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Automatic Exposure Control

FAIRUZ BINTI MOHD NASIRRADIOGRAPHY , FPSK

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Lecture Overview Introduction on AEC

Purpose of AEC

Types of detectors Technical consideration with AEC

Limitation of AEC

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Introduction The Automatic Exposure Control (AEC) requires a special

understanding on the part of the radiologic technologist.

AEC is one method for setting exposure factors to ensure thata quality radiograph image is produce.

The AEC is a device that measures the quantity of radiation

that reaches the image receptor.

It automatically terminates the exposure when the imagereceptor has received the required radiation intensity.

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Automatic exposure control devices can assist theradiographer in producing consistent radiographic imagesfrom patient to patient, regardless of size or presence of

pathology.

The advantages of this consistency are numerous andinclude:

decreased repeat rate;

decreased patient exposure; and

increased department efficiency.

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Purpose of AECAEC is a system used to consistently control

radiographic density by terminating the length ofexposure based on the amount of radiation reachingthe image receptor.

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When an AEC device is used to terminate anexposure, the technologist sets the kVp and mA,

but the time of the exposure is automaticallydetermined by the machine.

The AEC device differs from a manual timer

because the AEC does not stop the exposureuntil the film has reached an appropriate density.

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X Ray tube

Collimator

Beam

Soft

tissueBoneAir Patient

Table

Grid

Cassette

AEC detectors

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AEC should produce consistently optimal densityradiographs because based on sensitometric,specific amount of radiation to the film producespecific density.

If the x-ray exposure is terminated when the

exposure corresponding to optimal density isreached, the resultant radiograph shoulddemonstrate optimal density.

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AEC SystemsAEC devices work by the same principle of operation:radiation is transmitted through the patient

converted into an electrical signal

terminating the radiographic exposure.

Two types of AEC have been used:Phototimers

Ionization chamber (most common)

Regardless of the specific type of AEC system used,almost all systems use a set of three detectors, arrangein same specific manner.

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Phototimer type AEC Units

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Phototimer Type AEC Units Consist of

a fluorescent screen,

a photomultiplier tube (PMT) or photo tube and a complex circuit connected to a timer designed to terminate the

exposure.

The fluorescent screen is adjacent to the photomultiplier

tube and both lie behind the Bucky grid and cassette.

considered as exit-type devices because the detectors

are positioned behind the image receptor

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Phototimer Type AEC Units Light paddles coated with fluorescent material; converting

radiation to the light.

The PMT receives the light energy and converts it toelectrical energy

The timer is tripped and the radiographic exposure isterminated when a sufficient large charge has been

exposed

The amount of light produced by the screen depends onthe amount and energy of the X-rays reaching thefluorescent screen.

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Phototimer AEC

X-RAY TABLE

X-RAY TUBE

CASSETTE

SCREEN

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PM TUBELIGHT

ELECTRONS

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Ionization chamber system

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Ionization chamber system Ionization chamber is a hollow cell that contains air

and is connect to the timer circuit via electrical wire.

Ionization chamber AEC devices are consideredentrance-type devices because the detector arepositioned in front of image receptor.

Compare to phototimers, ion chambers are lessaccurate, but there are prone to failure.

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IONIZATION CHAMBER AEC

X-RAY TABLE

X-RAY TUBE

BUCKY

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X-rayspenetrate the

thinaluminumshell and

strike the airmolecules.

Electronreleased

andionizationoccurs.

The electronsare attracted

to thepositively

chargedelectrode and

are carriedthrough anelectrical

circuit

Thechargetravel

alongthe wireto thetimer

circuit.

The timer istripped and the

radiographicexposure isterminated

when asufficient large

charge hasbeen exposed

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Even though the ionization chamberand the phototimer operate

differently, they both have thesame function:

convert radiation into an electrical

signal which will be used to

automatically stop the exposure

when the film has reached the

proper density

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Technical Considerations with AEC

To use AEC to its advantage,

radiographers must be awareof some important technical

consideration peculiar to AEC

system.

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FACTORSINFLUENCING

AEC

Patientpositioning

Detectorselection

Densitysetting

CollimationBackuptimer

Effects ofmA, kVand SID

Buckyselection

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Patient Positioning AEC units use detectors to receive radiation passing

through the patient, it is important to properlyposition the anatomical part of interest over the

detector. If the detector receives radiation coming from

another anatomical area, the exposure can be eithertoo long or too short, overexposing or underexposing

the film. If the radiographer positions the anatomical part

correctly, the AEC unit will produce a radiographexhibiting proper density.

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Patient Positioning If the radiographer positions the anatomical part correctly, the

AEC unit will produce a radiograph exhibiting proper density.

Without a technologist who is very knowledgeableabout anatomy and positioning, automatic timers are

worthless.

In fact, they may actually decrease departmentefficiency because of the increased amount of repeatradiographs that will result if used improperly.

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Detector Selection Proper detector selection must be made for an AEC unit to

produce the desired radiographic density.

There are usually 3 detectors that are connected to a

complex electrical circuit.

Any single detector or a combination of two or three

detectors, can be selected for a radiographic exposure.

Failure to select the proper detectors will increase patient

dosage because the exposure must be repeated to correctthe underexposure or overexposure of he first radiograph.

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Density Setting The AEC devices are equipped with density

controls allow the radiographer to fine tune theradiographic density that is produce by the unit.

These are generally are in the form of buttons on thecontrol panel that are numbered -2, -1, +1, +2.

These button changes exposure by somepredetermined amount or increment expressed aspercentage.

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+4

+3

+2

+1

0

-1

-2 -3

-4

+100

+75

+50

+25

0

-25

-50 -75

-100

D. SETTING % DENSITY CHANGE

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Backup Timer The backup timer is safety device that prevents

excessive exposure to the patient in cases of

timer switch failure or radiographer error.

The backup timer must limit the exposure to a

maximum of 600mAs

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Collimation Collimation affects the production of scatter radiation.

As collimation increases, the field size decreases and the quantity ofradiation decreases.

The detector is unable to distinguish transmitted radiation fromscattered radiation.

Because the detector is measuring both type of radiation, the timer isturn off too soon when scattered is excessive, which result inunderexposure of area of interest.

The radiographer should open the collimator to the extent that the partof being radiographed is image properly, but not so much as cause tostop the AEC device to stop the exposure before the area being image isproperly exposed.

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Bucky selection The Bucky must be correctly selected.

Failure to do so results in the patient and the imagereceptor being exposed to excessive radiation.

The backup time is reached, the exposureterminated.

The radiographer should be certain to deactive the

AEC system and use manual technique whenperforming any radiographic study where the imagereceptor is located outside the Bucky.

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Effects of mA, kV and SID When using the AEC units, the radiographer does not

set the exposure time.

But the radiographer can influence the exposure time

according to the mA and kV setting he chooses. If the mA is changed, X-ray quantity also changes, high

mA, produces more X-rays so the exposure time will beshorter and vice versa.

kV setting also influences exposure time when using

AEC. High kV setting, higher energy X-rays are produced,

more of the X-rays will also penetrate the part ,resulting shorter required exposure time. Vice versa.

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EFFECTS OF mA, kVp, SID ON AEC

mA AEC TIME

kVp AEC TIME

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Interchangeability of film-screen

system Different film-screen systems cannot be interchanged

easily once an AEC is calibrated to produce specificdensities.

When calibration is performed, it is done for aparticular film-screen speed.

The AEC device cannot sense when the radiographer

use different film or screen, resulting in too much ortoo little density.

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Minimum Response Time The minimum response time represents the shortest

exposure time possible.

Minimum response time usually longer with AECsystem.

This can be problem with some segment of patientpopulation, such as pediatric patient and

uncooperative patients.

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Lack of calibration Failure to maintain regular calibration of the unit and

AEC device result in radiographs that lack consistence,reproducible, and appropriate density.

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