Questions and Answers
What is the principal function of the automatic exposure control?
Automatic exposure control eliminates the need for the radiographer to set an exposure time.
What are the typical number and configuration of the ionization chambers?
Three-chamber configuration.
What is the purpose of an operational amplifier?
The operational amplifier sums the voltages received from each cell and divides by the number of cells that have been activated.
When should the density controls be used?
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How can changes in subject density result in AEC problems?
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How can collimation affect AEC image quality?
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Define minimum response time.
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What do ionization chambers measure the exposure to?
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What is the only function of the automatic exposure control (AEC)?
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How many chamber configurations does the ionization chamber usually contain?
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Where should tissue be placed for the automatic exposure control (AEC) to produce a diagnostic exposure?
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What does the radiographer need in order for the automatic exposure control (AEC) to produce quality results?
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What is the minimum response time for modern automatic exposure controls (AECs)?
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At what percentage of the anticipated manual exposure should the backup timer be set?
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At what milliampere-seconds (mAs) does U.S. public law state that generators automatically terminate above 50 kVp?
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What will result in an increased repeat rate in automatic exposure controls (AECs)?
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When the correct voltage for a diagnostic quality exposure is reached, how is it terminated?
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What automatic exposure device is rarely used in present-day radiography?
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When there is a problem with density and contrast in an automatic exposure system, what is usually the cause?
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What is an example of a condition that would cause the automatic exposure control (AEC) to stay on longer?
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Study Notes
Automatic Exposure Control (AEC) Overview
- AEC minimizes the need for radiographers to set exposure time, maintaining control over all factors except time and mAs.
- Typically utilizes a three-chamber configuration, with the center chamber positioned centrally and the lateral chambers raised slightly.
Operational Mechanisms
- An operational amplifier computes the sum of voltages from activated cells and determines when a diagnostic-quality exposure is achieved to terminate the exposure.
Density Control Usage
- Density controls are necessary when ionization chamber configurations cannot be adapted, especially in cases of overexposure, requiring adjustment despite properly positioned patients and chambers.
Subject Density Challenges
- AEC may struggle with unexpected subject densities that can either increase or decrease required exposure times, such as fluid in the lungs, resulting in overexposure of aerated areas.
Collimation Effects on AEC
- Close collimation near ionization chambers can misrepresent tissue density, leading to longer exposure times and overexposure. Wider collimation can result in underexposure from scatter radiation affecting exposure termination.
Technical Parameters
- Minimum response time for AEC systems is crucial, defined as the time required to respond to ionization and send termination signals, typically around 0.001 seconds.
Image Quality Factors
- AEC assesses exposure primarily to the image receptor and requires good radiographer positioning skills to yield optimal results.
- Backup timers should be set at 150% of the anticipated manual exposure for safety measures.
Regulatory and Practical Considerations
- U.S. regulations mandate automatic termination of exposure at 600 mAs for levels above 50 kVp to prevent overexposure.
- Poor positioning leads to increased repeat rates in AEC systems, highlighting the importance of correct technique.
AEC Device Usage
- Phototimer devices are now rarely utilized in modern radiography settings, replaced by ionization chambers.
- Issues with density and contrast in the system are often linked to unexpected subject density, necessitating careful assessment of conditions such as fluid accumulation in the lungs.
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Description
This quiz focuses on Chapter 7 of radiography, specifically on Automatic Exposure Control (AEC) principles and functions. It tests knowledge on the key concepts related to AEC functionality and the configuration of ionization chambers used in radiography. Perfect for enhancing your understanding of AEC systems.