Radiography: AEC, Technique Charts, and Exposure Factors

A PA chest is taken using the center AEC chamber instead of the two outer chambers. What is the likely result; why?

Excessive receptor exposure; The center chamber is positioned under the mediastinum/heart/spine region rather than the lung fields. Dense anatomy slows radiation reaching the chamber, so exposure time increases. The lungs may receive excessive exposure.

A lateral C-spine is positioned correctly with the vertebral column over the center chamber. Just before the exposure, the patient moves slightly anteriorly, so soft tissue is now over the chamber. What is the likely result; why?

Insufficient receptor exposure; The chamber is now under soft tissue/air instead of the cervical spine. Radiation reaches the chamber too quickly, so exposure terminates too soon. The C-spine may be underexposed.

An AP pelvis is taken using AEC. A lead shield is unintentionally placed over one active AEC chamber. What is the likely result; why?

Excessive receptor exposure / increased patient dose; The lead shield blocks radiation from reaching the active chamber. The chamber fills slowly, exposure time increases, mAs increases, and patient dose increases.

A knee is imaged using AEC, and all three AEC chambers are selected. What is the likely result; why?

Insufficient receptor exposure; The knee is too small/narrow to cover all three detectors. Uncovered detectors may receive radiation too quickly, causing the exposure to terminate early.

An abdomen image is taken using AEC while positive contrast media is present over the selected chamber. What is the likely result; why?

Excessive receptor exposure; Barium is additive/positive contrast and absorbs radiation. If it is over the chamber, the chamber receives radiation too slowly, increasing exposure time and dose.

A very small patient is being imaged using AEC. The department protocol allows the technologist to select a density setting. What should happen to the density setting and why?

Decrease density; A small bucket/negative density setting terminates the exposure sooner. This decreases exposure time, receptor exposure, and dose.

A very large patient is being imaged using AEC. The radiographer needs to ensure adequate receptor exposure. What should happen to the density setting and why?

increase density; A larger bucket/positive density setting requires more radiation before termination, increasing receptor exposure, exposure time, and dose.

An AEC lumbar spine image demonstrates acceptable contrast but low receptor exposure. What density adjustment should be made and why?

increase density; When using AEC, increasing mA or kVp doesn't directly increase receptor exposure. The direct AEC adjustment is the density/IR exposure setting.

A technologist accidentally leaves a +2 density setting selected from the previous patient. What happens to exposure time if the density settings remain as is and why?

Increases; A +2 setting increases the AEC target exposure. The chamber must receive more radiation before the exposure terminates.

A technologist accidentally leaves a +2 density setting selected from the previous patient. What would happen to patient dose if the density settings remained as is

increase in dose

What does the acronym AEC stand for?

Automatic exposure control

State the primary purpose of an AEC device.

To produce consistent receptor exposure

How does an AEC device differ from a PBL device?

PBL auto-collimates the primary beam to not go outside of a given parameter; AEC auto-shuts off exposure when enough photons hit a particular chamber.

What is the most common type of AEC device used in modern radiography?

Ionization chamber

Where is the modern AEC device located in relation to the image receptor?

Entrance-type (located before the IR)

Which technical factor does the AEC device directly control?

Exposure time

Which technical factors and setup decisions must still be selected by the radiographer?

kVp, SID, mA, grid, chamber selection, density/exposure setting, positioning, centering, & collimation

In an AEC system, when is the exposure terminated?

When the selected chamber has received the preset amount of radiation needed for the desired receptor exposure

In a manual technique, the RE is primarily controlled by

mAs (chosen by the rad tech)

Using an AEC technique, the RE is primarily controlled by

density and IR exposure settings (AEC controls time to maintain RE)

What effect will changing kVp or mA have on receptor exposure when using AEC correctly?

When AEC is functioning correctly, changing kVp or mA should maintain receptor exposure since the AEC adjusts exposure time to keep consistent receptor exposure.

True/False: kVp can still affect subject contrast, scatter, and dose with an AEC technique

true

What is the only direct way to intentionally increase or decrease receptor exposure when using an AEC system?

Density controls

With a small/thin patient, exposure time should be

shorter

With a small/thin patient, the overall mAs will be

lower

With a small/thin patient, why would the mAs be lower and exposure time be shorter

less tissue = less attenuation = more radiation reaching the chamber /s = shorter exposure

With a large/thick patient, exposure time should be

longer

With a large/thick patient, the overall mAs will be

higher

With a large/thick patient, why would the mAs be higher and exposure time be longer

More tissue = more attenuation = less radiation reaching the chamber /s = longer exposure

What is the common MRT for most modern AEC systems?

1 ms (0.001 second)

Why does an AEC system require a backup timer?

for safety; protects a patient if AEC fails or is used improperly

What is the maximum exposure for an AEC unit?

150% of expected exposure or 600 mAs

Situation: The x-ray tube is positioned over a patient who is supine on the table, but the technologist accidentally activates the wall bucky AEC chambers. Would the patient be overexposed or underexposed? Explain why.

Overexposed; the wall bucky would be waiting to receive an exposure, but all the primary radiation would be hitting the table bucky. The chambers on the table bucky are not registering the radiation, and the backup timer would be reached, resulting in an overexposed image.

Which factors influence AEC exposure termination? Select all that apply. (a) Tissue thickness, (b) Tissue density, (c) Positioning of anatomy over the detector, (d) Collimation of the primary beam

a, b, c, and d

Describe how pathology can affect exposure time when using AEC

Additive pathology can increase attenuation, resulting in fewer photons hitting the chamber per second and increasing exposure time/prolonging termination. Destructive pathology can decrease attenuation, resulting in more photons hitting the chamber per second and decreasing exposure time/quickening termination

If a patient has emphysema and the AEC is set for an average patient, what will happen to exposure time

decrease

If a patient has emphysema and the AEC is set for an average patient, will the exposure be over or underexposed; why

underexposed; Emphysema decreases tissue density, so more XRs reach the AEC chamber sooner. The exposure will terminate earlier, resulting in an underexposed image for all other anatomy

If a patient has positive contrast/barium and the AEC is set for an average patient, what will happen to exposure time

increase

If a patient has positive contrast/barium and the AEC is set for an average patient, will the exposure be over or underexposed; why

overexposed; Barium is highly attenuating, so fewer XRs reach the AEC chamber. The AEC keeps the exposure running for longer, increasing the # of photons attenuating all other anatomy and producing an overexposed image

List three advantages of using an AEC system

Maintains receptor exposure despite varying patient thickness, reduces patient dose (when used correctly), and improves efficiency and reduces repeat images

What does APR/APT stand for

Anatomically programmed radiography

What does APR/APT do

uses pre-programmed technical factors organized by body part and projection to simplify technique selection

What is the purpose of a technique chart

Technique charts provide standardized manual exposure factors for specific exams.

What information is commonly included on a technique chart

body part and projection, kVp, mAs, SID, grid use or grid ratio, FSS, pt. size or part thickness

What equipment/instrument should be used to obtain an accurate measurement of part thickness?

Caliper

True/False: Technique charts replace radiographer judgement.

False

True/False: Technique charts should be created for each fixed/mobile unit.

True

True/False: Technique charts rely on accurate part measurement.

True

True/False: Technique charts account for extreme pathology/body habitus.

False

In a fixed kVp technique chart, which technical factor is adjusted? State the rule for adjustment.

mAs changes; double mAs for every 4-5 cm increase in tissue thickness

In a variable kVp technique chart, which technical factor is adjusted? State the rule for adjustment.

kVp changes with part thickness; increase kVp by 2 for every 1 cm increase in tissue thickness

True or False: Technique charts are primarily useful when AEC is activated

False; used for manual techniques when AEC is unavailable/inappropriate

Describe the appropriate AEC chamber pattern for the following anatomic region: PA chest

Two outer chambers only

While imaging an AP pelvis, the lead shield is unintentionally placed over one of the AEC chambers. How will this affect exposure time?

exposure time will increase

Describe the appropriate AEC chamber pattern for the following anatomic region: AP oblique knee

center chamber only

In which system does the radiographer select the anatomic part from the console menu?

APR/APT

Which of the following can be used to increase the receptor exposure on a radiograph during a normal AEC exposure? (1) An increase in the backup timer, (2) An increase in the density settings, (3) A decrease in the SID

2 only

An AEC system has ________ different ionization chambers that the radiographer can select prior to patient exposure

3

Describe the appropriate AEC chamber pattern for the following anatomic region: AP humerus

center chamber only

A radiograph of the abdomen is taken using an AEC system. Which of the following is likely to occur during a second exposure after the administration of two cups of barium?

the time of the exposure is likely to increase

The radiographic accessory used to measure the thickness of body parts to determine optimal selection of exposure factors is the

caliper

Describe the appropriate AEC chamber pattern for the following anatomic region: Lateral chest

center chamber only

Describe the appropriate AEC chamber pattern for the following anatomic region: AP shoulder

center chamber only

Which of the following factors determine AEC exposure termination? (Select all that apply.) (1) tissue thickness and density, (2) positioning of the anatomy with respect to the AEC chamber, (3) beam restriction, (4) the use of a fixed kVp technique chart

1, 2, and 3

The AEC device operates on which of the following principles? (1) Delivery of the required exposure time, (2) motion of magnetic fields inducing current in a conductor, (3) an ionization chamber charged by XR photons, (4) the anode target angle remaining below 45 degrees

1 and 3

An exposure of 600 mA and 70 kV has been selected for an AP projection of the abdomen using AEC. If the backup time is 80 ms and the minimum response time is 35 ms, the mAs will be at least

21

A radiograph of a lateral coccyx was made using AEC but is overexposed. This is most likely a result of

incorrect centering of the anatomy

Describe the appropriate AEC chamber pattern for the following anatomic region: PA oblique wrist

AEC is not recommended for this anatomy

A device used to ensure reproducible radiographs, regardless of tissue density variations, is called the

automatic exposure control

Describe the appropriate AEC chamber pattern for the following anatomic region: Lateral skull

center chamber only

Which of the following effects will occur when using AEC to image a hypersthenic patient

increased exposure time

Describe the appropriate AEC chamber pattern for the following anatomic region: AP pelvis

two outer chambers only

AEC improves consistency by primarily compensating for differences in

patient size and tissue thickness

True/False: AEC compensates for differences in patient size by adjusting exposure time

true

In an ionization chamber, radiation produces an electrical current by interacting with

has inside the chamber

Why are AEC detectors positioned between the patient and the image receptor

to measure radiation exiting the patient before it reaches the image receptor

Why is minimum response time especially important for small or low-density body parts

The exposure may be too short for the AEC system to respond properly

Which technical factor must still be selected carefully because it affects beam penetration and radiographic contrast

kVp

True/False: Using very high kVp with AEC may increase patient dose and positively affect radiographic contrast

false

If the wrong AEC detector is selected for the anatomy being imaged, the most likely problem is

the image may be exposed based on the wrong area of anatomy

Which situation may cause underexposure when using AEC

a detector receives radiation too quickly because it is not covered by enough tissue

Why can AEC be difficult to use with anatomy that varies greatly in tissue density across the image?

the detector response may not represent the entire anatomy of interest

Match each AEC limitation or consideration with the best explanation: small anatomy

May not completely cover the detector

Match each AEC limitation or consideration with the best explanation: poor centering

Detector may not be under the anatomy of interest

Match each AEC limitation or consideration with the best explanation: large field size

May increase scatter reaching the detector

Match each AEC limitation or consideration with the best explanation: metal prosthesis

May block radiation from reaching the detector

Why is collimation especially important when using AEC

detectors cannot distinguish between primary radiation and scatter radiation

Why does the reading emphasize that radiographers still need to understand manual technique

AEC is not available or appropriate in every situation

When reviewing an image produced with AEC, an exposure indicator that is outside the acceptable range should prompt the radiographer to

investigate possible causes before repeating or adjusting technique

What is anatomically programmed radiography designed to do

Simplify technical factor selection by using preset techniques for specific exams

The most critical factor in obtaining diagnostic quality images using an AEC device is the use of accurate

positioning and/or centering

The minimum response time of an AEC system

is the shortest possible exposure time

The purpose of the backup timer in an AEC system is to

prevent excessive exposure by terminating the exposure in case of error

What is the recommended minimum response time for modern AEC systems

1ms

Where are the AEC detectors located in most modern radiographic systems?

between the patient and the IR

AEC detector type that measures radiation before it reaches the IR

ionization chamber

Safety feature that prevents excessive exposure if AEC fails

backup timer

Shortest exposure time needed for AEC to accurately terminate exposure

minimum response time

Control used to increase or decrease the amount of exposure reaching the IR

density/IR exposure adjustment

True/False: AEC eliminates the need to select technical factors

false