AEC image production 2

What is AEC

Automatic Exposure Control

Why was AEC developed?

To assist the radiographer in setting the appropriate technical
factors for a variety of exams and patient variations. Ultimately to consistently produce
quality images.

What is the principle of operation in an AEC device

Once a pre-determined amount of
radiation is transmitted through a patient, the x-ray exposure is terminated. This
determines the exposure time and therefore the total amount of radiation exposure to
the IR.

What factor is the variable when using AEC? (what changes) Think of the three factors;
mA, time and kVp

Time is the variable.

Photomultiplier tube

“exit” device - light paddles coated with a fluorescent material -
produce light in response to x-ray exposure - light is converted to electric signal and
timer is tripped after a relatively large charge is received by sensor/detectors which is
located behind IR (see image in text). Not used anymore

Ionization chamber device

Hollow cell contains air that is connected to a timer circuit
by an electrical wire. “entrance device”. Detectors are positioned in front of IR. Upon
exposure to radiation, the air in the chamber becomes ionized, creating a charge.
Charge travels along wire to timer. Timer is tripped when a large amount of charge is
received. Ionization chambers are less accurate and less sophisticated than phototimers
but less prone to failure so they are used today

What is “mAs readout” display

The mAs used will display after the exposure is
terminated, may be only for a few seconds so radiographer may need to look right
away

What is the advantage to mAs readout?

To assist radiographers in applying
consistent quality exposure factor

A) What determines the length of exposure time with an AEC device?

KVp is constant and mA will determine the amount of time in which the exposure takes place.

Increasing the mA value will have what effect on the exposure time AND decreasing
the mA will have what effect on the exposure time?

Higher mA will decrease the
amount of time needed and decreasing mA will increase the amount of time needed for
a quality exposure.

s this an inverse process or direct process?

Inverse

What is “minimum response time” or “minimum reaction time”?

the shortest exposure
time an AEC can produce. (1 ms with modern systems) may not work with smaller
patients and body parts since those exams are generally table-top (non-bucky)

What is “back up time”?

the maximum exposure time allowed during an AEC assisted
exposure. Back up time is usually set at 150-200% of the expected exposure time
needed.

When would the radiographer need to adjust the exposure?

When there are
compensating factors such as devices for example: pacemaker in a chest, prosthesis in a
knee, shoulder, hip, metal dental devices etc. Radiographer may need to use manual
techniques

What are density controls used for?

excessive differences in patient thickness, allows
the radiographer to adjust the preset amount of radiation detection. Can be
represented as +1, +2, +3.... Or -1, -2, -3 .... Each button changes exposure by about 25%
in one increment or + or -50% min two increments. Allows the system to increase
exposure

Can one AEC system be used for a variety of imaging systems?

An AEC device MUST be calibrated to the type of system in which it is being used. CR
and DR techniques are different, the AEC device is not intuitive “at least not yet”

What are two factors that are extremely important when using AEC?

Centering and
detector selection. Radiographer should be aware of part thickness and part make-up,
( air vs fluid etc.

Is detector size fixed or variable in an AEC system?

Fixed, may not be able to use with
very small parts

When experiencing compensating issues, what may determine whether or not you may
use AEC? Provide an example;

Smaller than normal part size

What factor is crucial and standard to reducing scatter in the image and protecting the
patient from additional radiation exposure?

Collimation

What may happen to the AEC when scatter is excessive

It may trip the timer too soon
resulting in a suboptimal image

What is APR? Anatomical Programmed Radiography -

it can be incorporated with an AEC
in a unit. Radiographer selects from a menu which has preset technical factors ( not
AEC) for the part and projection. Factors may need to be adjusted for precision

Is APR and AEC the same thing

No, but may be used together in some systems. AEC
controls the exposure, APR only provides technical factors and may need adjustment.