms, increased radiation happens

Backup Time

• It defines the maximum length an X-ray exposure runs when utilizing an AEC system
• Technologists are responsible to set the backup time
• Backup time serves as a safety feature during AEC failure or incorrect equipment use
• It safeguards the patient from excessive exposure and shields the tube from exceeding heat load capacity
• If automatically set, the limit stops at 600 mAs when the equipment runs over 50 kVp
• If the technologist chooses the backup time, setting it at 150% to 200% of the anticipated exposure duration is advised

Exposure Adjustment

• Permits technologists to modify preset radiation detection thresholds using density settings
• Adjustment options on the control panel are labeled from -2 to +3
• Each selection alters exposure time by a pre-set increment
• A 25% increment means the timer may be either raised or lowered from standard in this measure
• For example, +2 raises the timer by 50%

Detector Selection

• AEC Systems allow various detector combinations or offer five detectors for flexibility
• Selected detectors actively assess radiation when exposing the image and the electric signals are averaged
• Detectors measuring the most exposure influence the total exposure greatly
• Radiographers should position the detectors in line or overlapping with anatomical regions, based on interest
• Incorrect detector selection leads to either over or underexposure of images
• Technologist need to check image quality, mAs and exposure while using the AEC System
• If the region of interest is not precisely aligned with operational sensors
• It can cause exposure issues, resulting in a computer adjusting image flaws and risks image/exposure issues
• Underexposure can cause quantum noise
• Overexposure results in higher radiation hurting the image displayed

Detector Size

• Detector sizes and shapes are fixed in AEC systems and cannot be changed
• If the target body part is smaller than chosen detectors, the exposure will end quickly
• The primary beam exposes some detectors prematurely, causing underexposure
• For smaller parts, the manual technique should be used

Compensating Issues: Patient Considerations

• Increased patient thickness requires longer exposure with AEC
• Conditions like gas in the bowels lead to quicker termination and underexposure
• Additive pathology, contrast media, metal or prosthetic devices may lengthen exposure

Compensating Issues: Collimation

• Inadequate collimation can affect radiation exposure levels
• Insufficient collimation causes increase scatter at the detectors, causing quick termination
• Excessive collimation increases exposure time, as detectors need sufficient exposure initially

Compensating Issues: Image Receptor Variations

• Exchanging IR types can cause problems when devices are preset
• Since calibration sets the AEC device to specific IR/radiation amounts
• AEC devices cannot sense IR types and will produce inaccurate exposure settings

Anatomically Programmed Technique

• This system enables the technologist to select an anatomical area on the console panel
• After selecting the anatomical part (e.g., chest) and projection (e.g., PA or Lat), the system displays pre-programmed exposure settings

Exposure Technique Charts

• Exposure technique charts are pre-established guidelines that assist technologists in selecting the appropriate exposure factors for x-ray examinations
• It includes kVp, mAs, IR type, grid, and SID
• Technique charts promote consistent image quality, decrease the amount of retakes needed, and lessens patient exposure
• The charts are designed for average patients and overlook unusual conditions
• Part measurement accuracy is vital
• Calipers measure part thickness at the location of the central ray midpoint or the thickest portion of the area being radiographed
• Wide exposure latitude equals image quality

Types of Technique Charts

• Primary types are variable kVp/fixed mAs and fixed kVp/variable mAs

Variable kVp/Fixed mAs Technique Chart

• Easy to formulate, decreasing subject contrast and may not ensure penetration
• With this technique chart part measurement must be accurate to ensure that the 2 kVp adjustment is accurately applied
• kVp increases as part thickness increases
• For every 1cm (0.4 inch) increase in part thickness, kVp increases by 2 and mAs stays constant
• Baseline kVp then adjusts for part thickness
• For example if settings are 70 kVp and 10 mAs when imaging a knee but part size goes from 10 cm and up, settings change to 72 kVp/10mAs
• Less accurate for extreme ranges in part size

Fixed kVp/Variable mAs Technique Chart

• The Fixed chart selects the ideal kilovoltage and sets adjustment of mass
• MAs should be chosen depending on thickness
• For every few centimeters, the mast increase or decrease
• Example, if the Kilovoltage is at 70 and mass is at 10 for a patient
• One might have to select twenty mass and keep the Kilovoltage to use adequate settings

Exposure Technique Chart Development

• Calipers, phantoms, and calculations are used for tool chart development
• After test images, extrapolate exposure techniques
• Comparative anatomy says parts of the same side can be read with similar exposure if kVp is well set

Special Considerations: Pediatric Patients

• Pediatric patients require lower kVp and mAs values
• Shortened exposure times are required because of children's inability to suspend breathing
• AEC use may be limited by minimum response time
• Smaller size may not cover the detector, which can lead to inaccurate technique

Special Considerations: Geriatric Patients

• Aging patients may have physical changes, such as sensory and thin broken skin
• Exposure techniques may be decreased

Special Considerations: Bariatric Patients

• An obese patient is someone with a BMI higher than 30
• Imaging a bariatric patient means considering table weight limits and equipment size
• Higher kVp and mAs is needed
• Grids are useful to reduce scatter

Special Considerations: Projections and Positions

• Exposure factors are altered per specific projection
• Lateral knees warrant more technique

Special Considerations: Casts and Splints

• Casts are made from attenuating materials
• Fiberglass casts do not need exposure adjustments
• Plaster casts require exposure increase
• Splints require no adjustments

Special Considerations: Pathologic Conditions

• Additive diseases increases an anatomical part's absorption characteristics, such as ascites, tumors etc
• Destructive decreases absorption, such as atrophy, emphysema ect
• Kilovoltage should increase if adding, and decrease if decreasing
• Change should be at least 15%

Special Considerations: Soft Tissue

• In digital imaging, brightness and contrast can be used instead
• A decrease in mass can display the larynx

Special Considerations: Contrast Media

• Positive contrast calls for higher exposure and are radiopaque
• Negative is air and are normally less exposed

Mastering exposure variable effects is essential for technologists