Exposure factors2

1. EXPOSURE
FACTORS
DR Hussein Ahmed Hassan

2. Exposure factors are factors that control
density (blackening) and contrast of
radiographic image.
They are some of the tools that
technologists use to create high-quality
radiographs

3. Exposure Factors Controlled by
the Operator
kVp
mA times Exposure Time = mAs
Determines the quality and
quantity of the exposure
FFD (SID), Focal Spot and
Filtration are secondary factors

4. 1-EXPOSURE
FACTORS:
 KVP. :
It controls the quality of the beam, i.e.
PENETRATION .
It influences :
a: penetration power, i.e. beam
quality;
kVp. a penetration power.
b: Radiographic contrast;
kVp. a 1/radiographic
contrast.
 
c: Radiation dose to patient.
kVp. a 1/radiation dose.

5. 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.

6. 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.

7. 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.

8. High kVp.
low radiographic
contrast
Low kVp.
High radiographic
contrast

9.  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.

10. MA
Patient dose is also directly
proportional to the mA with a fixed
exposure time.
A change in mA does not affect
kinetic energy of the electrons
therefore only the quantity is
changed.

11. 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

12. MA
More expensive three phase
machines will have a higher
maximum mA.
A General Electric MST 1050 would
have 1000 mA and 150 kVp.

13.  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.

14. 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.

15. 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.

16. (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;

17. 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.

18. 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.

19. 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.

20. 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.

21. 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

22. (5) Focal spot:
Most x-ray tubes offer two focal
spot sizes:
a.Fine focus:
b.Broad focus:

23. 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 FFD ... etc).

24. 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.

25. Two focal spot

26. FOCAL SPOT SIZE
The focal spot size limits the
tube’s 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
patient movement.

27. FOCAL SPOT SIZE
If the mA is properly calibrated,
the focal spot will have no impact
on the quantity or quality of the
beam.

28. (6) F.F.D. :
The intensity of x-ray beam reduces
with increased FFD.
It follows the Inverse Square Law
( I.S.L.) .
I a 1/d
2
.

29. DISTANCE
Distance affects the intensity of
the x-ray beam at the film but has
no effect on radiation quality.
Distance affects the exposure of
the image receptor according to
the inverse square law.

30. INVERSE SQUARE LAW
mAs (second exposure) SID2 2nd
exposure
---------------------------- =
------------------------
mAs (first exposure) SID2 1st
exposure

31. DISTANCE
The most common source to image
distances are 40” (100 cm) and
72”(182 cm)
Since SID 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.

32. DISTANCE
Increasing the distance will impact
the geometric properties of the
beam.
Increased SID reduces
magnification distortion and focal
spot blur.
With the need to increase the mAs
3.5 times for the 72” SID, tube
loading becomes a concern.

33. DISTANCE
72” SID is used for Chest
radiography and the lateral
cervical spine to reduce
magnification.
72” SID used for the full spine to
get a 36” beam.

34. (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;

35. FILTRATION
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.

36. 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.

37. 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.

38. FILTRATION
CHIROPRACTIC RADIOGRAPHY IS
A LEADER IN THE USE OF
COMPENSATING FILTERS. WE
HAVE TOTAL CONTROL OVER
COMPENSATING FILTRATION.
IN AREAS OF THE BODY WITH
HIGH SUBJECT CONTRAST OR
WIDE DIFFERENCES IN DENSITY,
COMPENSATING FILMS IMPROVE
IMAGE QUALITY AND REDUCE
PATIENT EXPOSURE.

39. THE END