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AUTOMATIC EXPOSURE
CONTROL
I. INTRODUCTION
Automatic exposure
control devices can assist the radiographer in producing
consistent radiographic images from patient to patient,
regardless of size or presence of pathology. The advantages of
this consistency are numerous and include: decreased repeat
rate; decreased patient exposure; and increased department
efficiency. The most important benefit being decreased repeat
rate. According to Chesney's, Equipment for Student
Radiographers, "Surveys conducted on a wide scale have drawn
conclusions that inaccurate exposures have been the most common
cause for radiographs needing to be repeated" (1994).
Although automatic timers have
the potential to decrease the amount of films and increase
department efficiency, this can only be accomplished if the
equipment is operated by a skillful technologist. Even though it
is called an "automatic" exposure device, a technologist must be
very knowledable about automatic timing devices to produce high
quality radiographs.
This course is
designed to review the basic operation of Automatic Exposure
Control devices and offer suggestions to the participant
on how to utilize AEC devices to obtain optimum radiographs.
1.1 METHODS
OF TERMINATING AN EXPOSURE
There are two ways
that a radiographic exposure can be terminated: manually or
automatically. When an examination is manually timed, the
technologist sets the kVp, mA, and time. After the predetermined
time has elapsed, the exposure is terminated. If the equipment
is operating properly and the correct technique was used for the
appropriate patient thickness, one can expect a properly exposed
radiograph.
When an AEC device is used to
terminate an exposure, the technologist sets the kVp and mA, but
the time of the exposure is automatically determined by the
machine. The AEC device differs from a manual timer because the
AEC does not stop the exposure until the film has reached an
appropriate density. Unlike manual timers, which simply stop the
exposure after the preset time has elapsed.
A major benefit of the AEC device
is its ability to consistently obtain accurately exposed
radiographs, even in the presence of pathology. While manual
timers terminate the exposure at the preset time, regardless of
pathology or achievement of proper film density. The following
example demonstrates the difference:
Two patients may
come to the radiology department for chest x-rays. They both may
measure 18 centimeters, one may have normal lung fields, while
the other may have a pleural effusion. Since both patients
measure the same thickness, the radiographer would most likely
use the same technique on both patients when manually timing.
Chances are the
radiograph of the fluid-filled lungs will be lighter than the
healthy lungs and therefore it would have to be repeated. If AEC
would have been used in this situation, the exposure time would
have been automatically increased to compensate for the fluid in
the lungs . A diagnostic radiograph would have been produced,
therefore eliminating the need for a repeat radiograph.
II. AEC PHYSICS AND
INSTRUMENTATION
AEC devices are common in today's
radiographic equipment. When AEC devices were first introduced,
they were strictly used for fluoroscopic spot films. As advances
were made in technology, automatic timers were re-designed to be
used in wall and table buckys. Today, AEC devices find
application in general, fluorosopic and even portable
radiographic equipment. Before discussing AEC devices any
further, it is important to review the basic operation of an
automatic timer.
2.1 TYPES OF
AUTOMATIC EXPOSURE CONTROLS
The most common type of AEC
devices used in today's radiographic equipment is the ionization
chamber. In older equipment, the phototimer was most commonly
used for the automatic timing mechanism.
| Even
though the ionization chamber and the phototimer operate
differently, they both have the same function: convert
radiation into an electrical signal which will be used to
automatically stop the exposure when the film has reached
the proper density. |
2.2 THE
IONIZATION TIMER
The ionization timer utilizes an
ionization chamber, capacitor, and exposure terminating switch
to automatically terminate the exposure after the film has
reached the predetermined density. A brief review of how the
ionization timer operates, will be beneficial at this point.
An ionization chamber is a
radiation detection device that produces a small electrical
current when struck by radiation. Inside the chamber are two
conducting plates which are separated by air. When radiation
strikes the chamber, the air inside the chamber is ionized and
the electrons migrate toward the plates, thus producing a small
electrical current. This electrical current is then used to
charge a capacitor. When the capacitor, an electronic storage
device, reaches a predetermined charge the exposure terminating
switch is activated and the radiographic exposure is terminated.
It is important to remember that in this situation, the exposure
was terminated by sensing the amount of radiation reaching the
film, and not by a preset time.
When an x-ray machine utilizes
the ionization chamber as its automatic timer, three chambers
are used in the configuration that is demonstrated in Figure
2.2. The ionization chambers are usually located behind the grid
and in front of the cassette.
Figure 2.2
2.3 THE PHOTOTIMER
Another type of automatic
exposure device that may be used in radiographic equipment is
the phototimer. The phototimer was commonly used in older x-ray
equipment and consisted of a fluorescent screen, photomultiplier
tube, capacitor and an exposure terminating switch. Although the
components of the phototimer are different, the theory of
operation is similar to that of the ionization timer which is
discussed in section 2.3.
The ionization timer and
phototimer both convert radiation into an electrical signal
which is used to terminate the exposure when the film has
reached the proper density. However, since the phototimer is a
bit less sophisticated than the ionization chamber, there are a
few more steps involved in the conversion of radiation into an
electrical signal.
In the phototimer assembly, a
fluorescent screen is placed behind the bucky. A photomultiplier
tube is then placed directly behind the fluorescent screen. The
fluorescent screen converts the radiation that exits from the
patient and cassette into light. The photomultiplier tube then
converts the light emitted from the screen into an electrical
current which is used to charge a capacitor. When the capacitor
is charged to the predetermined level, the exposure is
terminated.
As with the ionization timer, the
length of the exposure is based on the time it takes to charge
the capacitor to the predetermined level and not a time set by
the technologist. If a patient is larger than "average" more
radiation is absorbed by the patient, and less is converted to
an electrical signal. Therefore, the exposure will be longer
since it will take longer to charge the capacitor to the
predetermined level. Likewise, if a patient is very thin, there
is minimal absorption of the beam which results in more
radiation being converted into an electrical signal. This in
turn will charge the capacitor more quickly and terminate the
exposure more rapidly. Since the xray tube is "on " while the
capacitor is being charged, it should become obvious that the
length of time that it takes to charge the capacitor to the
predetermined level is directly related to film density.
As mentioned earlier, the
ionization timer is used more commonly in modern radiographic
equipment than the phototimer. However, the term "phototiming"
has become synonymous with either type of automatic exposure
control.
Now that the basic operation of
AEC devices have been reviewed, it is time to discuss how to
properly use them.
III. TECHNOLOGIST'S
DECISIONS
When using an automatic exposure
control device, there are many important decisions a
technologist must make in order to ensure that a diagnostic film
will be obtained. The two most important are patient
positioning and proper detector selection. As
discussed earlier, the Phototimer or Ionization Chamber which
are known as detectors collect the radiation coming from
the patient and convert it into an electrical signal. For the
film to have the proper density, the detector must sample the
radiation coming directly from the area of interest. If the
detector samples radiation from another area, the film will not
have the proper density. This then explains why proper
positioning is so important when when using AEC devices:
Incorrect positioning will lead to a film with incorrect
density.
3.1 PATIENT
POSITIONING
The following diagrams will help
illustrate the importance of proper patient position.
In the diagram of an incorrectly
positioned shoulder, the radiograph will not have the proper
density because the shoulder joint is not directly over the
detector. Due to the poor positioning, a portion of the detector
is completely outside of the body and will be directly exposed
by the beam. This will charge the capacitor very quickly,
resulting in a radiograph that is too light to fully demonstrate
the shoulder.
In Figure 3.1-1, the lateral
spine radiograph will be too light because the detector is
sampling radiation from soft tissue along with radiation
emerging from the spine. Because the soft tissue is easily
penetrated, a "large" electric current will be produced in the
timing circuit. The capacitor will be charged quickly resulting
in a radiograph that is too light because the exposure time was
too short.
Figure 3.1-1
| Without
a technologist who is very knowledgeable about anatomy and
positioning, automatic timers are worthless. In fact, they may
actually decrease department efficiency because of the
increased amount of repeat radiographs that will result if
used improperly. |
3.2 DETECTOR
SELECTION
Along with proper positioning,
proper detector selection also influences the operation of the
AEC device. Auto timers may have one to three detectors in their
circuits, most table and wall buckys have three. Because there
are three, the question often arises, "Which detector should
I use?" If one keeps in mind that the detector must sample
radiation coming from the area of interest, the decision
of which detector to select becomes an easy one.
Here are some general guidelines
to follow for determining detector selection:
When the vertebral column is the main area of interest, the
center detector should be selected.
When using AEC for
joints such as the shoulder or knee, the center detector
should be selected. If the outside detectors were selected,
the radiograph may not have the proper density because the
outer detectors may be collimated out of the field or they may
detect too much radiation coming from the soft tissue. Either
one of these situations will result in a radiograph that does
not have the proper density.
When the pelvis is being
radiographed using AEC, the two outside detectors should be
selected. When a pelvis is properly positioned the two outer
detectors will be directly below the ilia.
Detector selection for a chest x-ray is a bit more challenging
because several factors influence this decision. Radiologist's
preference, pathology, and surgical intervention play the
biggest role in choosing which detector to use.
When the lungs are the area of
interest, the right or both outer cells may be
selected. The use of both outer cells for a PA chest
radiograph will result in a slightly darker radiograph, since
the left cell will take longer to accumulate radiation due to
absorption by the heart. Therefore, when deciding between
right or both outer cells for a PA chest radiograph, one
should consider if the radiologist prefers darker or lighter
chest films.
As a general rule, use of the
left or center detector for a PA chest radiograph will result
in an over exposed radiograph except in the presence of
certain chest pathology or surgical intervention.
If a large pleural effusion is
present in a lung and the detector over the affected lung is
selected, an overexposed radiograph will result. This is
explained by the fact that the fluid in the affected lung will
absorb a greater amount of radiation, which in turn will
result in less radiation getting to the detector. Since less
radiation reaches the detector, the exposure will continue
longer, and an overexposed film will result. In this
situation, the cell opposite the affected lung should be
selected.
If a patient has undergone a
pneumonectomy, the detector on the unaffected side should be
selected when using AEC for the PA chest radiograph. If the
cell under the affected side was selected in this situation,
an underexposed radiograph would result. Since the side of the
surgical intervention would offer little absorption of the
radiation, the detector would accumulate radiation very
rapidly resulting in a short exposure. The film would be
undiagnostic because the remaining left lung and mediastinum
would not be visualized adequately.
The center cell should be
selected if the mediastinum is of interest on the PA chest
radiograph, and also for the lateral chest xray.
When using AEC for the abdomen, the technologist's choice for
detector selection is once again related to the radiologist's
preference for darker or lighter radiographs. Although the
center, outer two, or all three detectors maybe used for the KUB;
selection of the outer two cells is most technically accurate.
The KUB radiograph is most commonly ordered to evaluate the soft
tissue structures of the abdomen. Selection of the two outer
detectors will sample the radiation coming from the soft tissue
structures only, resulting in a properly exposed radiograph for
the area of interest. Selection of the center cell will result
in a slightly darker radiograph because the lumbar spine will
attenuate a greater portion of radiation compared to soft
tissue, therefore resulting in a darker radiograph. Finally, use
of all three detectors will result in a radiograph having
density midway between a radiograph taken with the outer
detectors and a radiograph taken using only the center detector.
This is explained by the fact that the detectors are sampling a
portion of the radiation coming from the soft tissue and
bony structures. Therefore, an electronic averaging
occurs between those structures.
Figure 3.2-1 demonstrates the
effect of proper cell selection and its affect on density.
Radiograph A was taken with the the two outer cells selected,
while radiograph B with take with the center cell selected.
Figure 3.2-1
When using AEC for an upright
abdomen, the center detector should be selected. Use of the
outer two or all three detectors is not a good choice because of
the configuration of the detectors. Since the outer two
detectors are positioned higher and more laterally than the
center detector, there is a chance these detectors may sample
radiation coming from the base of the lungs. Because the lungs
are easily penetrated, the radiographic exposure will be
terminated prematurely, resulting in a radiograph that is too
light. Figure 3.2-2 demonstrates that when using the outer cells
for an upright abdomen, the film will lack sufficient density.
figure 3.2-2
* It should be
noted that these suggestions for cell selection are based on
operational theory. Due to differences in equipment and
calibration, results may vary from machine to machine.
Cassette size is another
important factor that should be considered when choosing which
detector to use for an exposure. When using cassette sizes that
are smaller than 10" x 12", only the center detector should be
used. Cassette sizes smaller than 10" x12 " have such a small
area of coverage that a portion of the two outer detectors lie
outside of the collimated area. If the outer detectors where
selected in this situation, a portion of the radiation would
never be able to reach the entire detector therefore resulting
in a longer time to charge the capacitor. This would result in
an unnecessarily longer exposure time and an overexposed film.
| A
thorough understanding of the AEC makes detector selection
less threatening. Simply remember the AEC must sample
radiation coming from the area of interest. |
3.3 DENSITY
SELECTOR
Along with proper positioning
skills and knowledge of detector selection, the radiologic
technologist must understand the function of the density
selector to fully utilize AEC.
When the automatic timer is
installed, the capacitor in the circuit is set to terminate the
exposure when it has acquired a specific charge. Remember, that
as long as the capacitor is charging, the x-ray tube is
producing radiation. The precise charge on the capacitor which
terminates the exposure is determined by phantom studies
conducted by the service engineer and radiologist input. The
service engineer then adjusts the Neutral setting of the density
control to correspond to an acceptable radiographic density.
When a radiographic imaging unit is properly calibrated, the
density selector should be kept on the "neutral" setting for the
majority of procedures. If you find that you are not using the
"neutral" setting for most of the exams, the unit most likely
needs to be recalibrated.
There are occasions when the
radiographic machine is properly calibrated, but using the
neutral setting does not result in properly exposed radiographs.
Some of the reasons for this will now be discussed.
Probably the biggest culprit of
improper film density while using AEC is inaccurate positioning.
As mentioned earlier, if the part is not properly positioned
over the detector, the film will not have the correct density.
The density control cannot be blamed for inadequate film density
if the patient is not properly positioned!
In figure 3.3-1, the detectors
have been outlined with lead wire. These radiographs demonstrate
how improper positioning affects density. Radiograph B is
properly positioned and therefore has sufficient density. In
radiograph A, the central ray is directed too far posteriorly
and in radiograph C, the central ray is directed too far
anteriorly.
A B
C
figure 3.3-1
Poor Collimation can be another
cause for obtaining improperly exposed radiographs while using
the AEC The detectors cannot tell the difference between the
primary radiation coming from the patient and scattered
radiation. Therefore, if large amounts of scattered radiation
are being produced, it will be picked up by the detector(s) and
cause the exposure to terminate too soon, resulting in a
radiograph that has insufficient density. This radiograph will
be too light, even though it was taken with density selector set
at Neutral.
The radiographs in
Fig. 3.3-2 demonstrate that as you increase the amount of
collimation and shielding, the amount of scattered radiation
reaching the detector decreases, resulting in a darker
diagnostic radiograph.
Figure3.3-2
Although, little thought about,
another factor that can affect the density of a radiograph while
using AEC is use of the proper film/screen combination. When the
automatic timer is calibrated, it is adjusted so the Neutral
setting will produce a properly exposed film for a specific
film/screen combination. If a cassette with a different
film/screen combination is used, the film will not have the
correct density because the AEC device cannot recognize changes
in film/screen speed. Figure 3.3-3 demonstrates how film/screen
speeds affect density.
Figure 3.3-3
The AEC device
was calibrated to be used with the film screen combination
demonstrated on the left. The film on the right has a
film/screen speed combination that is sixteen times slower
(extremity) than the film on the left. Both radiographs were
taken at the "N" setting.
Figure 3.3-4 is another example
of how film/screen combinations affect density. The machine was
calibrated to be used with the film/screen combination of the
radiograph on the left. The film on the right was taken with a
faster film/screen speed combination, resulting in a darker
film.
Figure 3.3-4
| When
using AEC devices, it is important to remember to use only
the film/screen combinations that were calibrated to be used
with the machine. |
It is evident that the previously
discussed causes of improper film density can be attributed to
"user error". All too frequently AEC devices "take the rap"
for improperly exposed radiographs. More often than not, AEC
errors are most likely the result of poor positioning and/or
collimation. However, there are situations where the
technologist performed the procedure correctly but ended up with
an improperly exposed film when using an AEC device at the
neutral density setting. Certain patient conditions such as
obesity, and ascites may affect the operation of the AEC.
Because of excessive scatter produced in these situations, the
AEC device may terminate the exposure prematurely.
Surgical intervention may also
have an affect on the ability of the AEC device to operate
properly. As discussed earlier, if a patient has had a
pneumonectomy and the detector that is selected is on the same
side as the pneumonectomy the image will not have the proper
density.
Another factor that may influence
the performance of the AEC is the presence of a prosthesis.
Should the detector be directly behind a prosthesis, the
radiograph will be too dark. This is easily explained by the
fact that the prosthesis will prevent radiation from reaching
the detector, which in turn increases the length of the
radiographic exposure.
Finally, one must also consider
machine failure if films taken using an AEC device do not have
the proper density.
3.3.1
CHANGING THE DENSITY SETTINGS
When a radiograph needs to be
repeated because of improper density, and it is not due
to user error such as incorrect positioning or improper
collimation, the density selector switch must be
used to change the density. Since the density of a radiograph
taken using AEC is determined by how long it takes to charge a
capacitor to the predetermined level, the mA and time selectors
no longer control the density of the radiograph as it did in
manual timing. Basically, when the density selector switch is
changed, the resistance in the timer circuit is also changed.
Increasing the density selector by using +1 or +2 increases the
resistance in the circuit, which in turn causes the capacitor to
take longer to be charged to its predetermined level. Therefore,
since it will take longer to charge the capacitor, the
radiograph will have greater density. Likewise, choosing -1 or
-2 on the density selector, deceases the resistance in the timer
circuit which allows the capacitor to be charged faster than
normal. This results in a film having decreased radiographic
density.
At this point, it would be
beneficial to review how the characteristics of kVp, mA and time
change when using AEC, compared to manual timing. This will
assist in understanding how to make corrections for improperly
exposed radiographs obtained while using AEC devices.
As with manual timing, kVp still controls contrast when using
AEC devices. However, the practice does exist where
technologists increase kVp to increase density when an
underexposed film is obtained while using AEC. This is an
incorrect solution to the problem since kVp primarily controls
contrast. A repeat film taken with higher kVp may appear
darker than the original, however that is due to the decreased
contrast of the film.
Figure 3.3.1-1 demonstrates
that as you increase kVp, the films do appear darker, but as
mentioned above, it is result of the change in the scale of
contrast.
Figure
3.3.1-1
|
Increasing kVp to produce a darker film should be avoided
because the resultant change in the scale of contrast may
affect interpretation of the radiograph. |
During
the use of the AEC device, the function of the mA control
changes dramatically. No longer does the mA setting govern
density, as it did while using manual timing. When using an
automatic timer, the mA control now influences the time of the
exposure. The mA selector still controls the quantity of
radiation produced, however when using AEC one must remember
the length of exposure is determined by the time is takes to
charge the capacitor to a specified level. Therefore when
using AEC devices, milliamperage influences the time of
exposure because when a higher mA is selected more radiation
is present. When more radiation is present, the capacitor is
charged more rapidly resulting in a shorter exposure.
The
timer control becomes inoperable when using AEC. This makes
sense because the main purpose of the AEC is to terminate the
exposure automatically. Certain manufacturers design their
equipment so that when AEC is selected, the timer control
becomes the backup timer control. This will be discussed in
greater detail later in this unit.
Based on the above information,
it should become obvious that when using the AEC device, density
changes should only be made by using the density selector.
The density selector allows the
technologist to increase or decrease the density in
predetermined increments. It is a good idea to know how each
density step affects the density of the radiograph so logical
predictions can be made of which setting to use should a repeat
be necessary. Most radiographic equipment is calibrated to that
+1 corresponds to a 25% increase in density from the "N" setting
and +2 corresponds to a 50% increase. The -1 setting corresponds
to 25% decrease in density compared to the "N" setting, while
the -2 setting corresponds to a 50% decrease. Be aware that the
density settings may be adjusted for any desired increments by
the service engineer.
Figure 3.3.1-2 demonstrates how
density selector changes directly affect film density. The
density settings are listed below each knee image. Evaluation
of these images with a densitometer did indicate the
following:
A 50% decrease in density
from Radiograph C (Neutral)
B 25% decrease
in density
D 25% increase
in density
E 50% increase
in density
A
B C
D E
-2
-1 N
+1 +2
IV. MINIMUM
RESPONSE TIME
Occasionally a technologist may
obtain an overexposed film on the -2 density setting. Even if
the technologist positioned the patient properly and correctly
collimated, there are times when the radiograph may still be too
dark. This situation is due to a property of automatic timers
known as minimum response time.
After the capacitor in the
automatic timer circuit has been charged to the predetermined
level, a signal is sent to terminate the exposure.
Unfortunately, the exposure is not terminated instantaneously.
Anywhere from three to thirty milliseconds may elapse before all
the electronics and relays actually stop the exposure. This
"lag" time is referred to as Minimum Response Time (MRT). MRT
poses a problem to the radiographer when the MRT is longer
than the time required for the exposure. In situations where
the MRT is longer than the required exposure time, the
radiograph will be too dark regardless of the density setting.
An example will help clarify this concept:
The minimum response time of a
machine is 5 milliseconds. An elderly woman with emphysema
requires a chest xray. As a result of her condition, the woman's
chest x-ray will only require a 2 millisecond exposure. Even
though the required time for the exposure is only 2
milliseconds, the actual exposure time will be 5 milliseconds
because it takes the machine a minimum of 5 milliseconds to
terminate an exposure. Therefore, the radiograph will be dark
when using the "N", "-1" or "-2" settings of the density
selector because the machine simply cannot shut off in two
milliseconds.
Problems with MRT are more common
in older equipment since the MRT of these units can be as long
as 30 milliseconds. Newer equipment is not faced with MRT
problems as frequently as older units because advances in
technology have reduced MRT's to as little as 1 millisecond.
4.1 SOLUTIONS
Problems associated with long
MRT's can be solved by the use of a lower mA setting, decreased
kVp, or simply using manual timing. The use of a lower mA
setting is the preferred method to correct MRT problems.
When using a lower mA setting,
the required time for the exposure is increased (less radiation
is present, therefore a longer exposure). The goal is to make
the present exposure time longer than the MRT of the machine. By
making the the exposure time longer than the MRT, the automatic
timer can now terminate the exposure at the proper time which in
turn will result in a film with the correct density.
If the mA cannot be selected
while using AEC (i.e. portable units equipped with AEC), the kVp
can be lowered which will lead to an increased exposure time.
Lower kVp settings result in decreased x-ray production,
therefore it will take longer to charge the capacitor to
appropriate level resulting in a longer exposure. However one
must be careful when using kVp to change exposure time because
of the affect on contrast.
Finally,the technologist should
always consider one definite way to solve problems associated
with the MRT of automatic timer and that is to use manual
timing.
V. THE BACK-UP TIMER
One other feature that must
addressed when discussing AEC devices is the back-up timer. In
order to minimize errors which lead to repeat radiographs, the
technologist must be fully aware of the purpose and operation of
the back-up timer.
The back-up timer is a safety
device which prevents the patient from receiving an excessive
dose of radiation should the automatic timer fail due to
mechanical or operator errors.
The back-up timer automatically
terminates a phototimed exposure if it exceeds 600mAs (Federally
mandated). Some radiographic units allow the operator to set the
back-up time, while others are fixed at 600 mAs. If the
radiography machine allows the back-up time to be adjusted, a
good rule of thumb to follow is to set the back-up timer for two
to three times the estimated mAs of the exposure. It is poor
practice to always set the back-up timer to the maximum level
because if the automatic timer fails, the patient will be
unnecessarily exposed to the radiation produced by a 600 mAs
exposure before radiation production is actually terminated. If
the machine allows the user to select the back-up time for the
exposure, the mA selector and timer control automatically become
the back-up timer selector.
5.1 REASONS
WHY THE BACKUP TIMER IS ACTIVATED
The are several reasons why the
back-up timer may be activated and the exposure terminated.
Excessively large patients or pathologic conditions may activate
the back-up timer. However, it is reasonably safe to say that
the main reason the back-up timer is activated during an
exposure is a careless technologist. At this point, it will be
helpful to review the most common errors that cause the back-up
timer to be activated:
Wrong
Detector Selected: If a patient is being
radiographed on the table and the chest board detectors are
accidently selected, the film will be too dark and the back-up
timer will be activated. This occurs because the selected
detectors on the chest board did not receive any radiation so
the exposure continued until it was terminated by the back-up
timer. Unfortunately, should this occur, the patient is exposed
by the primary beam much longer than is necessary.
Wrong
Tube Selected: If a procedure room has two
radiographic x-ray tubes and the tube over the bucky is not the
one that is energized, the exposure will continue until it is
terminated by the back-up timer. This is explained by the fact
that tube over the selected detector is not producing any
radiation, therefore the exposure would continue indefinitely if
not terminated by the back-up timer.
Incorrect
Tube/Bucky Alignment: If the x-ray tube is not
centered to the bucky, the back-up timer may be activated. Since
the detectors are located in the bucky assembly, they cannot
sense any radiation if they are not aligned with the tube.
Therefore, the exposure will continue longer than necessary.
Incorrect
Back-up Timer Setting: If the back-up timer is set
for too short of a time, the back-up timer will terminate the
exposure prematurely, resulting in an underexposed radiograph.
This happens most frequently if the previous exam used a short
manual time and the back-up timer was not set before the
exposure was made.
An important point to remember
about the back-up timer is that it is a safety feature which is
used to prevent the patient from receiving excessive radiation
due to machine failure. More often than not, the back-up timer
is activated because of technologist error instead of equipment
failure. Unfortunately, when the back-up timer is activated the
patient has already been improperly exposed to radiation, and a
repeat radiograph will be necessary resulting in additional
unnecessary radiation exposure to the patient. Therefore, a
radiologic technologist should always make sure all the
automatic timer controls are properly set before the
radiographic exposure is made.
VI. SUMMARY
The development of automatic
exposure control has brought with it the possibility of
decreased repeat rates and increased productivity. However, this
is only possible with a technologist who fully understands all
of the steps that are necessary to produce a diagnostic film
while using AEC. Technologists who do not have a complete
understanding of AEC may actually find themselves with an
increased repeat rate, since its use requires precise
positioning and thorough knowledge of the equipment. Perhaps a
better term for automatic exposure control should be "assisted"
exposure control because in reality the technologist does assist
the automatic timing device in producing a diagnostic image.
The following is a summary of the
steps to success when using AEC:
The performance of any automatic timer is dependent on the
knowledge and skill of the technologist.
Positioning is crucial when using AEC devices.
Choose the detectors that are directly below the dominant
area of interest.
The detectors cannot differentiate between secondary
and scattered radiation. Therefore, proper collimation is very
important.
The back-up timer is a safety feature and should be set
approximately 2-3x greater than the estimated mAs of the
exposure.
Density changes should only be made by changing the density
selector.
When using AEC, kVp changes should only be made to vary
contrast.
To decrease the density of dark films due to long minimum
response times, lower the mA.
If used properly, automatic timers can decrease repeat
rates and increase department efficiency.
VII.
REFERENCES/SUGGESTED READINGS
Carter, P.H., ed. Chesneys'
Equipment for Student Radiographers. Cambridge, MA:
Blackwell Scientific Publication, Inc., 1994.
Cornuelle, A.G. and Gronefeld, D.H.
Radiographic Anatomy Positioning: An Intergrated Approach.
Stamford, CT: Appelton & Lange, 1998.
Curry, T.S.; Dowdey, J.E.; Murry, R.C.
Christensen's Physics of Diagnostic Radiology 4th ed.
Malvern, PA: Lea & Febiger, 1990.
Thomspson, M.A.; Hattaway, M.P.; et
al. Principles of Imaging Science and Protection.
Philadelphia, PA: W.B. Saunders Company, 1994.
© 1998 Endless Education
Ventures.
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