18 AEC_PRESENTATION post - Duckskull.org

marblefreedomAI and Robotics

Nov 14, 2013 (3 years and 4 months ago)

76 views

AUTOMATIC EXPOSURE
CONTROL

DMI 63

What Is Automatic Exposure
Control
(AEC)?


Any device that measures quantity of
radiation reaching image receptor


Then automatically terminates exposure when
optimal density is reached


You give up control of exposure density to a
machine!





Purpose of AEC

To deliver consistent, reproducible
exposures across a wide range of:



Anatomical thicknesses


Anatomical parts


Equipment


Users

Automatic Exposure Control


Most technologists refer to AEC as
“phototiming”



AEC encompasses phototiming and all
other devices currently in use

3 DIFFERENT TYPES OF AEC

1. Phototimer


2. Programmed exposure
(computer controlled)


3. Anatomically programmed exposure

(computer
controlled




All three of 3 types of AEC units still use
some type of
radiation detector and
backup timer



Difference is that a computer controls
termination of exposure with Programmed
exposure and APR

Types of Detectors



All AEC use 2 basic types of detector
devices:



Photomultiplier tube (photocell)



Ionization chamber


How a photocell works


Radiation goes through Pt and
image receptor



Through photomultiplier tube



Which lights up when hit by
radiation



Converts light to electric signal




Exposure cuts off when signal
reaches a predetermined
intensity

photcell

Photocell

Ionization Chamber

Most common type

Plate parallel plate
between

pt and image
receptor

Radiolucent so doesn’t
show up on image

Note: chamber
in front

of IR


Ionization Chambers
cont’d



These chambers contain
cells filled with air



During exposure air is
ionized



When charge reaches
certain level, exposure
terminates



Location of ionization
chambers shown by small
rectangles on table or
upright image receptor

Photodiode

Solid state instead of
photomultiplier tube


Note: Photodiode is
after

image receptor

Comparison

Photodiode

Ionization Chamber

Proper Cell Selection


Generally 2 or 3 cells


Tech
must

select cells
appropriate to area of
anatomical interest


Using 2 cells or even 3
creates a signal that is
averaged from for more
uniform density

Image receptor

What cells would you select for
-



PA Chest
-


Lateral Chest
-


Pelvis
-


Pelvis with Left prothesis
-


AP Lumbar spine
-




Image receptor detector positions

1. Photo
timing

Earliest


still widely used


Uses photocell







Technique Selection


Generally, 2 second backup time


Different levels of density can

be selected


Adjusts mAs upward or downward in increments of 25
-


30% per step


Tech selects kVp based on body part
thickness

2. Programmed Exposure


Uses
microprocessor



Microprocessor allows tech to digitally
select any kVp or mAs



then microprocessor automatically chooses
mA station and time


Backup times programmed in




Programmed Exposure
cont’d


Falling
-
load generator

occasionally used


microprocessor begins exposure at a maximum mA
value then drop in mA as exposure continues



beginning mA could be 1000, but drop to 400


readout of actual mAs used will be displayed on
console


Goal: reduce exposure time to minimize
motion blur on radiograph


3. Anatomically

Programmed Exposure

(APR)

Similar to Programmed Exposure


Uses touch screen with
picture

of anatomic part


(Essentially a computerized technique chart)


Microprocessor selects predetermined kVp & mAs


parameters can be changed and saved in some models until
optimal exposure density is reached



Can be used without AEC

AEC
does not

relieve tech of following
obligations:

Skill in positioning


Technique selection
: still need to select mA
and kVp and backup time (newer models
build it in)


Anatomic recognition

-
different parts require
different settings


Important to remember!


Positioning accuracy is critical!



Anatomy must be placed directly over detector



Certain anatomy works well (abdomen)



Certain anatomy does not (shoulder)



Why might it not work well on children?


Important to remember!

Never put contrast filled anatomy over
photocell!


Or
breast implant

Or
metal prosthesis

Or
shielding



All rooms aren’t calibrated the same

Rooms change over time

Service engineer must calibrate initially then periodically recheck



Upside of using AEC



More efficient setting exposure level
-
speeds up exam



Improves exposure accuracy, as long as proper
positioning is used



More consistency in density



Lowers repeat rate


Downside of using AEC


Technologists come to depend upon system
and when it crashes, can’t remember manual
techniques


Over
-
confidence in system may cause
technologist to become neglectful and commit
errors



Obsolete?


Digital equipment will override image density produced
by AEC
(techs will use too high techniques intentionally!
)



AEC controls pt exposure
-

good for pt. and image



Digital equipment

controls appearance of image
-

pt
doesn’t matter



Should be used together!

The End