Radiography Chapter 7: Automatic Exposure Control

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?

When the configuration of the ionization chamber cells cannot be adapted to the necessary positioning. Signup and view all the answers

How can changes in subject density result in AEC problems?

AEC problems occur whenever an unexpected density is present or when an expected density is lacking. Signup and view all the answers

How can collimation affect AEC image quality?

Tightly collimate near ionization chamber locations should be avoided. Signup and view all the answers

Define minimum response time.

The length of time necessary for the AEC to respond to the ionization and send a signal to terminate the exposure. Signup and view all the answers

What do ionization chambers measure the exposure to?

The image receptor. Signup and view all the answers

What is the only function of the automatic exposure control (AEC)?

Eliminates the need to set exposure time. Signup and view all the answers

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?

Over the chamber. Signup and view all the answers

What does the radiographer need in order for the automatic exposure control (AEC) to produce quality results?

Good positioning skills. Signup and view all the answers

What is the minimum response time for modern automatic exposure controls (AECs)?

0.001 second. Signup and view all the answers

At what percentage of the anticipated manual exposure should the backup timer be set?

150 percent. Signup and view all the answers

At what milliampere-seconds (mAs) does U.S. public law state that generators automatically terminate above 50 kVp?

600 mAs. Signup and view all the answers

What will result in an increased repeat rate in automatic exposure controls (AECs)?

Poor positioning. Signup and view all the answers

When the correct voltage for a diagnostic quality exposure is reached, how is it terminated?

By an operational amplifier. Signup and view all the answers

What automatic exposure device is rarely used in present-day radiography?

Phototimer. Signup and view all the answers

When there is a problem with density and contrast in an automatic exposure system, what is usually the cause?

Unexpected density. Signup and view all the answers

What is an example of a condition that would cause the automatic exposure control (AEC) to stay on longer?

Fluid in the lungs. Signup and view all the answers

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.