filters grids beams
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Transcript of filters grids beams
IMAGING
KARTHIK . A
2K13MP01
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Outline
1. Filters
2. Beam restrictors
3. Grids
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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.
Types of Filtration
• There are two types of filtration:
• Inherent Filtration
• Added Filtration:
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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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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
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Added Filter
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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3 Types of beam-restricting devices
• Aperture Diaphragm
• Cones or Cylinders
• collimator
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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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DISADVANTAGE
• Produce large penumbra
PENUMBRA
Away from the target[tumor]
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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
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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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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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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
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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
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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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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
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Types of grid
1. Linear grid
2. Crossed grid
3. Focused grid
4. Parallel grid
5. Moving grid
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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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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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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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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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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
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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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Thank you
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