X ray Filters

1. Filters, Collimators and Grids
Christensen’s physics of
diagnostic radiology

2. Filters
Filtration is the process of shaping the
X-ray beam to increase the ratio of
photons useful for imaging to those
photons that increase patient dose or
decrease image contrast

3. Diagnostic X-ray beam is
polychromatic, comprising of whole
spectrum of energies. The mean
energy will vary from 1/3rd to one half
of the their peak energy. The first few
centimeters of the tissues receive
much more radiation than the rest of
the body tissues of the patient

4. Filters
Filters are sheets of metal, attached
at the opening of tube housing, which
will absorb the low energy photons
from the X-ray beam before it reaches
the patient

5. Types of filtration
The X-ray beam is filtered by absorbers at
three different levels
Inherent Filtration
Added filtration
Patient

6. Inherent filtration
Glass envelope
The insulating oil surrounding the tube
Window in the tube housing

7. Added filtration
Aluminum At.No. 13
Copper At. No. 29
Compound Filter copper + Aluminum

8. Advantage of compound filtration
Copper is used to cut down the thickness
of filter
Copper will absorb high energy photons
and Aluminum will absorb the
characteristic radiation from copper
(8 keV)

9. Measurement of filtration
Filtration is measured in Aluminum
equivalents which is defined as the
thickness of the Aluminum that would
produce the same degree of
attenuation as the thickness of
material in question

10. Effects of filtration
Patient exposure and
Exposure factors

11. 60 –kVp beam
Aluminum Exposure dose Decrease in
filtration (mm) to skin (mR) exposure dose (%)
None 2380
0.5 1850 22
1.0 1270 47
3.0 465 80

12. Inherent filtration at the tube housing is
0.5 to 1.0 Al.Eq.
Below 50 kVp 0.5 mm Aluminum
50-70 kVp 1.5 mm Aluminum
Above 70 kVp 2.5 mm Aluminum

13. Effect on exposure factors
There will be reduction in the intensity of
X-ray beam as the filters absorb some
photons at all energy levels.
To Compensate the loss of high energy
photons, increase in the exposure
factors (mAs) is required

14. Types of filters
Single Filter Aluminum
Compund Filter Aluminum +Copper
Wedge Filter Used in the
angiography
Molybdenum Filters used in
Mammography.
K. Edge filter

15. Molybdenum filters
Used in molybdenum target X-ray tubes
used for mammography
17.5 kev K alpha and 19.6 kev K beta
characterstic radiation of Mo
When operated at 30-40 kVp, Mo will
produce bremsstrahling with energies
higher than 20 kev
Mo filter attenuates these high energy rays

16. K- Edge filters
These filters make use of K absorption edge
of elements with atomic No. greater than 60.
The purpose of heavy metal filters or K edge
filters is to produce an X-ray beam that has a
high number of photons in the specific energy
range
Enhance contrast for Iodine and barium,
reduce patient dose, and increase tube
loading

17. X-ray beam restrictors

18. X-ray beam restrictors
An X-ray beam restrictor is a device
that is attached to the opening in the
X-ray tube housing to regulate the
size and shape of the X-ray beam

19. Types of restrictors
Aperture diaphragms
Cones & cylinders
Collimators

20. Aperture diaphragm
It consists of a sheet of lead with a hole in
the center. The size and shape of the hole
determine the size and shape of the X-ray
beam.

21. Cones
Cones are usually flare shaped
Ideal geometric configuration for an
X-ray beam restrictor.
The flare of the cone is greater than the
flare of the x ray beam.

22. Cylinders
Beam restriction with cylinder takes
place at the far end of the barrel, so
there is less penumbra.

23. Aperture diaphragm cone cylinder
P

24. Disadvantages
Penumbra: Partially exposed periphery of
the X-ray field is called penumbra
Another major disadvantage with cones
and cylinders is severe limitations they
place on the number of available field
sizes

25. Collimators
Collimator is the best all round X-ray
beam restrictor
These are two types:
Manual collimator
Automatic collimator or PBL (Positive
beam limitation device or automatic light
localised variable aperture collimator)

26. Advantages
It provides infinite variety of rectangular
or square X-ray fields
The light beam shows the exact center
and configuration of X-ray field

27. Construction of collimator
X ray
filter
mirror
X ray
&light
beam
bulb
Lower
shutter
Collimator shutters

28. Testing of X- ray beam and light
beam alignment
Periodic check of alignment of
X-ray beam and light beam is
essential, because the mirror gets
out of adjustment due to frequent
daily use.

29. Automatic collimators
When the cassette is loaded in the Buckey
tray the sensors in the tray identify the size
and alignment of the cassette and relay
the information to collimator motors, which
positions shutters to the exact size of the
film used.

30. Functions of X ray beam restrictors
Protects the patient from unnecessary
radiation
It decreases the scatter radiation

31. The number of scattered radiation depends upon
field size
Small fields generate little scatter, as the field
increases scatter increases
Collimators are only successful in decreasing the
scatter radiation with small fields, so we should
reduce the size of X-ray beams as much as
possible

32. GRIDS

33. The radiographic grid consists of a series of
lead foil strips separated by X ray transparent
spacers.
It was invented by DR.GUSTAVE BUCKY in
1913.
Grid is still the most effective way of removing
the scatter radiation from large radiographic
fields

34. Primary radiation is oriented in the same axis as
the lead strips and passes between them .
Scatter radiation arises from many points within
the patient and most of it is absorbed by the lead
strips

35. The interspaces of the grids are filled either
with aluminium or some organic compound.
The main purpose of the interspace
material is to support the thin lead foil
strips.

36. Is defined as the ratio between the height of
the lead strips and the distance between them.
GRID RATIO

37. The lead strips are approximately 0.05 mm
thick ( lead foil).
The interspaces are much thicker.
Grid ratios are usually expressed as two
numbers, such as 20:1
Ratios usually range from 4:1 to 16:1
the Higher the ratio, the better the grid
functions.

38. Grid pattern
Is the orientation of the lead strips in their
longitudinal axis.
The two basic grid patterns are :
Linear and
Crossed.

39. They allow us to angle the x-ray tube along the
length of the grid .
Linear grid

40. A crossed grid is made up of two superimposed linear
grids that have the same focussing distance
The grid ratio of crossed grids is
equal to the sum of the ratios of
the two linear grids.
A crossed grid made up of two 5:1
linear grids has a ratio of 10:1.
Crossed grids cannot be used with oblique
techniques requiring angulation of the X-ray tube
Crossed grids

41. Is a grid made up of lead strips that are angled slightly so
that they focus in space.
A focussed grid may be either linear or crossed.
Linear focused grids converge at a line in space called
the convergent line.
Crossed grids converge at a point in space called the
convergent point.
The focal distance is the perpendicular distance
between the grid and the convergent line or point.
Focussed grid

42. Focussing range
Indicates the distance within which the grid can be used
without significant loss of primary radiation
It is fairly wide for a low-ratio grid and narrow for a high
ratio grid.
A 5:1 grid focused at 40 inches has a focusing range of
approximately 28 to 72 inches.
While a 16:1 grid focused at 40inches has a range of only 38
to 42inches.

43. Parallel grid
A parallel grid is one in which the lead strips are parallel
They are focused at infinity.
can only be used with either very small Xray fields or long-
target grid distances.
They are frequently used in fluoroscopic spot film
devices.

44. Lines per inch
Is the number of lead strips per inch of the grid.
Lines per inch = 25.4/D+d
D= thickness of the interspaces
d=thickness of the lead strips(both in millimeters)

45. Grid cassette
Usually used for portable radiography ,
with a grid built in to the front of the
cassette.
Are focussed and
Have a grid ratio of 4:1 to 8:1

46. Evaluation of grid performance
The three methods of evaluating grid
performance:
1.Primary transmission(Tp)
2.Bucky factor (B)
3.Contrast improvement factor(K)

47. Primary transmission
Is the percentage of primary radiation transmitted
through the grid.
Ideally , a grid should transmit 100% of the primary
radiation.
The first measurement is made
with the grid in place
The second measurement is
made after removal of the grid

48. A ratio of the intensity with the grid to the
intensity without the grid gives the
fractional transmission, which is
multiplied by 100 to give the percentage of
transmission.
intensity with grid Ip
Tp = _____________________ X100
intensity without grid I’p
There is a significant loss of primary
radiation with grids, more with cross grids

49. Measured primary tranmission
The measured primary transmission is
always less than the calculated primary
transmission:
absorption by the interspace material
manufacturing imperfections

50. Bucky factor
Is the ratio of the incident radiation falling on the grid to the
transmitted radiation passing through the grid.
It indicates how much we must increase exposure factors
when we change from a non grid to a grid technique.
The Bucky factor indicates the absorption of both primary and
secondary radiation.
It is determined with a
large X-ray field and a
thick phantom

51. The Bucky factor( B) is a measure of the total
quantity of radiation absorbed from an X-ray
beam by a grid
incident radiation
B = ____________________
transmitted radiation
The transmitted radiation is measured with the
grid in place ,and
The incident radiation is measured after the grid
has been removed

52. Grid ratio 70 kVp 120 kVp
No grid 1 1
5:1 3 3
8:1 3.5 4
12:1 4 5
16:1 4.5 6
High ratio grids absorb more scatter radiation
and have larger Bucky factors than low-ratio
grids
Higher energy beams generate more scatter
radiation and place a greater demand on a grid’s
performance than lower energy radiation.

53. The higher the Bucky factor,the
greater the exposure factors and
radiation dosage to the patient.
If the Bucky factor for a particular grid-
energy combination is 5, then exposure
factors and patient exposure both
increase 5 times

54. Contrast improvement factor (K)
The contrast improvement factor(K) is the ratio of
the contrast with a grid to the contrast without a
grid.
contrast with a grid
K = _________________
contrast without a grid
Is the ultimate test of grid performance.
The contrast improvement factor is dependent on
kVp,field size and phantom thickness.
These three factors determine the amount of
scatter radiation

55. The larger the amount of scatter radiation,the
poorer the contrast, and the lower the
contrast improvement factor.
It is more closely related to the lead content
of the grid than any other factor.
Generally, the higher the grid ratio,the
higher the contrast improvement factor.

56. Lead content
The lead content of a grid is expressed in gm/cm2
Imagine cutting a grid up in to 1 cm squares and then
weighing one square.
Its weight in grams is the lead content of the
grid(ignoring the interspace material)
The amount of lead in a grid is a good indicator of
its ability to improve contrast.
There is a definite relationship between the grid
ratio,lead content and the number of lines per inch.

57. when grids are constructed with many lines per
inch, both the thickness and height of the lead
strips are decreased.
These grids are thinner, and improve contrast
less than grids of comparable ratios with fewer
lines per inch.

58. Grid cut off
Grid cut off is the loss of primary radiation that occurs when
the images of the lead strips are projected wider than they
would be with ordinary magnification
It is the result of a poor geometric
relation ship between the primary
beam and the lead foil strips .
The resultant radiograph will be
light in the area in which the cutoff
occurs.
With linear grids there may be
uniform lightening of the whole film,
one edge of the film ,or both edges
of the film,depending on how the
cutoff is produced.

59. The amount of cutt off is always greatest with high
ratio grids and short grid focus distance
There are 4 situations that produce grid cutt
off
1.Focussed grids used upside down
2.Lateral decentering
3.Focussed grid distance decentering
4.Combined lateral and focus grid distance
decentering

60. Upside down focussed grids
When a focused grid is used upside down, there is
severe peripheral cutoff with a dark band of
exposure in the center of the film and no exposure at
the film’s periphery

61. Lateral decentering
Results from the X-ray tube being positioned
lateral to the convergent line but at the correct
focal distance

62. All the lead strips cutoff the same amount of primary
radiation,so there is a uniform loss of radiation over
the entire surface of the grid ,producing a uniformly
light radiograph
This is probably the most common kind of grid
cutoff, but it cannot be recognised by inspection of
the film.
All we see is a light film (that is usually attributed to
incorrect exposure factors.)
The films become progressively lighter as the
amount of lateral decentering increases.

63. Three factors affect the magnitude of
cutoff :
Grid ratio
Focal distance and
Amount of decentering
The equation for calculating the loss of primary radiation
with lateral decentering is
L=rb/fo x 100
L= loss of primary radiation(%)
r=grid ratio
b=lateral decentering distance (inches)
fo =focal distance of grid (inches)

64. When exact centering is not
possible , as in portable
radiography, low ratio grids
and long focal distances should
be used whenever possible

65. Off level grids
When a linear grid is tilted , as it frequently is
in portable radiography, there is a uniform loss
of primary radiation across the entire surface
of the grid

66. Focus-grid distance decentering
In focus -grid distance decentering , the target of the X-ray
tube is correctly centered to the grid , but it is positioned
above or below the convergent line.
If the target is above the convergent line ,it is called FAR
focus-grid distance decentering
If the target is below the convergent line , it is called NEAR
focus-grid distance decentering.
The cut off is greater with NEAR than with FAR focus-grid
distance decentering.
The cutoff becomes progressively greater with increasing
distance from the film center.

67. The central portion of the film is not affected, but the
periphery is light.

68. Parallel grids are focused at infinity
A film taken with a parallel grid has a dark center
and light edges because of near focus-grid distance
decentering

69. Combined lateral & focus-grid distance decentering
The most commonly recognised kind of grid cutoff is from
combined lateral and focus grid distance decentering
It causes an uneven exposure,resulting in a film that is light
on one side and dark on the other side.
The projected images of the lead strips directly below the
tube target are broader than those on the opposite side, and
the film is light on the near side.
Cutoff is greatest on the side directly under the Xray tube.

70. The projected images of the lead strips are broader
on the side opposite the tube target than on the
same side, and the film is light on the far side.
Cutoff is least on the side under the Xray tube.

71. Moving grids
Grids are moved to blur out the shadows cast by
the lead strips.
Most grids are reciprocating,which means they
continuously move 1 to 3 cms back and forth
through out the exposure.
They start moving when the Xray tube anode
begins to rotate.
They eliminate grid lines from the film

72. Moving grids precautions
The grid must move fast enough to blur its lead
strips
The transverse motion of the grid should be
synchronous with the pulses of the Xray
generator

73. Disadvantages
They are costly
Subject to failure
May vibrate the Xray table
Put a limit on the minimum exposure
time because they move slowly
increase the patient’s radiation dose

74. Grid selection
Usually 8:1 grid will give adequate results below 90kVp.
Above 90kVp,12:1 grids are preferred.
There is little decrease in
transmitted scatter beyond an
8:1 ratio grid,
And almost no change
between 12:1 and 16:1
For this reason12:1 grids are
preferable to 16:1 grids for
routine radiography

75. Air gap technique
Scattered radiation arising in a patient
from comption reactions is dispersed in all
directions.
With an air gap the concentration of
scatterd radiation decreases because
scatters photons fail to reach the film

76. Used in 2 clinical situations
• Magnification radiology
• Chest radiology
With magnification techniques the object -film
distance is optimised for the screen focal spot
combination and the air gap technique reduces
the scatter radiation
In chest radiography the focal film distance is
usually lengthened from 6-10 ft to restore
sharpness

77. Exposure factors with air gaps
X-ray tube exposure must be increased for
the air gap technique because of larger
focal film distance
Patients exposures are usually less with
air gap technique
The air gap loses less primary radiation,
so the patient’s exposure is less

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