Chapter 16 Beam-Restricting

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Chapter 16 Beam
-
Restricting
Devices


Three factors contribute to an increase in
scatter radiation:


Increased kVp


Increased Field Size


Increased Patient or Body Part Size
.

X
-
ray Interactions


a


some interact with the
patient and are scattered
away from the patient.


b


some are absorbed


c
-

some pass through
without interaction


d


some are scattered in
the patient


c & d are image forming
x
-
rays.


Relative Contributions of Scatter to
the Radiographic Image

Percent Interaction of Scatter and Percent
Transmission through 10 cm of Tissue

kVp

Photoelectric

Compton

Total

%

Transmission

60

70 %

30%

>99%

<1%

70

60 %

40%

> 99%

< 1%

80

46%

52%

98%

2%

90

38%

59%

97%

3%

100

31%

63%

94%

6%

110

18%

83%

93%

7%

Beam
-
Restricting Devices


There are two principal means to reduce
scatter radiation:


Beam Restricting Devices
limit the field
size to reduce scatter and primary
radiation.


Grids
to absorb scatter before it reached
the image receptor.

Beam
-
Restricting Devices


There are three principal types of beam
restricting devices:


Aperture Diaphragm


Cones & Cylinders



Collimators

Production of Scatter Radiation


Two kinds of x
-
rays are responsible for the
optical density, or degree of blackening on
a radiograph.


Those that pass through the patient
without interacting called remnant ray.


Those that are scattered through Compton
interaction.

Kilovolt Peak


As x
-
ray energy increases, the
relative

number

of x
-
rays that undergo Compton
Scattering
increases
.


The
absolute number

of the Compton
interactions
decrease

with increasing
energies but the number of photoelectric
interactions decreases more rapidly.

Field size


The size of the field or
area being irradiated
has a significant
impact on scatter
radiation.


Field size is
computed in square
inches or square cm

Field size


Scatter radiation
increases as the field
size increases.


The relative intensity
of the scatter varies
more when the field
size is small than
when the field is
large.

Field size


When the field size is reduced, the
resulting reduction in scatter will reduce
the density on the image.


The mAs must be increased to maintain
density.


The reduced scatter will improve contrast
resolution resulting in improved image
quality.

Field size


To change from a 14” x 17” to a 10” x 12”
increase mAs 25%.


To change from a 14” x 17” to a 8” x 10”
increase mAs 40%.

Patient or Part Thickness


More scatter results from imaging thick
body parts compared to thin body parts.


There will be more scatter for a lumbar
spine film compared to a cervical spine
film.


As tissue thickness increases, more of the
rays go through multiple scattering.

Tissue Thickness


The relative intensity
of scatter radiation
increases with
increasing thickness
of the anatomy.


The amount of
primary radiation also
increases to
compound the
scatter.

Patient thickness


Normally body thickness
is out of our control but
we can change the
method of imaging to
improve image quality.


With obese patients,
tissue thickness is
reduced when taking the
film recumbent due to
compression.


Be sure and measure the
patient recumbent.

Types of Beam
Restricting Devices


There are three types
of beam restricting
devices.


Diaphragms


Cones


Collimators

Types of Beam Restricting
Devices


Large field sizes
result in more scatter
radiation that reduces
image contrast.

Aperture Diaphragm


Aperture diaphragms
are basically lead or
lead lines metal
devices placed in the
beam to restrict the
x
-
rays emitted from
the tube.

Aperture Diaphragm


Apertures are the
simplest form of
collimation.


In this case, the
aperture is used to
reduce exposure to
the breast tissue.

Aperture Diaphragm


The width or size of
the aperture is fixed
and can not be
adjusted.


The operator must be
careful when placing
the aperture in the
beam.

Cones and Cylinders


Cones and cylinders
are modifications to
the aperture.


Cones are typically
used in dental
radiography.


Cones and Cylinders


Most cone produce a
round image on a
rectangular film.


Cones are very
effective at reducing
scatter.


Hard to center.


Variable Aperture Collimator


Proper collimation of
the x
-
ray beam has
the primary effect of
reducing patient dose
by restricting the
volume of tissue
irradiated.

Variable Aperture Collimator


Proper collimation
also reduces scatter
radiation that
improves contrast.

Light Localizing Collimator


The light localizing
variable aperture
collimator is the most
common beam
restricting device in
diagnostic
radiography.

Collimator


Not all of the x
-
rays
are emitted precisely
from the focal spot.


These rays are called
off
-
focus radiation
and they increase
image blur.

Collimator


First stage shutters
protrude into the tube
housing to control the
off
-
focus radiation.


Adjustable second
stage shutter pairs
are used to restrict
the beam.

Collimator


Light localization is
accomplished by a
small projector lamp
and mirror to project
the setting of the
shutters on the
patient.

Collimator


The light field and x
-
ray beam should
match to avoid
collimator cut
-
off.


A scale on the
collimator is used to
match the beam to
the film size at fixed
SID’s.

Collimator


Many newer
collimators a bright
slit of light is provided
to properly center the
beam and the film.


Units manufactured
between 1974 and
1994 has motorized
shutters.

Collimator


A sensor in the Bucky
and the motor were
used to automatically
collimate the image to
film size. This was
called a positive
-
beam limiting (PBL)
device.


Required by the FDA.

Collimator


Requirement was
repealed in 1994.


If the beam is not
centered to the film,
collimator cut
-
off will
occur on the top or
bottom of the image.

Collimator


If the tube is not
centered to the Bucky
or the film is not
pushed into the
Bucky, side to side
collimator cut
-
off will
occur.

Collimation Rules


California required three borders of
collimation to be seen on the film.


Collimation must be slightly less than film
size or to the area of clinical interest,
whichever is smaller.


ANY exposure beyond the film is
unnecessary patient exposure.

End of Lecture

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