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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• 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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• 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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