Imaging Concepts Ch 7: Automatic Exposure Controls

T/F: The density controls should not be used to compensate for patient part thickness or kVp changes.

True

What impact will having the incorrect chamber selected, or the patient with a less anatomical dense tissue over the activated chamber?

Undercutting/ underexposed

Typical AEC density control labels are

-3, -2, -1, 0, 1, 2, 3 1/4, 1/2, N, 1 1/4, 1 1/2

Modern AECs have a minimum response time in the region of ____

0.001 seconds

What is "undercutting" and what impact does it have on IR exposure?

Open collimation too much Underexposed image

What is the effect of collimating off part of an activated AEC chamber?

Overexposing the image

Explain how to modify technical factors on an anatomically programmed control unit.

Override technique

What are the two types of AEC systems and which is modern? What is are the advantages/ disadvantages?

Phototiming devices Ionization chambers- modern Disadvantage- no control over mAs, must be correctly positioned Advantage- minimizing radiation

Who introduced AECs?

Russell H Morgan

What is the result of an activated chamber being struck by the primary beam?

Terminate early

What is the difference between APR and AEC?

Will expose at specific exposure factors that they think are average for that size patient where AECs will pick a kV but then expose until they reach a certain amount of radiation

What is the most common relationship of the three chambers on an AEC?

center chamber at the center of the image receptor with right and left chambers slightly higher

What happens when wide collimation is used?

the full primary beam will produce scatter radiation that may undercut the patient.

the exposure is terminated by what?

the operational amplifier

Backup times should be set at what percent of the expected manual technique time?

150%

What year were AECs introduced?

1942

how many chambers are on most AECs?

3

How many different combinations of activated chambers are there?

7

Describe how to modify image receptor exposure when using an automatic exposure control (AEC).

Can adjust the amount of radiation necessary to send the exposure termination signal. -3 to 3

What are the "density" controls to be used for? Why should they not be used to compensate for body habitus?

Configuring of the ionization chamber cells cannot be adapted to the necessary positioning. You positioned correctly but the technique is off. AEC compensates for patient habitus automatically

What is the function of AEC?

Control exposure time

Discuss the advisability of the creative use of AECs.

Creative to make sure that the right amount of tissue is over the chamber because AEC will not give you a precise answer on how to fix and repeat an exposure

Provide solutions to minimum response time and back-up time problems when working with bariatric patients.

Minimum response time: the length of time necessary for the AEC to respond to the ionization and send a signal to terminate the exposure. Backup time: establishes the maximum exposure time for the system in order to prevent overexposure mA should be decreases backup time should be increases

What is cell selection based upon?

The radiographer, based on their knowledge of anatomy and positioning

With respect to AEC chambers, what does the term averaging mean?

The signals from the cells are sent to a special operational amplifier, which sums the voltages received from each cell and divides by the number of cells that have been activated

What is the single function of an AEC?

To eliminate the need for the radiographer to set an exposure time

Explain effect of collimation on AEC response.

Too tight of collimation can overexpose the image If you open the collimation too much it can cause a lot of scatter and terminate prematurely

Describe various common subject density and subject contrast problems when using AECs.

Unexpected subject density is present or when an expected subject density is lacking Fluid in the lungs Emphysema Osteoporosis

Describe configurations size, shape, and position for various brands of ionization chambers.

Usually 3 chambers, one in the center and one on each side (right and left) slightly above Seven different combinations of the 3 chambers

AEC will provide diagnostic exposures for what structures?

Whatever is exactly over the chamber

Explain why the art of automatic exposure control is the art of positioning.

You have to be able to position exactly the right amount of tissue over the chambers.

What should be done if a decrease in exposure is wanted but the patient and ionization chambers are properly positioned?

adjust density controls

when taking a chest radiograph where do the right and left chambers line up on the patient?

away from the mediastinum and completely within the lobes of the lungs

when taking an abdominal radiograph where does the center chamber line up on the patient?

below the duodenum and transverse colon

What is the backup time set at and what is its function?

establishes the maximum exposure time for the system in order to prevent overexposure

What is THE MOST CRITICAL element in using AECs?

exact position of the ionization chambers because positions differ for various brands and models of equipment

What do ionization chambers serve to measure?

exposure to the receptor

The center chamber gets rid of what artifacts in the bowel?

gastric and bowel gas

What happens if you tightly collimate near the ionization chambers?

if the primary beam is collimated from an activated chamber, the chamber will operate as if the tissue is extremely dense. This will create an overexposed image.

When using AECs poor positioning skills can result in what?

increased repeat rate

phototiming actually refers to the use of what?

ionization chambers

What does the technologist select on a manual technique? What about an AEC technique?

kVp and the mA must select which chambers will be active

When using AEC, what impact does changing kVp, mA, and/ or s, have on the IR?

kVp- contrast mAs- exposure

What does the technologist "lose control" of using an AEC technique?

mAs

the art of using AECs is the art of _____

positioning

What will undercutting scatter cause?

the AEC will terminate the exposure while some areas of the image are still underexposed

When more than one cell is activated which will have the most influence on overall exposure?

the cell receiving the most radiation will contribute the greatest electrical signal and therefore will have the most influence on overall exposure

Russell Morgan designed the first AEC for what body part?

the chest

What is the minimum response time?

the length of time necessary for the AEC to respond to the ionization and send a signal to terminate the exposure.

When using an AEC what 2 things are critical?

the location of the ionization chamber be determined precise positioning of tissue over that location be achieved

What does the term averaging mean?

the signals from the cells are sent to a special operational amplifier, which sums the voltages received from each cell and divides by the number of cells that have been activated

AECs provide diagnostic-quality exposures for which structures?

those only positioned directly above the ionization chambers

when do AEC problems with subject density and contrast occur?

when an unexpected subject density is present or when an expected subject density is lacking Examples: fluid in the lungs, emphysema