This document discusses three key components of x-ray imaging systems: filters, beam restrictors, and grids. It describes how filters like aluminum are used to absorb low energy rays and reduce patient exposure. It explains the three main types of beam restrictors - aperture diaphragms, cones or cylinders, and collimators - and how they define the size and shape of the x-ray beam. It also outlines the purpose of grids in removing scattered radiation to increase image contrast, and the different grid types including linear, crossed, focused, and moving grids.
3. 3
X-ray Filtration
• X-ray machines have metal filters inserted
into the useful beam.
• The primary purpose is the remove the low
energy beam that reach the patient and are
absorbed superficially
• Metals such as aluminum will absorb the soft
low energy rays.
• This reduces the patient exposure.
4. Types of Filtration
• There are two types of filtration:
• Inherent Filtration
• Added Filtration:
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5. Inherent Filtration
• The absorption of
X-rays by the X-ray
tube and its housing is
called inherent
filtration
• In diagnostic x-ray
tubes the glass is equal
to about 0.5 mm Al.
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6. Added filtration
• One or two mm of
aluminum is added
filtration placed in the
collimator. This
filtration is generally
placed on the mirror of
the collimator
• The minimum filtration
for tube operated above
70 kVp is 2.5 mm Al
equivalence 6
Added Filter
7. BEAM RESTRICTORS
• X Ray beam restrictors is a device that is
attached to the X-ray tube housing, to
regulating the size and shape of an X-ray
beam
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8. 3 Types of beam-restricting
devices
• Aperture Diaphragm
• Cones or Cylinders
• collimator
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9. Aperture Diaphragm
• Diaphragm consists of a sheet of lead with a
hole in the center
• The size of the hole determines the size and
shape of the X-ray beam
• The aperture can be altered to any size and
shape
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11. Cones or Cylinders
• It will reduce the
penumbra
• Flare of the cone is
greater than the flare of
the x-ray beam
• These systems provide
limited number of field
sizes 11
12. Collimator
• It is the best X-ray restrictors
• It defines the size and shape of X-ray field
• It attached to the tube
• Collimator consists of two sets of shutters.
Namely Longitudinal and Transverse
• Which can be moved independently
• Shutter consists of four or more lead plates
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13. Cnt……
• The collimator also has a light and mirror
arrangement, to illuminate the X-ray field
• The light bulb is positioned laterally and the
mirror is mounted in the path of the X-ray
beam at an angle 45 degree
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14. Grids
• A beam of X-ray passes through the beam is
absorbed and scattered
• The absorbed primary beam gives a useful
shadow
• Scattered radiation will tend to spoil the
shadow
• Scattered radiation will increase the noise in
the image
14
15. Cnt….
• The ratio between the amount of scattered
radiation energy to the amount of primary
radiation energy at a point is called as
scattered to primary ratio(SPR)
• scattered radiation must be removed, in
order to increase the image contrast
15
16. Cnt…..
• The scattered radiation can be removed by a
grid
• The grid is placed between the film and the
patient
• Cardboard, aluminium, or wood are low
attenuating materials
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17. Cnt….
• Primary radiation is parallel in direction
• Scattered radiation is non-parallel direction
Grid ratio
• The ratio of the height to the width of the
spacer between the lead strips
Grid ratio=h/b
Range =4 to 16 strip line
Densities =25 to 60 lines per cm 17
18. Types of grid
1. Linear grid
2. Crossed grid
3. Focused grid
4. Parallel grid
5. Moving grid
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19. Linear Grid
• Lead strips are parallel to each other in their
longitudinal axis
• Most of X-ray table are provided with
linear grids
• It is possible to rotate the X-ray tube along
the length of the grid
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20. Crossed Grid
• Two superimposed linear grids that have
same focusing distance
Focused grid
• Focused grid made up of lead strip that are
angled slightly so that they focus in space
• A focused grid may be linear or crossed
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21. Parallel grid
• A parallel grid is one in which the lead
strips are parallel, when viewed in a cross
section
• It can be used with small X-ray fields
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22. Moving grid
• When a focused or parallel grid is used,
each lead strip will appear on the
radiograph as a very fine line.
• These lines may spoil the information in the
film.
• These lines may be removed by moving
grid during the radiographic exposure
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23. Cnt….
• The grid may be made to move
continuously in one direction
• The grid motion is timed by the exposure
control of the X-ray machine
• The travelling period should be greater than
the exposure time
• The use of grid will always increase the
exposure, because it will absorb some of the
primary radiation 23
24. Cnt…
• In order to reduce the exposure, grid with
smaller ratios should be preferred
• Low energy grids such as 8:1
• High energy grids such as 12:1
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