Beam restriction devices in x rays by sushil pattar

BY
Sushil Pattar(Radiographer)

Introduction
Beam restricting Devices
Filter and types
Grids
Conclusion
CONTENTS

 The X-Ray beam emerging from tube head is
composed of large numbers of photons that travels in
straight line in all direction.
 It is important to limit the size of the primary beam
and prevent exposure of body region which are not
required in image production.
 Large X-Ray fields result in unnecessary exposure,
thus scatter radiation is increased and image contrast is
decreased.
 Beam restricting devices limit the field size to reduce
scatter and primary radiation.
INTRODUCTION

 X-Ray tube are designed so that
the projectile electrons interact
with the target.
 Some of the electrons bounce
off the focal spot and interact
with other areas of target,
causing X-Ray production
outside of the focal spot.
OFF FOCUS RADIATION

 The scatter radiation arises from an object after
interaction of primary beam.
 In diagnostic radiology scatter radiation leads to Fog
in the X-Ray image.
SCATTER RADIATION

SCATTER RADIATION
 The secondary radiation arises from an object
after interaction of primary beam.
 In diagnostic radiography scatter radiation has no
use, it makes unfavourable contribution to the
formation of the xray image and it seriously
deteriorates contrast in the radiograph.
 Scatter radiation increases with..
- Increased kVp and mAs
- Increased field size
- Thicker body part.

Diaphragm apertures
Cones and cylinders
X-ray collimators
BEAM RESTRICTIVE DEVICES

 It is a simplest device.
 A sheet of lead with a hole
in the centre.
 Determination of size and
shape of X-Ray beam.
DIAPHRAGM APERTURE

DISADVANTAGE
 The size and shape is fixed.
 It produced large penumbra.
 Reduce Sharpness and resolution.

 Modification of the aperture.
 Heavy Metal Base like Lead.
 Copper Lined Barrel.
CONES AND CYLINDERS
cylinder cone
base
barrel

Advantage
 Better restriction to the x-ray field.
 Produce less penumbra than aperture diaphragm.
 Round image on a rectangular film.
 Reduce scatter Radiation.
CONES AND CYLINDERS

 Most effective beam restricting device.
 Two stage of shutters to control the Beam.
 The shutters are made up of lead.
COLLIMATOR

Fieldenter indicator
 To identify the centre of the x-ray field a thin plastic
sheet having a painted cross line is mounted at end of the
collimator.
COLLIMATOR

OR
.
Four TYPE OF COLLIMATOR
 Manually operated collimator
 Electrical collimator
 Automatic collimator
 CT collimator

Advantages
 Light beam shows the center & exact configuration of
the X-Ray field.
 Helps to limits penumbra.
 Provides adjustable rectangular or square field.
 Reduce scatter radiation that improves image contrast.
 It reduce patient dose.
COLLIMATOR

 Filtration is the process of increasing the mean
energy.
Filter
 A sheet of metal placed in the path of X-ray
beam
 It absorb low energy radiation before it
reach to the patients.
 Reduce skin exposure dose up to 80%.
RADIOGRAPHIC FILTER

There are two type of filter-
 Inherent filter
 Added filter
TYPES OF FILTER
Inherent filter
Added filter

Inherent filter
 Glass envelope window, tube cooling oil and tube
housing window are responsible for the inherent
filtration it is equivalent of 0.5 to 1 mm Al.
Added filter & types
 Aluminium filter
 Compound filter
 Wedge filter
 Peripheral filter
 Molybdenum filter
INHERENT FILTER
Inherent filtration

Aluminium filter
 Aluminium filter is the most commonly use in
diagnostic radiology.
 It is available in different thickness, less than 50 kVp
we are use 0.5 mm,50 to 70 kVp use 1.5 mm and
above 70 kVp we are use 2 to 2.5 mm Al filter.
Compound filter
 Copper and aluminium use for compound filter, it is
useful of high energy X-ray.
 Copper thickness is 0.1 to 0.9 mm and aluminium
filter thickness is 1.0 mm.
TYPE OF FILTER

Wedge filter
 To obtaining uniform density of part like we are doing
angiography of head, neck and skull lateral radiograph to
insert a wedge shape filter.
TYPE OF FILTER

Peripheral filter
 Peripheral filter use for lower limb angiography study.
 Function of peripheral filter is to provide uniform density of
part and prevent over exposure.
 Made up of plastic and putting a thin layer of silicon
copper.
TYPE OF FILTER

Molybdenum filter
 Molybdenum filter use in mammography, it is work as
special k-edge principle.
 Mo target transmit 17.5 kev K-alpha and 19.6 kev K-beta
characteristic radiation to produce good contrast of breast
image.
TYPE OF FILTER
17.5
6

Molybdenum filter
 Bremsstrahlung radiation is continuous photon energy
spectrum with a maximum energy determine by the selected
Kvp value.
 It attenuate the above 20 kev bremsstrahlung radiation and
convert into characteristic radiation.
 Molybdenum filter always use with molybdenum target and
thickness of Mo filter is 0.03mm.
TYPE OF FILTER

Advantage
Absorb low energy X-Ray photon.
Reduce unnecessary radiation dose to the patient.
It increase the quality of radiograph.
Disadvantage
Decrease some energy of X-ray photon.
Increased exposure factor.
Higher tube loading required.
FILTERS

• Radiographic grid was invented by Dr
Gaustav Buckey in 1930.
• Grid is a device use to eliminate scattered
radiation from radiographic field before
reaching on IR.
• The use of grid improve the radiographic
contrast, but at the cost of increased
radiation dose.
• Placed in between object & Image receptor.

Construction
• Grid consists of series of lead
strips separated by strips of
radioparent material.
• The radio-parent material may
be of aluminium, organic
compound or carbon fibres.
• Lead strips (foils) of grid are
arranged in such a way that
they allows most of the primary
radiation to pass through and stops
the scattered radiation originating from object, before reaching
to the detector system i.e. Film.

GRID RATIO
Ratio of height of lead strips and
distance between them.
If d= thickness of lead strips
D= distance between two lead strips
h= height of lead strips
GR = h/D.
Grid ratio is a parameter which is wisely
used to express the grid ability to remove
scatter radiation.
Grids are available with GR from 4:1 to
16:1or more.
Higher the grid ratio, more will be the
patient dose.

Grid lattice or Grid frequency
• Grid frequency is expressed as the number
of lead strips per inch measures across the
grid
• Grid with high grid frequency shows less
distinct grid lines on a radiograph than grid
with low frequency.
• Higher the frequency, thinner its strips of
inter space material must be.(general
thickness of lead strips is 0.05 mm)
• Most of the grid have frequency in the

Grid lattice or Grid frequency
• Grid frequency can be calculated if
thickness of inter space material & lead
strips is known
• Grid frequency = 25.4/D+d
D = Thickness of interspaces
in mm
d = thickness of lead strips in
mm
Grid used in mammography have low grid
ratio (2:1-4:1). These grids have grid

LEAD CONTENT
• Lead content of a grid is expressed in g/cm2 .
• The amount of Pb in a grid is a good indicator of its ability to
improve contrast, provided the grid is well designed. Poor
designed will be solid sheet of lead.
There is a definite relation between the grid ratio, lead content
& number of lines per inches.
Keeping grid ratio constant, if we increase the No of lines/inch
the lead content will decrease. (either by decreasing thickness
of lead strips or interspaces).This puts a limitation on the No of
lines/inch a grid may have and still be effective.
A 133-line 10:1 grid improves contrast about the same as an
80-line 8:1 grid.

Types of grid
Based on grid pattern Linear and Cross
Grid orientation Parallel and Cross
On movement Stationary and Moving

The pattern of grid can be seen from the top view.
The two basic grid patterns are linear & crossed.
In a linear grid the lead strips are parallel to each
other in their longitudinal axis.
Most X-Ray tables are equipped with linear grids.
Linear grid allow us to angle the X-Ray tube along the
length of grid without loss of primary radiation from grid
“cutoff’’. Can not eliminate scatter radiation along the
length of grid.
LINEAR
GRID

CROSSED GRID
Linear grid ratio 5:1
Crossed grid ratio 10:1
Crossed grid is formed by superimposing two linear grids
at right angle .
The grid ratio of crossed grid is equal to the sum of the
ratios of the two linear grid.
Eliminates scatter radiation in all direction.
Tube angle not possible.
Can be used for radiography of small area.
CROSSED
GRID

FOCUSING GRID
In focused grid the lead strips are slightly angled so that they
focus in space.
Focused grid may be either linear or crossed.
Linear focused grid at a line in space is called convergent line.
Cross grid converge at a point in the space is called convergent
point.
The perpendicular distance between grid & convergent line or
convergent point is called focal distance.
The distance within which a focused grid can be used without a
significant loss of primary radiation is called focusing range.
Focusing range is fairly long for low ratio grid & narrow for
high ratio grid.(5:1 grid focused at 40 inch has focusing range 28 -72 inch whereas
16:1 grid focused at 40 inch has a range 38-42 inch)

An ideal grid is that which allow all primary radiation to pass through and
absorb all secondary radiation, but the ideal grid does not exist. The price of
better contrast is increased patient exposure. The grid performance can be
evaluate by following three methods:
Primary Transmission (Tp): It is measurement of the percentage
of primary radiation transmitted through a grid.
Intensity with grid
Intensity without grid
Bucky factor (B) : is the ratio of the incident radiation falling on
the grid to the radiation transmitted through the grid.
Incident radiation
Transmitted radiation
Bucky factor is similar to primary transmission except for one difference. Primary
transmission indicates only the amount of primary radiation absorbed by a grid,
whereas the Bucky factor indicate the absorption of both primary and secondary
radiation.
EVALUATION OF GRID PERFORMANCE
Tp = X 100
B =

Contrast Improvement Factor (K): The ratio of the contrast with a grid to the
contrast without a grid.
Contrast improvement factor depends upon the KVp, field size & part thickness.

CONCLUSION
Ultimately, it is the responsibility of the radiographer to
use the proper collimation and under no circumstance should
exceed the exposure field the size of the image receptor.
 The radiographer should always limit the field to the part
being examined thereby improving the image quality and
minimizing the patients dose.
It is a way to achieves “ALARA”(As Low As Reasonably
Achievable) principle by using beam restrictive devices and
radiographic filter.

Beam restriction devices in x rays by sushil pattar

Beam restriction devices in x rays by sushil pattar

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Beam restriction devices in x rays by sushil pattar