Unit 4: Exposure Technique Selection (AEC)

automatic exposure control

what does AEC stand for?

• compensate for differences in patient size

• ensures the IR receives the exact necessary amount of receptor exposure

what is the primary purpose of the AEC device?

ionization chamber

what is the most common type of AEC device?

before the IR, inside of the bucky

where is the ionization chamber type AEC device located?

exposure time

which technical factor does the AEC device control?

kVp, mA

what technical factors must be set by the technologist?

once the appropriate charge within the ionization chamber has been reached

when is the exposure terminated in an AEC system?

shorter exposure time, lower overall mAs

when imaging a small patient using AEC, the exposure time and mAs results in:

longer exposure time, higher overall mAs

when imaging a large patient using AEC, the exposure time and mAs will result in:

mAs

when using a MANUAL TECHNIQUE, receptor exposure is controlled by:

density settings

when using an AEC system, receptor exposure is controlled by:

no effect

what effect will changing kVp or mA have on receptor exposure when we use the AEC?

change density settings

the ONLY way to change receptor exposure when using an AEC system is:

exposure time will decrease

if you use the density setting to decrease the size of the bucket, will the exposure time increase or decrease?

exposure time will increase

if you use the density setting to increase the size of the bucket, will the exposure time increase or decrease?

shortest possible exposure time

define the minimum response time (MRT)

1 ms

what is the common MRT for most AEC systems?

safety precaution that terminates exposure if the AEC circuit fails. protects the patient from excessive exposure.

why does an AEC system require a backup timer?

150% of expected exposure time or 600 mAs

what is the maximum exposure of an AEC unit?

patient will be overexposed

if the wrong bucky is chosen and the wrong AEC chambers are activated, what happens to the patient?

anatomy of interest

what part of the body should be placed over an active AEC chamber?

lead shield, orthopedic device, metal

what should never be placed over an active AEC chamber?

• decrease repeat rates

• decrease patient dose

• increases consistency among images

what are three advantages to using an AEC device?

additive

what type of pathology will cause exposure time to increase?

destructive

what kind of pathology will cause exposure time to decrease?

pre-programmed technical factors for each type of exposure

what is APR?

automatically programmed techniques

what does APR stand for?

pre-established manual factors that are to be used for particular exams

describe a technique chart

kVp, mAs, grid, and SID

which technical factors are typically included on a technique chart?

calipers

what instrument should be used to measure part thickness?

false

(T/F) technique charts include recommendations for extreme variations in body habitus and pathology

true

(T/F) a technique chart should be created for each fixed and mobile x-ray tube

mAs

in a fixed kVp technique chart, which is the only technical factor that can be adjusted?

kVp

in a variable kVp technique chart, which is the only technical factor that can be adjusted?

long scale contrast

fixed kVp technique charts produce what kind of contrast?

short scale contrast

variable kVp charts produce what kind of contrast?

variable kVp chart

which technique chart produces a higher patient dose?

mAs increases by a factor of 2 for every 4-5 cm increase in tissue thickness

state the rule for a fixed kVp technique chart

kVp increases by +2 for every 1 cm increase in tissue thickness

state the rule for variable kVp technique chart

• incorrect bucky activated

• exposure time less than MRT

• density control left on from previous patient

• incorrect chamber selected and/or chamber is under anatomy that is more dense or thicker than anatomy of interest

• orthopedic device is over chamber

• presence of external artifacts

causes of overexposure using AEC

• backup timer set shorter than necessary

• density control left on from previous patient

• lack of proper collimation (excessive scatter)

• incorrect chamber selected and/or chamber is under tissue that is less dense or thinner than anatomy of interest

• chambers not completely covered by anatomy of interest

causes of underexposure using AEC

after the IR

where are phototimer type AEC devices located?

top 2 chambers

what ionization chambers do we use for a PA chest?

all 3 chambers

what ionization chambers do we use for an AP abdomen?

top 2 chambers

what ionization chambers do we use for an AP pelvis?

no AEC, if we were to use it would be the middle chamber

what ionization chamber do we use for an AP humerus?