RSAP 180 Learning Outcome #8 Review

The most critical factor in obtaining diagnostic quality images using automatic exposure control (AEC) is the use of accurate____________.

• Positioning and centering

• Focal spot size

• Side markers

• Backup time

Answer explanation

Automatic exposure devices provide diagnostic quality exposures only for structures positioned directly above the ionization chambers. If the anatomy isn’t properly placed directly above the ionization chamber, the resulting image can be under or over exposed.

The art of using AEC is the art of positioning

TRUE or FALSE: the preset amount of radiation exposure is reached sooner with high kVp. 

• True

• False

Answer explanation

Because high kVp radiation is more penetrating (reducing the total amount of x-ray exposure to the patient because more x-ray photons exit the patient) and the detectors are measuring quantity of radiation, the preset amount of radiation exposure is reached sooner with high kVp. 

The ionization detectors that are used in most automatic exposure controlled units are located between _________ and ___________.

• Tube; collimator

• Collimator; patient

• Filter; grid

• Patient; image receptor

Answer explanation

Modern AECs are ionization chambers, located before the image receptor and after the grid.

TRUE OR FALSE: When using AEC, the radiographer loses control over kVp.

• True

• False

Answer explanation

When utilizing AEC, the radiographer loses control over time, and as a result mAs when using an AEC.

If you have too much collimation and collimate an ionization chamber out, the resulting image would be:

• Creates an under exposed image

• Creates an over exposed image

• No affect

• System will not allow an exposure

Answer explanation

If the primary beam is collimated from an active chamber, the chamber operates as if the tissue is extremely dense (dark image)

TRUE OR FALSE: AEC is a system used to consistently control the amount of radiation reaching the image receptor by terminating the length of exposure

• True

• False

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

• Prevent excessive exposure by shutting the exposure off in case the AEC's timer switch failure

• Controls the length of time required for the AEC chamber to respond to the radiation and terminate the exposure

• Determines the total amount of mAs required to produce the appropriate exposure to the image receptor

• Permits the choice of an anatomical region and the projection

Answer explanation

Minimum Response Time: Controls the length of time required for the AEC chamber to respond to the radiation and terminate the exposure

mAs Readout: Determines the total amount of mAs required to produce the appropriate exposure to the image receptor

APR: Permits the choice of an anatomical region and the projection

TRUE or FALSE: Phototimers is the most common type of AEC system

• True

• False

Answer explanation

Ionization Chambers are the most common type of AEC system

Once radiation is transmitted through the patient, the air inside the chamber becomes ionized, creating an electrical charge.

This charge travels along a wire to the timer circuit and the exposure is automatically terminated when the total amount of radiation measured reaches a pre-set level; this level corresponds to the required density

What are the typical number and configuration of the ionization chambers?

• 1 chamber configuration

• 2 chamber configuration

• 7 chamber configuration

• 3 chamber configuration

Answer explanation

TRUE OR FALSE: The most common errors when using AEC are Improper positioning over the AEC cell and improper cell selection

• True

• False

Answer explanation

The anatomical area of interest needs to be precisely positioned over the ionization chamber(s)

When not enough of the structure of interest is positioned over the activated chamber, the AEC cell will attempt to produce a diagnostic quality exposure for whatever it is over.