Download Beam Restricting Devices

Beam Restricting Devices
Sergeo Guilbaud
School of Radiologic
Technology
Abstract
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Beam limiting devices are used to control the amount
of scatter radiation that is created as a result of
Compton’s interaction with the body’s tissues.
Scatter hinders the visualization of detail by adding
additional density to the radiographic image. Scatter
is also responsible for increasing the patient’s
radiation dosage, decreasing image contrast and
impairing the visibility of detail. By controlling the
amount of scatter radiation that is produced, there is
improvement in detail, enhancement of contrast and
a reduction in radiation dosage to the patient. Beam
limiting devices are types of radiographic tools used
to reduce the production of scatter radiation.
Objectives
At the conclusion of this lecture, each participant should
be able to:
1.
Identify the principle factors that are responsible
for the creation of scatter radiation.
2.
Identify the three types of Beam limiting devices.
3.
Identify the advantages and disadvantages of each
beam limiting device.
4.
Identify the rationale for using Positive Beam
Limitation (PBL).
5.
Describe the ancillary devices that may be used to
further reduce the amount of scatter radiation
reaching the image receptor system.
Principle factors creating scatter
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Kilovoltage (kV)
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Irradiated Tissues
Principle factors creating scatter
Kilovoltage (kV)
Affects beam penetrability
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As kVp increases, the percentage of x-ray
photons that undergo Compton’s
interaction increases.
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B/C Compton interactions are responsible for
creating scatter, as the kVp increases, the
percentage of primary photons that will
scatter also increases.
Principle factors creating scatter
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As kVp increases, the percentage of
photons that undergo photoelectric
absorption decreases.
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Thus, the patient receives significantly less
radiation.
Principle factors creating scatter
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Scattered photons from Compton’s interaction
are of no value to the radiographic image b/c
they do not assist in demonstrating the
anatomic structures of interest. (add
additional density to film)
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The majority of the overall radiographic density on a film
is created by scattered photons.
Thus, a decrease in Compton’s scatter will improve the
image quality by removing the scatter that reaches the
film.
Principle factors creating scatter
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Irradiated Tissue
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Tissue volume and atomic number affect
the amount of scatter produced during a
radiographic exposure.
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Tissue volume irradiated is controlled by the
radiographer.
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When the amount of tissue volume increases, so
does the amount of scatter radiation.
The larger the field size, the more photons to
interact with tissue, this results in greater amount of
scatter production.
Principle factors creating scatter
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Remember:
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More scatter is created in bone than in soft tissue.
When the beam is restricted, less scatter radiation
is produced thus, less scatter reaches the film.
The technical factors may also have to be
manipulated.
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There may need to be an increase in the technical
factors to compensate for the reduction in the overall
density on the film.
Beam Restrictors
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Three types:
1. Aperture diaphragms.
2. Cones/cylinders.
3. Collimators.
Aperture Diaphragm
A flat sheet of metal with a hole cut in the
middle attached to the tube head.
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The simplest of beam limiting devices both
in design and ease of use.
This was the original type of beam limiting
device.
Use is now limited to dedicated chest units,
where field size and SID do not vary.
Aperture Diaphragm
Aperture Diaphragm
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Advantages:
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1. Simple design
2. Low cost
3. Ease of use
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Disadvantages
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1. Increase in
penumbra.
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This is a result of the
diaphragm’s close
proximity to the target.
An increase in off-focus
(stem radiation)
radiation reaching the
film. This can result in
images, similar to
shadows beyond the
field.
Aperture Diaphragm
Formula for determining diaphragm field size.
Projected image size =
SID X diameter of diaphragm
dx from focal spot to diaphragm
e.g. A diaphragm is placed 5” from the FS. The
diameter of the opening in the aperture is a 2” circle.
What would be the projected image size if the SID is
40”?
40 x 2 = 80
-------- --- = 16 inches
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Cones/Cylinders
Circular aperture diaphragms with metal
extensions.
Cone – has a flaring extension w/ upper
diaphragm smaller than bottom (flared end).
Cylinder – does not flare but, may be
equipped w/ an extension sleeve which can
expand or collapse to cavy the beam
restriction.
Cones/Cylinders
Cones/Cylinders
Advantages:
1. Inexpensive.
2. Simple to use.
3. Reduce penumbra and off-focus
radiation.
4. Provide better beam restriction at a
greater distance from the focal spot.
Cones/Cylinders
Disadvantages:
n Fixed field sizes unless equipped with
extension sleeve.
n Flared cones are no better at reducing
penumbra than aperture diaphragms.
Cones/Cylinders
Formula for determining Cone field size.
Projected image size =
SID x lower diameter of cone
Dx from focal spot to bottom of cone
e.g. The diameter of the lower rim of a cone is 3”. The bottom of the cone
is 16” from the focal spot. What will be the size of the projected image
at 40” SID?
40 x 3
120
Projected image =
-------- =
----- = 7.5 “ circle
16
16
Collimators
Collimators
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Consists of two sets of lead shutters at right angles
to one another which move in opposing pairs.
Each set moves symmetrically from the center of the
field.
They can be adjusted to correspond to an infinite
number of squares or rectangular field sizes.
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Bottom shutters serve to reduce penumbra along the
periphery of the beam.
Upper shutters help reduce amount of off-focus radiation.
Collimators
Construction:
Two sets of shutters moving in
opposite pairs.
Light field provided by light
source.
Mirror mounted at 45 degrees
within path of beam to
reflect light source. Provides
additional beam filtration.
Collimators
Advantages:
1. Permit infinite number of field sizes.
2. Provides light source as an aid to
proper tube, part and film alignment.
3. Only one device providing all said
functions.
Collimators
Disadvantages:
1.
2.
3.
4.
Expensive to manufacture.
Light requires replacement with heavy
usage.
Whenever light source is changed, proper
alignment must be checked (light-field
congruency test.
Collimator accuracy testing must be
performed at regular intervals.
Positive Beam Limitation (PBL) Devices
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Automatic Collimators
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Automatically reduce the collimated area to the
size of the film placed in the Bucky tray.
When operating properly, they leave a small
unexposed area on all four borders of the film.
State regulations allows the collimation to fall
within plus or minus 2% of the SID.
Can be overridden by the radiographer.
Assure that area exposed does not exceed image
receptor size.
Ancillary devices used to reduce the scatter reaching the film
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Normally designed to fill a special need.
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Lead blockers
Lead Masks
Ancillary Devices
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Lead Blockers
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A sheet of lead-impregnated rubber that
can be cut to a size or shape.
Especially useful in preventing scatter
radiation from reaching the film especially
when imaging the Thoracic & Lumbar
spines in the projection and the shoulder
when done on the table.
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Lead Masks
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A sheet of lead-impregnated rubber that
can be cut to a size or shape to correspond
to a particular field size then, secured to
the end of a collimator.
Most common: lead mask used during
cerebral angiography.
References
Bushberg et al, The Essentials of Physics and Medical Imaging,
Williams & Wilkins Publisher.
Bushong, S., Radiologic Science for Technologists, Physics, Biology
and Protection, 8th Edition, C.V. Mosby Company.
Carlton et al, Principles of Radiographic Imaging, An Art and
Science, Delmar Publishing.
Curry et al, Christensen’s Introduction to the Physics of Diagnostic
Radiology. 4th Edition, Philadelphia, PA, Lea and Febiger, 1990.
Selman, J., The Fundamentals of X-Ray and Radium Physics, 8th
Edition, Charles C. Thomas Publisher.