Radiographic exposure and image quality

1. Radiographic Exposure
•Exposure Factors influence and determine
the quantity and quality of the x-radiation
to which the patient is exposed.
•Radiation quantity refers to the radiation
intensity referred to as mR or mR/ mAs.
•Radiation Quality refers to the beam
penetrability and measured in HVL.

2. Radiographic Exposure
•The radiographic exposure factors are
under the control of the operator except
for those fixed by the design of the x-ray
machine.
•There are two choices for focal spot.
•With the exception of compensating filters,
added filtration is fixed.
•The type of high voltage power is also
fixed.

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

4. kVp
•As we have discussed in the laboratory,
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.

5. 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.

6. 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.

7. mA
•1 Ampere = 1 C/s = 6.3 x 10
18
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.

8. 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.

9. 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

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

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. mAs
•If we know the mR/mAs, multiply that
figure times the mAs. or
•If we know the mR for a given exposure
at a given kVp, we can divide the
exposure by the mAs to get the mR/ mAs.
•To compute exposure we need to know
what the mR/mAs is for the kVp used and
the SID.

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

20. Inverse Square Law
–mAs (second exposure) SID
2
2nd
exposure
–----------------------------= -------------------------
–mAs (first exposure) SID
2
1st exposure

21. 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.

22. 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.

23. 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.

24. Imaging System Characteristics
•Operator has limited control.
•The following will impact the technical
factors based upon the type of machine.
–Focal Spot Size
–Filtration
–High-voltage Generation

25. Focal Spot Size
•Most machines limited to two focal spot
sizes.
•Common office focal spots are 1.0 mm for
the small and 2.0 mm for large.
•Highly detailed radiography such as
mammography use micro-focus tubes with
0.1 mm and 0.3 mm focal spot sizes.

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
•For single phase machines, the small focal
spot use is limited to extremities and the
cervical spine.
•With high frequency, most views can be
done on the small focal spot except for
larger patient and ones that cannot hold
still.
•My limit is exposure times less than 1/2 s.

28. Focal Spot Size
•If the mA is properly calibrated, the focal
spot will have no impact on the quantity or
quality of the beam.

29. 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.

30. 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.

31. 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.

32. High-voltage Generation
•You will determine the type of high-voltage
generation when you purchase your x-ray
machine.
•The type of generator will determine the
efficiency of the generator or the amount
of ripple in the wave form.
•Single phase has 100% ripple.

33. Three Phase Generation
•Three phase has a 14% so it is significant
improvement in efficiency increasing both
quality and quantity of the beam.
•More x-rays per mAs with higher energy.
•Cost to provide 3 phase power is very high
so not practical in office.

34. High Frequency Generation
•Virtually no ripple ( less than 1%.)
•Inexpensive and can use normal incoming
power.
•Provides significant reduction is mAs or
kVp compared to single phase. Reduction
of mAs by 50% compared to single phase
techniques.

35. Chapter 19 Radiographic
Quality
•Radiographic Quality refers to the fidelity
with which the anatomic structures being
examined are images on the film.
•Three main factors:
–Film Factors
–Geometric Factors
–Subject Factors

36. Radiographic Quality
•Characteristic of radiographic quality:
–Spatial Resolution (Recorded Detail)
–Contrast Resolution (Visibility of Detail)
–Noise (Visibility of Detail)
–Artifacts

37. Spatial Resolution
•Spatial Resolution is the ability to image
small structures that have high subject
contrast such as bone-soft tissue
interface.
•When all of the factors are correct,
conventional radiography has excellent
spatial resolution.

38. Contrast Resolution
•Contrast resolution is the ability to
distinguish structures with similar subject
contrast such as liver-spleen, fat-muscle.
•Computed tomography and MRI have
excellent contrast resolution. Convention
radiology is fair to poor.

39. Noise
•Noise is an undesirable fluctuation in
optical density of the image. Two major
types:
–Film Graininess- no control over
–Quantum Mottle- some control over

40. Film Graininess
•Film graininess refers to the distribution in
size and space of the silver halide grains
in the film emulsion.
•Similar to photographic film. 400 ASA film
is more graininess than 100 ASA film.
•Similar to structure mottle that refers to
the size and shape of the phosphors in the
intensifying screens.

41. Quantum Mottle
•Quantum mottle refers to the random
nature of how the x-rays interact with the
image receptor.
•It is the primary form of radiographic
noise.
•The use of high mAs and low kVp reduced
quantum mottle.

42. Quantum Mottle
•Very fast screens have higher quantum
mottle because it takes fewer x-rays to
make the image.

43. Speed
•Resolution and noise are intimately
connected with speed.
•While the speed of the images receptor is
not apparent on the image, it influences
both resolution and noise.

44. Radiographic Quality Rules
•Fast Image receptors have high noise and
low spatial and contrast resolution.
•High spatial and contrast resolution
require low noise and slow image
receptors.
•Low noise accompanies slow image
receptors with high spatial and contrast
resolution.

45. Film Factors of Quality
•Characteristic curve
–Density
–Contrast
–Latitude
•Processing
–Time
–Temperature

46. Sensitometry
•Sensitometry is the study of the
relationship between the intensity of
exposure of the film and the blackness
after the film is processed.
•Unexposed film is clear with a blue tint
after processing.
•Exposed film is black after processing.

47. Sensitometry
•Two principles involved.
–Exposure of the film
–Amount of light transmitted through the
processed film of optical density.
•Used to describe the relationship of
radiation exposure and blackness or
density on the film.

48. Characteristic Curve
•This relationship is
called the
characteristic curve or
H & D curve of the
film.
•H & D stands for
Hurter and Driffield.

49. Parts of the Characteristic
Curve
•Toe and shoulder
where large changes
in exposure results in
small changes in OD.
•Very high and very
low variations of
exposure make very
small changes in
density.

50. Parts of the Characteristic
Curve
•The straight line or
intermediate area is
where very small
changes in exposure
results in large
changes in density.
•This is the important
part of the curve in
radiography.

51. Log Relative Exposure (LRE)
•X-ray films responds
to a wide range of
exposure from 1 mR
to 1000 mR.
•Exposure is
represented on
logarithmic manner.

52. Optical Density Range
•The optical density
range is from 0.0 for
no density to 4.0 for
absolute black.
•Useful range in
general radiography
is from 0.5 to 2.25.
•Image range is 0.5 to
1.25 OD

53. Base fog or base density
•The tint of the base of
the film and the
inadvertent exposure
of the during
processing.
•Range is from 0.1 to
0.3. Should be never
above 0.30 most is .
21 OD

54. Items that Impact Base Fog
•Film storage
•Film exposure to wrong spectrum of light
or light intensity.
•Chemical contamination.
•Improper processing.
•High Base fog levels reduce contrast.

55. Contrast
•Radiographic Contrast is the combined
result of image receptor contrast and
subject contrast.
•Image receptor contrast refers to the
contrast inherent in the film and influenced
by the processing of the film.

56. Contrast
•Subject contrast is determined by the size,
shape and x-ray attenuating
characteristics of the subject being
examined and the energy (kVp) of the x-
ray beam.

57. Image Receptor Contrast
•Inherent to the film and screen
combination but is influenced by:
–Range of Optical Density
–Film Processing Technique
•Film type is determined by the type of
intensifying screens used but many
dealers sell off brands of film.

58. Image Receptor Contrast
•The slope of the
straight line portion of
the H & D curve is the
receptor contrast.
•The average gradient
is a straight line
drawn between the
densities of 0.25 and
2.00 + base fog.

59. Average Gradient
•The average gradient
is a straight line
drawn between 0.25
OD and 2.0 OD
above base plus fog.
•This is the normal
range of density in a
radiograph

60. Speed
•Speed is the ability of
the receptor to
respond to low x-ray
exposure.
•The H & D curse is
useful in comparing
speed when selecting
film or screens.

61. Speed
•A relative number of 100 given to Par
Speed Calcium Tungstate Screens.
•High Speed Calcium Tungstate has a
speed of 200. Half of the exposure is
needed to produce the same image.
•Rare earth screen film combinations range
is speed from 80 to 1600.

62. Speed
•By knowing the Speed, sometimes
referred to as the Relative Speed Value, it
is easy to convert the technical factors for
one speed to another speed.

63. LATITUDE
•Latitude can be
observed on the H &
D curve.
•Latitude refers to the
range of exposure
that will produce a
diagnostic range OD.

64. Latitude
•Latitude and Contrast
are inversely
proportional.
•Wide latitude has a
wide gray scale or low
contrast. (B)
•Narrow latitude has a
short scale or high
contrast. (A)

65. Latitude
•Latitude is designed into some screen and
film combinations. With wide latitude, the
error factor in technique is wider.
•Latitude can also be impacted by the
technical factors.

66. Film Processing
•Radiographic Quality
is impacted by film
processing
parameters.
•The developer must
be at the proper
concentration and at
the correct
temperature.

67. Film Processing
•The film must also
spend the correct
amount of time in the
developer.
•This is the time &
temperature
relationship.

68. Processing
•Speed and base fog increase with the
temperature.
•Contrast will increase to a point and then
drop with the base fog increase.
•Manufactures set processing parameters
to optimize speed, contrast and low base
fog.

69. Processing
•In 9th Quarter we will discuss processor
quality control in detail.

70. End of Lecture
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