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EXPOSURE FACTORS
R Hussein Ahmed Hassan
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Exposure factors are factors that control
density (blackening) and contrast ofradiographic image.
They are some of the tools that
technologists use to create high-quality
radiographs
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Exposure Factors Controlled by the
Operator
kVp
mA times Exposure Time = mAs
etermines the quality and quantity of
the exposure
FF (SI ), Focal Spot and Filtration are
secondary factors
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1- EXPOSURE FACTORS:
VP. :
It controls the quality of the beam, i.e.
PENETRATION.
It influences :
a: penetration power, i.e. beam quality;
kVp. penetration power.
b: Radiographic contrast;
kVp. 1/radiographic contrast.
c: Radiation dose to patient.
kVp. 1/radiation dose.
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KVP
kVp controls radiographic contrast.
kVp determines the ability for the beam
to penetrate the tissue.
kVp has more effect than any other
factor on image receptor exposure
because it affects beam quality.
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KVP
To a lesser extent it also influences the
beam quantity.
As we increase kVp, more of the beam
penetrates the tissue with higher energy
so they interact more by the Compton
effect.
This produces more scatter radiation
which increases image noise and
reduces contrast.
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KVP
50 kV 79% is photoelectric, 21%
Compton, < 1% no interaction
80 kVp 46% is photoelectric, 52%
Compton 2% no interaction
110 kVp 23% photoelectric, 70%
Compton, 7% no interaction
As no interaction increases, less
exposure is needed to produce the
image so patient exposure is decreased.
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High kVp.
low radiographic contrast
Low kVp.
High radiographic contrast
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MA.:
1 Ampere = 1 C/s = 6.3 x 1018 electrons/
second.
The mA selected for the exposure
determines the number of x-rays
produced.
The number of x-rays are directly
proportional to the mA assuming a fixed
exposure time.
100 mA produced half the x-ray that 200
mA would produce.
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MA
Many x-ray machines are identified by
the maximum mA or mAs available.
A MP 500 has a maximum mAs of 500
mAs.
A Universal 325 has a maximum mA of
300 and maximum kVp of 125
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MA
More expensive three phase machines
will have a higher maximum mA.
A General Electric MST 1050 would have
1000 mA and 150 kVp.
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EXPOSURE TIME
The exposure time is generally always
kept as short as possible.
This is not to reduce patient exposure
but to minimize motion blur resulting
from patient movement.
This is a much greater problem with
weight bearing radiography.
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EXPOSURE TIME
Older machine express time as a
fraction.
Newer machines express exposure time
as milliseconds (ms)
It is easy to identify the type of high
voltage generation by looking at the
shortest exposure time.
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EXPOSURE TIME
Single phase half wave rectified fasted
exposure time is 1/60 second 17 ms.
Single phase full wave rectified fastest
exposure time is 1/120 second or 8 ms
Three phase and high frequency can
provide exposure time down to 1 ms.
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(4) MAS. :
It affect the total number of x-ray
produced by the tube during exposure, i.e.
QUANTITY.
It is the product of two quantities;
mA. the tube current;
s. the exposure time;
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MAS
mA and exposure time is usually
combined and used as one factor
expressed as mAs.
mAs controls radiation quantity, optical
density and patient dose.
mAs determine the number of x-rays in
the beam and therefore radiation
quantity.
mAs does not influence radiation quality.
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MAS
Any combination of mA and time that will
give the same mAs should provide the
same optical density on the film. This is
referred to as the reciprocity law.
As noted earlier for screen film
radiography, 1 ms exposure and
exposure longer than 1 seconds do not
follow this rule.
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MAS
On many modern machines, only mAs
can be selected. The machine
automatically gives the operator the
highest mA and shortest exposure time.
The operator may be able to select mA
by what is referred to as Power level.
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MAS
mAs is one way to measure electrostatic
charge. It determines the total number
of electrons.
Only the quantity of the photons are
affected by changes in the mAs.
Patient dose is therefore a function of
mAs.
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20 mA. X 1.0 s = 20 mAs
40 mA. X 0.5 s = 20 mAs
80 mA. X 0.25 s = 20 mAs
200 mA. X 0.1 s = 20 mAs
400 mA. X 0.05s = 20 mAs
Ampere is 1 coulomb (C) of electrostatic
charge flowing each second.
1A = 1C/s = 6.3 X 10
18
electron/s
20 mAs = 0.2 Amperes.
This charge releases this No. of electrons:
6.3 X 10
18
X 0.2 = 1.26 X 10
18
electron/s
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(5) Focal spot:
Most x-ray tubes offer two focal spot
sizes:
a. Fine focus:
b. Broad focus:
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a/ Fine focus: (0.3 0.6 mm
2
)
It records fine details.
It can not withstand too much heat.
Its usage may require long exposure
time.
Used whenever geometric factors are
more (long subject-film distance, short
FF ... etc).
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a/ Broad focus: (0.6 1.2 mm
2
)
It can withstand too much heat.
Always used in combination with short
(s) and fast film/screen system.
Used whenever voluntary or
involuntary motion is highly expected.
Used when radiosensitive organ is
within exposed area or 10 cm from
collimation border.
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Two focal spot
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F
OCAL
S
POT
S
IZE
The focal spot size limits the tubes
capacity to produce x-rays. The
electrons and resulting heat are
placed on a smaller portion of the
x-ray tube.
The mA is therefore limited for the
small focal spot. This results in
longer exposure times with greater
chance of atient movement.
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FOCAL SPOT SIZE
If the mA is properly calibrated, the
focal spot will have no impact on the
quantity or quality of the beam.
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(6) F.F.D. :
The intensity of x-ray beam reduces with
increased FF .
It follows the Inverse Square Law ( I.S.L.) .
I 1/d
2
.
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D
ISTANCE
istance affects the intensity of the x-
ray beam at the film but has no effect on
radiation quality.
istance affects the exposure of the
image receptor according to the inverse
square law.
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INVERSE SQUARE LAW
mAs (second exposure) SI 2 2nd exposure
---------------------------- = ------------------------
mAs (first exposure) SI 2 1st exposure
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ISTANCE
The most common source to image
distances are 40 (100 cm) and 72(182
cm)
Since SI does not impact the quality of
the beam, adjustments to the technical
factors are made with the mAs.
To go from 40 to 72 increase the mAs
3.5 time.
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ISTANCE
Increasing the distance will impact the
geometric properties of the beam.
Increased SI reduces magnification
distortion and focal spot blur.
With the need to increase the mAs 3.5
times for the 72 SI , tube loading
becomes a concern.
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ISTANCE
72 SI is used for Chest radiography
and the lateral cervical spine to reduce
magnification.
72 SI used for the full spine to get a
36 beam.
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(7) FILTERATION:
Thin sheet of Al (aluminum) 1mm or 2mm
thick added to the pathway of radiation to
filter the low energy radiation.
Increasing filtration will increase the
quality and reduce the quantity of the
beam.
It removes low energy radiation:
Reduce skin dose;
Harden the beam;
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F
ILTRATION
All x-ray beams are affected by the
filtration of the tube. The tube housing
provides about 0.5 mm of filtration.
Additional filtration is added in the
collimator to meet the 2.5 mm of
aluminum minimum filtration required by
law.
2.5 mm is required for 70 kVp.
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FILTRATION
3.0 mm is required for at 100 kVp.
3.2 mm is required for operations at 120
kVp.
Most machines now are capable of over
100 kVp operation.
We have no control on these filters.
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FILTRATION
3.0 mm is required for at 100 kVp.
3.2 mm is required for operations at
120 kVp.
Most machines now are capable of
over 100 kVp operation.
We have no control on these filters.
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FILTRATION
C
HIROPRACTIC RA IOGRAPHY IS A LEA ER IN
THE USE OF COMPENSATING FILTERS
. W
E
HAVE TOTAL CONTROL OVER COMPENSATING
FILTRATION
.
IN AREAS OF THE BO Y WITH HIGH SUBJECT
CONTRAST OR WI E IFFERENCES IN ENSITY
,
COMPENSATING FILMS IMPROVE IMAGE
QUALITY AN RE UCE PATIENT EXPOSURE
.
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THE END
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